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UBC Theses and Dissertations

Eye of the harbour seal, Phoca vitulina 1970

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THE EYE OF THE HARBOUR SEAL, PHOCA VITULINA by GLEN STEWART JAMIESON B.Sc. ( A g r . ) , M c G i l l U n i v e r s i t y , . 1967 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n the Department o f Z oology We a c c e p t t h i s t h e s i s as c o n f o r m i n g t o the r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA Octo b e r , 1970 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f t h e r e q u i r e m e n t s f o r an a d v a n c e d d e g r e e a t t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r a g r e e t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by t h e Head o f my D e p a r t m e n t o r by h i s r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l n o t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . D e p a r t m e n t o f ^-p-p/a-tyx/ ' / The U n i v e r s i t y o f B r i t i s h C o l u m b i a V a n c o u v e r 8 , C a n a d a D a t e - A b s t r a c t The Eye o f the Harbour S e a l , — Phoca v i t u l i n a By Glen S t e w a r t Jamieson The s t r u c t u r a l and f u n c t i o n a l o r g a n i z a t i o n o f t h e p i n n i p e d eye, as r e p r e s e n t e d by t h e h a r b o u r s e a l , Phoca v i t u l i n a , i s much more complex than p r e v i o u s l y r e a l i z e d . The r e t i n a o f t h e h a r b o u r s e a l i s - s i m i l a r t o t h a t o f t h e h a r p s e a l , i n t h a t i t shows l i t t l e z o n a t i o n o f t h e i n n e r n u c l e a r l a y e r , p o s s e s s e s l a r g e h o r i z o n t a l c e l l s , and has s i m i l a r n u c l e a r d e n s i t i e s w i t h i n t h e d i f f e r e n t r e t i n a l l a y e r s t h r o u g h o u t the r e t i n a . The o u t e r n u c l e a r l a y e r i s t h e t h i c k e s t o f a l l t h e r e t i n a l l a y e r s . P h o t o r e c e p t o r c e l l s o f t y p e s B and C were o b s e r v e d w i t h a r a t i o o f r o u g h l y 1:23 r e s p e c t i v e l y . T h i s r a t i o and p h o t o r e c e p t o r d e n s i t y a l o n g w i t h a w e l l - d e v e l o p e d and e x t e n s i v e tapetum, i n d i c a t e s a h i g h l y l i g h t - s e n s i t i v e r e t i n a . The e x i s t e n c e o f r e t i n a l f o l d i n g i n m a rine mammals i s a l s o q u e s t i o n e d and d i s c u s s e d . R e f r a c t i v e o b s e r v a t i o n s o b t a i n e d t h r o u g h r e t i n o s c o p y show t h a t t h e h a r b o u r s e a l eye i s v e r y myopic and a s t i g m a t i c i n a i r , and h y p e r m e t r o p i c w i t h no d i s c e r n a b l e a s t i g m a t i s m i n w a t e r . The a x i s o f l e a s t c o r n e a l c u r v a t u r e i s h o r i z o n t a l ; c u r r e n t t h e o r i e s p r e d i c t t h a t the s t e n o p e i c p u p i l would thus c o r r e c t f o r a s t i g m a t i s m i n a i r . The f u n c t i o n a l s i g n i f i c a n c e o f the a s t i g m a t i s m , w h i c i s i n t h e o r d e r o f 7 t o 13 d i o p t e r s , has n o t been a d e q u a t e l y d e t e r m i n e d t o d a t e , however. These o b s e r v a t i o n s a r e i n agreement w i t h t h o s e r e c e n t l y r e p o r t e d f o r t h e h a r p s e a l . B e h a v i o u r a l s t u d i e s have a l s o c o n f i r m e d t h e h i g h s e n s i t i v i t y o f t h e h a r b o u r s e a l eye t o l i g h t , as s u g g e s t e d by the h i s t o l o g i c a l o b s e r v a t i o n s . Two c a p t i v e h a r b o u r s e a l s were t r a i n e d t o s e l e c t a two-bar p a t t e r n o ver a one-bar p a t t e r n i n b o t h a i r and w a t e r , d i s c r i m i n a t i n g f o r t h e p r e s e n c e o f a gap. T h e i r a b i l i t i e s t o c a r r y out t h i s t a s k were measured as t h e gap i n t h e two-bar p a t t e r n was g r a d u a l l y r e d u c e d i n s i z e . The l i g h t i n t e n s i t y 2 o f t h e w h i t e p a r t s o f t h e s t i m u l u s c a r d s was 284-369 candela/m . Under t h e s e c o n d i t i o n s , the h a r b o u r . s e a l can d e t e c t a gap as s m a l l as 1 mm a t a d i s t a n c e o f 1.7 m i n b o t h a i r and w a t e r . These s t u d i e s f a i l t o p r o v i d e v a l u e s o f the v i s u a l a c u i t y o f the h a r b o u r s e a l i n t h e s t r i c t sense, however, owing t o t h e p h y s i c s o f t h e p a t t e r n u t i l i z e d . R a t h e r , t h e y i n d i c a t e a measure o f the a b s o l u t e s e n s i t i v i t y o f t h e eye t o l i g h t . Acknowledgments: A p p r e c i a t i o n i s e x p r e s s e d t o Dr. A. Q. McCormick, Dept. o f Ophthalmology, U.B„C; D r s . D. Tennant and B. J e r v i s , Vancouver o p t o m e t r i s t s ; and Dr. G.G.E. Scudder, Dept. o f Zoology, U.B.C., f o r many h e l p f u l d i s c u s s i o n s . A l l a n M i t c h e l l , o f t h e P a t h o l o g y L a b o r a t o r y o f the Department o f Ophthalmology, U.B.C., and M i s s Joan M e r e d i t h , o f the Department o f Zoology, U.B.C., a r e thanked f o r t h e i r t e c h n i c a l a s s i s t a n c e . The s t a f f o f t h e Vancouver P u b l i c Aquarium i s a l s o thanked f o r t h e i r a s s i s t a n c e , as a r e Jane C r o c k e r and P e t e r C h a p p e l l who h e l p e d i n t e s t i n g the s e a l s . P a r t i c u l a r thanks a r e e x p r e s s e d t o Dr. H.' D. F i s h e r , my s u p e r v i s o r , and t o my w i f e , Dorothy, f o r t h e i r u n r e l e n t i n g s u p p o r t . T h i s r e s e a r c h was s u p p o r t e d i n p a r t by NRC r e s e a r c h g r a n t A-2091. ' i i c TABLE OF CONTENTS L i s t o f F i g u r e s . . . . . . . . . . i v L i s t o f T a b l e s . v i i I n t r o d u c t i o n 1 M a t e r i a l s and Methods 6 Morphology and H i s t o l o g y 6 R e f r a c t i o n 7 V i s u a l D i s c r i m i n a t i o n s 11 R e s u l t s . . . 19. Morphology and H i s t o l o g y 19 R e f r a c t i o n 29 V i s u a l D i s c r i m i n a t i o n s . . . . . 31 D i s c u s s i o n . 38 Summary . 63 G l o s s a r y o f terms . 65 L i s t o f R e f e r e n c e s 66 L i s t o f F i g u r e s F i g u r e 1. A p p a r a t u s used i n the underwater r e f r a c t i o n 9 o f t h e h a r b o u r s e a l eye. 2. M e a s u r i n g the r e f r a c t i o n o f t h e h a r b o u r s e a l 9 eye i n a i r . 3. Photo o f t h e t a n k s a t t h e Vancouver P u b l i c 13 Aquarium where t h e b e h a v i o u r a l a s p e c t s o f t h i s s t u d y were done. 4. A p p a r a t u s used i n t e s t i n g the h a r b o u r s e a l ' s 14 underwater d i s c r i m i n a t i o n o f one- and two-bar p a t t e r n s p r e s e n t e d s i m u l t a n e o u s l y . The r e l a t i o n s h i p s o f the t a n k s a r e shown i n b o t h d o r s a l (A) and l a t e r a l (B) v i e w s . F o r t e s t i n g i n a i r , the wa t e r was d r a i n e d from t h e s t i m u l u s p r e s e n t a t i o n tank (P) and the s t i m u l u s c a r d s were p r e s e n t e d a t the l e v e l o f t h e upper s e t o f windows ( W 2 ); the wa t e r l e v e l i n t h e t e s t i n g tank (T) remained t h e same b u t t h e s e a l s were t r a i n e d t o swim a t t h e s u r f a c e . One s e a l a t a time was l e t from the h o l d i n g tank (H) i n t o t h e t e s t i n g tank by means o f a s l i d i n g p a r t i t i o n . 5. a-5.b-5.c. Photos showing d e t a i l s o f the b e h a v i o u r a l 15 d i s c r i m i n a t i o n t a n k s and t h e p r e s e n t a t i o n o f a t y p i c a l problem. A. Photo o f the s t i m u l u s c a r d s , w i t h the p o s i t i v e c a r d a t the r e a d e r ' s r i g h t . In a no r m a l t r i a l , t he c a r d s would n o t be h e l d as h i g h , so t h a t t h e s e a l s would n o t be a b l e t o see t h e c a r d s . B. A c o r r e c t r e s p o n s e by t h e s e a l , as i t chose t h e r i g h t - h a n d l e v e r . - The s e a l ' s approach was made underwater, w i t h the c a r d s viewed t h r o u g h t h e lower window. C . Photo showing d e t a i l s o f t h e s t i m u l u s p r e s e n t a t i o n tank. The s t i m u l u s c a r d s were p r e s e n t e d j u s t t o the f o r e o f the b u c k e t , w h i c h c o n t a i n e d the h e r r i n g r e w a r d s . 6. a - 6 . b - 6 . c - 6 . d . Photos o f h i s t o l o g i c a l s e c t i o n s o f 2 0 p i n n i p e d eyes. A. Harbour s e a l . (Phoca v i t u l i n a ) B. N o r t h e r n f u r s e a l . ( C a l l o r h i n u s u r s i n u s ) i v C. Harp s e a l . ( P a g o p h i l u s g r o e n l a n d i c u s ) D. K i l l e r whale. (Grampus orca) 7. C l o s e - u p photo of the e x t e n s i v e f i b e r network 21 c h a r a c t e r i s t i c about the p e r i p h e r y o f the h arbour s e a l i r i s . 8. a-8.b-8.c. P u p i l shapes under d i f f e r e n t l i g h t 22 i n t e n s i t i t e s . A. V e r y h i g h l i g h t i n t e n s i t y . T h i s i s as s m a l l as the p u p i l may be made. B. H i g h l i g h t i n t e n s i t y . The p u p i l i s more t e a r - s h a p e d than i n A. C. E f f e c t o f a t r o p i n e . T h i s shows the maximum s i z e o b t a i n a b l e , and r e p r e s e n t s the p u p i l s i z e found under v e r y low l i g h t i n t e n s i t i e s . 9. V e r t i c a l c r o s s s e c t i o n o f the c e n t r a l a r e a o f 24 t h e r e t i n a . T o l u i d i n e b l u e . 10. V e r t i c a l s e c t i o n t h r o u g h the o p t i c n e r v e f i b e r 24 and g a n g l i o n l a y e r s . L a r g e r a d i a l f i b e r s o f M u l l e r can be seen v e r t i c a l l y t r a n s v e r s i n g t h e s e l a y e r s . Arrows p o i n t t o g a n g l i o n c e l l s and a s m a l l b l o o d v e s s e l . T o l u i d i n e b l u e . 11. V e r t i c a l s e c t i o n t h r o u g h a g i a n t g a n g l i o n c e l l . 24 H & E. 12. V e r t i c a l s e c t i o n t h r o u g h the i n n e r n u c l e a r l a y e r 24 showing l a r g e h o r i z o n t a l c e l l s (h) , amacrine c e l l s (a) , b i p o l a r c e l l s (b) and n e u r o g l i a l c e l l s ( n ) . H o r i z o n t a l c e l l a x i s c y l i n d e r s a r e marked by a r r o w s . T o l u i d i n e b l u e . 13. V e r t i c a l s e c t i o n t h r o u g h the i n n e r n u c l e a r l a y e r 24 showing t h e synapses between b i p o l a r c e l l s (b) and a p o l y s y n a p t i c p e d i c l e ( a r r o w ) . T o l u i d i n e b l u e . 14. V e r t i c a l s e c t i o n t h r o u g h th e o u t e r p l e x i f o r m and 26 o u t e r n u c l e a r l a y e r s showing o l i g o s y n a p t i c (o) and p o l y s y n a p t i c (p) t e r m i n a l p e d i c l e s . Arrow p o i n t s t o cone-type p h o t o r e c e p t o r n u c l e u s . T o l u i d i n e b l u e . 15. V e r t i c a l s e c t i o n t h r o u g h the o u t e r l i m i t i n g 26 membrane and l a y e r o f r o d s and cones, showing p h o t o r e c e p t o r i n n e r ( r i s ) and o u t e r (ros) segments. A r r o s p o i n t s t o c one-type p h o t o r e c e p t o r n u c l e u s . T o l u i d i n e b l u e . v 16. V e r t i c a l s e c t i o n t h r o u g h the pigment e p i t h e l i u m . 26 Arrows'point t o c y t o p l a s m i c p r o c e s s e s e x t e n d i n g towards the p h o t o r e c e p t o r o u t e r segments. T o l u i d i n e b l u e . 17. H o r i z o n t a l s e c t i o n ( s l i g h t l y o b l i q u e ) t h r o u g h 26 the r e g i o n o f t h e o u t e r l i m i t i n g membrane ( p o i n t s ) . Arrows p o i n t t o p h o t o r e c e p t o r n u c l e i . A z u r I I . 18. V e r t i c a l s e c t i o n t h r o u g h pigment e p i t h e l i u m and 26 tapetum. Arrows p o i n t t o c h o r i o - c a p i l l a r i e s t r a n s v e r s i n g t h e tapetum. T o l u i d i n e b l u e . 19. and 20. V e r t i c a l s e c t i o n t h r o u g h t h e r e t i n a 28 showing t h e s l i g h t form o f r e t i n a l f o l d i n g . Only t h e o u t e r n u c l e a r l a y e r and l a y e r o f r o d s . and cones a r e f o l d e d . H & E. 21. V e r t i c a l s e c t i o n t h r o u g h the r e t i n a showing t h e 29 complex form o f r e t i n a l f o l d i n g . A l l t h e r e t i n a l l a y e r s a r e f o l d e d . H & E. 22. a-22.b. Performance o f t h e two h a r b o u r s e a l s 32 d u r i n g c o n d i t i o n i n g . Each day's t r i a l s were b r o k e n down i n t o b l o c k s o f twenty t r i a l s each t o b e t t e r d e s c r i b e performance b e h a v i o u r . A. The performance o f t h e f o u r - y e a r o l d female, B l o n d i e . B. The performance o f t h e two-year o l d male, S c a r . 23. A comparison o f t h e a b i l i t y o f the f o u r - y e a r o l d 36 female h a r b o u r s e a l t o d i s c r i m i n a t e s m a l l d i f f e r e n c e s i n t h e gap s i z e o f the two-bar p a t t e r n i n b o t h a i r and w a t e r . A weighed l i n e a r r e g r e s s i o n was performed t o d e t e r m i n e the d i f f e r e n c e t h r e s h o l d w h i c h i s d e f i n e d as th e gap s i z e c o r r e s p o n d i n g t o the 75% (60 a r c s i n %) performance l e v e l . 24. A comparison o f t h e a b i l i t y o f the two-year o l d 37 male h a r b o u r s e a l t o d i s c r i m i n a t e s m a l l d i f f e r e n c e s i n t h e gap s i z e o f t h e two-bar p a t t e r n i n b o t h a i r and w a t e r . A w e i g h t e d l i n e a r r e g r e s s i o n was pe r f o r m e d t o d e t e r m i n e the d i f f e r e n c e t h r e s h o l d t h i c h i s d e f i n e d as t h e gap s i z e c o r r e s p o n d i n g t o t h e 7 5 % (60 a r c s i n %) performance l e v e l . v i L i s t o f T a b l e s T a b l e R e t r a c t i v e measurements o f t h e h a r b o u r s e a l as 3 0 o b t a i n e d i n t h i s s t u d y and by Johnson (1893) . Harp s e a l measurements were t a k e n by P i g g i n s (1970). These v a l u e s were o b t a i n e d i n a i r by means o f r e t i n o s - c o p y , w i t h t h o s e v a l u e s o b t a i n e d i n w ater n o t e d . Responses o f b o t h s e a l s i n b o t h a i r and w a t e r 3 5 f o r f i n a l t e s t i n g s . V a l u e s a r e g i v e n i n % c o r r e c t f o r each gap s i z e . E s t i m a t e d r e s o l u t i o n t h r e s h o l d s f o r b o t h s e a l s 5 7 i n a i r and w a t e r . V a l u e s i n p a r e n t h e s e s r e f e r t o the 95% c o n f i d e n c e l i m i t s . A comparison o f v i s u a l a c u i t y t h r e s h o l d s (V.A.T.) 60 and r e t i n a l s t r u c t u r e f o r a number of a n i m a l s from v a r i o u s s o u r c e s . v i i 1 I n t r o d u c t i o n P r e s e n t e v o l u t i o n a r y t h e o r y d i c t a t e s t h a t the v e r t e b r a t e s e v o l v e d i n w a t e r and hence the eye was i n i t i a l l y an a q u a t i c s t r u c t u r e : i t ' s b a s i c p l a n b e i n g e s t a b l i s h e d i n t h e f i s h e s . The p h y s i c a l p r o p e r t i e s o f w ater w h i c h i n f l u e n c e t h e eye and v i s i o n - p r o p e r t i e s such as t h e a b s o r p t i o n o f l i g h t , s c a t t e r i n g e f f e c t s and f r i c t i o n - a r e e s s e n t i a l l y e x a g g e r a t i o n s o f the same p r o p e r t i e s t h a t o c c u r i n a i r . Thus, when t h e v e r t e b r a t e s f i n a l l y took t o l a n d , t h e y were f a c e d p r i m a r i l y w i t h a q u a n t i t a t i v e d i s p a r i t y f o r most o f t h e p h y s i c a l p r o p e r t i e s w h i c h i n f l u e n c e v i s i o n . A few q u a l i t a t i v e changes were r e q u i r e d f o r a e r i a l v i s i o n , however, m o s t l y r e l a t i n g t o the d i f f e r e n c e i n r e f r a c t i v e i n d e x o f t h e medium. New f e a t u r e s i n the adnexa were a l s o r e q u i r e d t o p r o t e c t and m o i s t e n t h e now i m p o r t a n t r e f r a c t i v e s u r f a c e i n the eye, t h e c o r n e a . A l l the above m o d i f i c a t i o n s i n v o l v e d i n t h i s change o f media a r e q u i t e w e l l known ( W a l l , 1942) . \ The s econdary a d a p t a t i o n t o w a t e r w i t h an a e r i a l l y - a d a p t e d eye i s n o t so w e l l documented and u n d e r s t o o d . T h i s s econdary a d a p t a t i o n i s o b v i o u s l y p r e s e n t i n the c e t a c e a n s , w h i c h i n g e n e r a l may be c o n s i d e r e d c o m p l e t e l y a q u a t i c . I t i s i n t h o s e s p e c i e s w h i c h a r e amphibious, though, t h a t an a d a p t a t i o n p r o b l e m might a r i s e s i n c e somehow they must see w e l l i n b o t h a i r and w a t e r . Such forms a r e o f i n t e r e s t from an e v o l u t i o n a r y p o i n t o f v i e w s i n c e such a way o f l i f e may r e p r e s e n t s t a g e s i n t h e 2 I ! l a n d t o wa t e r t r a n s i t i o n . 1 There i s l i t t l e i n f o r m a t i o n on v i s i o n i n amphibious mammals, such as p i n n i p e d s , w h i c h b r e e d on l a n d b u t spend most o f t h e i r l i v e s i n w a t e r . B e i n g o n l y s e c o n d a r i l y adapted t o an a q u a t i c h a b i t a t , one would assume t h a t the v i s u a l d i f f i c u l t i e s o f amphibious mammals, i f any, l i e p r i m a r i l y u n derwater. The p r o b l e m o f a q u a t i c v i s i o n f o r t h e t e r r e s t r i a l eye i s one o f d e v e l o p i n g enough accommodation t o n e u t r a l i z e t h e h y p e r m e t r o p i a w h i c h r e s u l t s when t h e cornea i s i n c o n t a c t w i t h w a t e r . However, r a t h e r than u s i n g i n t r a - o c u l a r muscles t o i n c r e a s e t h e c u r v a t u r e o f t h e l e n s , as does t h e o t t e r ( W a l l s , 1942), p i n n i p e d s have a c h i e v e d a moderate degree o f h y p e r m e t r o p i a underwater ( P i g g i n s , 1970) by means o f a s p h e r i c a l l e n s . The c o m b i n a t i o n o f a s t e n o p e i c p u p i l a r r a n g e d a l o n g t h e a x i s o f g r e a t e s t c o r n e a l / c u r v a t u r e and a s e n s i t i v e r e t i n a i s tho u g h t t o make e x t e n s i v e accommodation i n a i r u n n e c e s s a r y ( W a l l s , 1942), a l t h o u g h i n a i r , t h e eyes a r e v e r y myopic and q u i t e a s t i g m a t i c (Johnson, 1893; P i g g i n s , 1970). In w a t e r , t h i s myopia and a s t i g m a t i s m d i s a p p e a r owing t o t h e s i m i l a r i t y o f t h e r e f r a c t i v e i n d i c e s of t h e co r n e a and w a t e r . To d a t e , most o f the emphasis on t h e s e n s o r y c a p a b i l i t i e s of p i n n i p e d s has been d i r e c t e d towards a c o u s t i c s and whether p i n n i p e d s employ a c t i v e sonar (eg. P o u l t e r , 1966; Schusterman e t a l . , 1967; Shaver and P o u l t e r > 1967), w i t h r e l a t i v e l y l i t t l e emphasis b e i n g d e v o t e d t o t h e s t u d y o f t h e o t h e r s e n s e s , p a r t i c u l a r l y v i s i o n . P r e v i o u s r e s e a r c h on the h a r b o u r s e a l , P_. v i t u l i n a 3 ( F e i n s t e i n and R i c e , 1966), and t h e C a l i f o r n i a sea l i o n , Z alophus c a l i f o r n i a n u s (Schusterman e t a l . , 1965), has been co n c e r n e d w i t h the a b i l i t i e s o f t h e s e a n i m a l s t o d i s c r i m i n a t e between t a r g e t s o f v a r i o u s s i z e s i n w a t e r . T h e i r s i z e - d i s c r i m i n a t i o n t e s t s have shown t h a t t h e s e s p e c i e s can make f i n e d i s c r i m i n a t i o n s and t h a t b o t h s p e c i e s p e r f o r m a l m o s t e q u a l l y w e l l . No t e s t s have been made t o d a t e , however, on t h e r e l a t i v e a b i l i t y o f p h o c i d s t o see above and below w a t e r , a l t h o u g h Schusterman and B a l l i e t have r e p o r t e d t h e v i s u a l a c u i t y o f t h e har b o u r s e a l and S t e l l e r sea l i o n underwater (1970a) and t h e v i s u a l a c u i t y o f t h e C a l i f o r n i a sea l i o n i n a i r (1970b) . A c c o u n t s on t h e s t r u c t u r a l o r g a n i z a t i o n o f t h e p i n n i p e d eye a r e a l s o few and f r a g m e n t a r y . I n i t i a l s t u d i e s by W i l s o n (1865) and Johnson (1893) on the h a r b o u r s e a l were p r i m a r i l y d e v o t e d t o t h e d e s c r i p t i o n o f the d i o p t r i c mechanism and c o n t r i b u t e d l i t t l e t o t h e u n d e r s t a n d i n g o f the r e t i n a . I t remained f o r P u t t e r (1903) t o d e s c r i b e the g r o s s morphology o f the r e t i n a , w h i c h he d i d f o r s e v e r a l p i n n i p e d s p e c i e s . However, owing t o t h e h i s t o l o g i c a l t e c h n i q u e s a v a i l a b l e t o him a t t h a t t i m e , he was u n a b l e t o d e s c r i b e i n d e t a i l t h e r e t i n a l o r g a n i z a t i o n a t t h e c e l l u l a r l e v e l . He d i d d e s c r i b e t h e tapetum and the g r o s s c h a r a c t e r i s t i c s o f the d i f f e r e n t r e t i n a l l a y e r s . He a l s o c o n f i r m e d Johnson's (1901) e a r l i e r s t a t e m e n t on the absence o f an a r e a c e n t r a l i s , w h i c h d i s c l a i m e d C h i e v i t z ' s (1889) r e p o r t o f the e x i s t e n c e o f such a f e a t u r e . 4 The most complete a c c o u n t t o d a t e on t h e s t r u c t u r e o f t h e p i n n i p e d r e t i n a i s by Nagy and R o n a l d (1970) on t h e h a r p s e a l , P a g o p h i l u s g r o e n l a n d i c u s . These a u t h o r s a l s o r e p o r t e d the absence o f an a r e a c e n t r a l i s and were a b l e t o o b s e r v e o n l y r o d p h o t o r e c e p t o r s , a l t h o u g h t e r m i n a l p e d i c l e s c h a r a c t e r i s t i c o f cones were n o t e d . However, as t h e y were u n a b l e t o d i s c e r n any f u r t h e r m o r p h o l o g i c a l d i f f e r e n c e s , t h e y c o n c l u d e d t h a t cones a r e n o t p r e s e n t i n t h i s s p e c i e s . The huge h o r i z o n t a l c e l l s and a p p a r e n t l a c k o f z o n a t i o n i n t h e i n n e r n u c l e a r l a y e r were a l s o p a r t i c u l a r l y emphasized. '• • - To d a t e , t h e r e has been no c o n c e r t e d a t t e m p t i n p i n n i p e d s t o r e l a t e v i s u a l a b i l i t y i n b o t h a i r and water w i t h eye s t r u c t u r e . Among t h e l o c a l p i n n i p e d s , t h e h a r b o u r s e a l (Phoca v i t u l i n a ) i s p e rhaps th e most c o n v e n i e n t f o r s t u d y . I t has l a r g e eyes and i s h a r d y . Thus, t h i s s p e c i e s was chosen f o r a s t u d y o f the v i s u a l a d a p t a t i o n s r e q u i r e d f o r an amphibious h a b i t . The o b j e c t i v e s were: 1. a h i s t o l o g i c a l and m o r p h o l o g i c a l s t u d y o f t h e h a r b o u r s e a l eye w i t h p a r t i c u l a r emphasis on the s t r u c t u r a l o r g a n i z a t i o n o f t h e r e t i n a . 2. a r e f r a c t i v e s t u d y o f t h e h a r b o u r s e a l eye i n b o t h a i r and w a t e r t o see i f the eyes a r e as a s t i g m a t i c and myopic as c l a i m e d by Johnson (1893). 3- a b e h a v i o u r a l i n v e s t i g a t i o n o f t h e h a r b o u r s e a l ' s d i s c r i m i n a t o r y a b i l i t i e s i n b o t h a i r and water w i t h the i n t e n t o f m e a s u r i n g v i s u a l a c u i t y . 5 W i t h t h i s i n f o r m a t i o n , t h e degree and n a t u r e o f t h e h a r b o u r s e a l ' s v i s u a l a d a p t a t i o n s t o an amphibious h a b i t w i l l be e v a l u a t e d and d i s c u s s e d . 6 M a t e r i a l s and Methods 1. Morphology and H i s t o l o g y : The s t r u c t u r a l a s p e c t s o f t h e h a r b o u r s e a l eye were s t u d i e d a t t h e l e v e l o f the l i g h t m i c r o s c o p e . Eyes o b t a i n e d from s e a l s caught i n the S t r a i t o f G e o r g i a were f i x e d whole i n 10% f o r m a l i n . Eyes were f i x e d w i t h i n a few minutes o f t h e a n i m a l ' s d e a t h , embedded whole i n B i o l o i d , 56-58° C. mp ( W i l l S c i e n t i f i c I n c o r p . , Ann A r b o r , M i c h . ) , and s e c t i o n e d a t 5-8 u i n t h e v e r t i c a l p l a n e , i e . p a r a l l e l t o t h e o r i e n t a t i o n o f the p h o t o r e c e p t o r s . S t a i n i n g was w i t h H a r r i s h a e m o t o x y l i n and e o s i n , p e r i o d i c a c i d - S c h i f f (PAS) , or t h i o n i n . In a d d i t i o n t o t h e above method, some eyes were removed from a n i m a l s a few m i n u t e s a f t e r a l l s i g n s o f body movement had ceased f o l l o w i n g i n j e c t i o n o f a l e t h a l dose o f B a r b - E u t h o l ( H a v e r - L o c k h a r t Lab., Kansas C i t y ) i n t o t h e e x t r a d u r a l v e i n . These eyes i m m e d i a t e l y a f t e r r e m o v a l were s l i t open around the o r a t e r m i n a l i s and t h e p o s t e r i o r hemisphere was c u t i n t o squares o f a p p r o x i m a t e l y one c e n t i m e t e r . Large s e c t i o n s were found n e c e s s a r y as w i t h s m a l l e r s e c t i o n s t h e r e t i n a d e t a c h e d i t s e l f f rom t h e c h o r o i d i n p o s t f i x a t i o n and became l o s t i n the s o l u t i o n . Owing t o the t h i n n e s s o f the r e t i n a l l a y e r , w h i c h was d i r e c t l y exposed t o t h e f i x a t i o n , no c a s e s o f poor p e n e t r a t i o n o f t h e f i x a t i v e a r o s e . F i x a t i o n was w i t h g l u t a r a l d e h y d e ( S a b a t i n i et. a_l. , 1963) o f ph = 7.4 i n Sorensen's phosphate b u f f e r ( C u l l i n g , 1962), f o l l o w e d by p o s t f i x a t i o n i n osmium 7 t e t r a o x i d e , d e h y d r a t i o n and embedding i n Epon ( L u f t , 1961). S e c t i o n i n g was done w i t h g l a s s k n i v e s on t h e S o r v a l l u l t r a - microtome, MT-1, a t 0.5-1 u i n b o t h the v e r t i c a l and h o r i z o n t a l p l a n e s . S t a i n i n g was w i t h a l k a l i n e t o l u i d i n e b l u e o r a l k a l i n e A z u r I I . C e l l t y p e s i n t h e r e t i n a were i d e n t i f i e d i n a c c o r d a n c e w i t h P o l y a k (1941) and c e l l numbers were d e t e r m i n e d w i t h t h e M e q u a t i o n P = A as d e s c r i b e d by A b e r c r o m b i e (1946) , L + M where P = t h e average number o f n u c l e a r p o i n t s p e r s e c t i o n , A = the cr u d e count o f n u c l e a r numbers seen i n s e c t i o n , M = t h e t h i c k n e s s o f t h e s e c t i o n i n u, and L = the average l e n g t h o f t h e n u c l e i i n u. F i v e c e l l c o u n t s (A) f o r each c e l l t y p e were made from p h o t o g r a p h s o f t h e c e n t r a l r e g i o n o f t h e r e t i n a . Eyes were a l s o o b s e r v e d i n l i v i n g i n d i v i d u a l s t h r o u g h o u t the s t u d y and p u p i l shapes w i t h d i f f e r e n t l i g h t i n t e n s i t i e s were n o t e d . 2. R e f r a c t i o n : The f o u r s e a l s c o n s i d e r e d i n t h i s s t u d y ranged i n age from one t o f o u r y e a r s and were a l l c a p t i v e a n i m a l s t h a t had been h e l d a t e i t h e r t h e U n i v e r s i t y o f B.C. o r the Vancouver P u b l i c Aquarium f o r a t l e a s t one y e a r . R e t i n o s c o p y measurements were t a k e n by two o p t o m e t r i s t s (Drs. D. Tenn'ant and B. J e r v i s ) and an o p h t h a l m o l o g i s t (Dr. A. McCormick), as I had n e i t h e r t h e e x p e r i e n c e nor t h e equipment t o u n d e r t a k e t h e s e s t u d i e s m y s e l f . The eyes were r e f r a c t e d under c y c l o p l e g i a , w i t h 3 d r o p s o f 1% a t r o p i n e s u l p h a t e s o l u t i o n (Alcon o f Canada 8 L t d . , Toronto) I n s t i l l e d i n each eye. C y c l o p l e g i a t y p i c a l l y t ook a t l e a s t f o r t y m i n u t e s t o complete and was ju d g e d by the absence o f a d i r e c t p u p i l r e f l e x t o l i g h t . A f t e r our f i r s t a t t e m p t a t r e f r a c t i o n , I f e l t t h a t more a c c u r a t e r e s u l t s c o u l d be o b t a i n e d i f t h e s e a l was f i r s t a n a e s t h e t i z e d , as a few were q u i t e a c t i v e and d i f f i c u l t t o h a n d l e . A n a e s t h e s i a was a c c o m p l i s h e d w i t h 2.5% sodium t h i o p e n t a l s o l u t i o n (Abbott Lab., M o n t r e a l ) w h i c h was i n j e c t e d i n t r a v e n o u s l y i n t o the e x t r a d u r a l v e i n . The s e a l was m a i n t a i n e d j u s t on t h e v e r g e o f complete a n a e s t h e s i a so t h a t b r e a t h i n g , w h i c h i s a v o l u n t a r y a c t i o n i n s e a l s , c o u l d be m a i n t a i n e d . Complete a n a e s t h e s i a r u n s t h e r i s k o f k i l l i n g t he a n i m a l i f m a i n t a i n e d f o r more than 5-10 m i n u t e s . R e f r a c t i o n was c a r r i e d o u t w i t h b o t h a s l i t r e t i n o s c o p e and a round, o r p o i n t , r e t i n o s c o p e , t h e o n l y d i f f e r e n c e between the two i n s t r u m e n t s b e i n g t h e shape o f the l i g h t beam produced. An a t t e m p t was a l s o made t o r e f r a c t the eyes underwater, w i t h t h e a n i m a l ' s head immersed i n a p l e x i g l a s s (methyl m e t h a c r y l a t e ) c o n t a i n e r f i l l e d w i t h w a t e r ( F i g . 1) . T h i s a p p a r a t u s , made o f 0.6 cm p l e x i g l a s s , c o n s i s t e d o f a r e c t a n g u l a r c o n t a i n e r , 15 cm by 18.5 cm by 60 cm, mounted a t an a n g l e o f 40°., The s e a l ' s head c o u l d be p l a c e d i n t h i s c o n t a i n e r and p o s i t i o n e d so t h a t i t was p o s s i b l e t o o b s e r v e the eye t h r o u g h t h e p l e x i g l a s s w a l l . In c o n j u n c t i o n w i t h t h e s e r e t i n o s c o p y measurements, a k e r a t o m e t e r was used t o measure t h e c u r v a t u r e o f t h e a n t e r i o r F i g u r e 1. A p p a r a t u s used i n the underwater r e f r a c t i o n o f th e h a r b o u r s e a l . F i g u r e 2. M e a s u r i n g the r e f r a c t i o n o f t h e h a r b o u r s e a l i n a i r .  10 s u r f a c e o f the c o r n e a . I t s p r i n c i p l e i s t h e r e f l e c t i o n o f an image o f f the a n t e r i o r c o r n e a l s u r f a c e , w h i c h i s then viewed a g a i n s t a s t a n d a r d measurement b u i l t i n t o t h e i n s t r u m e n t . I found I was u n a b l e t o o b t a i n even an a p p r o x i m a t e measure w i t h t h i s t e c h n i q u e , however, as due t o t h e extreme f l a t n e s s o f the c o r n e a , i t s measure o f c u r v a t u r e was w e l l o u t s i d e t h e c a l i b r a t i o n o f t h e k e r a t o m e t e r w h i c h was c a l i b r a t e d f o r human use. By use o f a P l a c i d o ' s d i s c , though, c o r n e a l a s t i g m a t i s m was o b s e r v e d . T h i s d i s c c o n s i s t s o f a l t e r n a t i n g b l a c k and w h i t e c o n c e n t r i c r i n g s and when h e l d i n f r o n t o f t h e eye and viewed t h r o u g h a h o l e i n i t s c e n t e r , e l l i p t i c a l images on t h e s u r f a c e o f t h e co r n e a r e s u l t i f c o r n e a l i r r e g u l a r i t y e x i s t s . C o r n e a l moulds o f t h e eye were then made t o measure t h i s r e f r a c t i v e e r r o r . The p l a n was t o make a p l a s t e r mould and from t h i s , a p l a s t i c c o n t a c t l e n s , w h i c h c o u l d be measured t h r o u g h u l t r a s o n i c t e c h n i q u e s . An i n i t i a l a t t e m p t t o make a p l a s t e r mould on a l i v e a n i m a l f a i l e d , f o r when the e y e l i d s were f o r c e d a p a r t t o t h e e x t e n t r e q u i r e d f o r a mould o f ^ t h e complete c o r n e a l s u r f a c e , the p r e s s u r e e x e r t e d on the g l o b e d i s t o r t e d the c o r n e a l shape. Two a t t e m p t s were then made on a n i m a l s w h i c h had j u s t been k i l l e d w i t h B a r b - E u t h o l . A f t e r r e m o v a l o f t h e e y e l i d s , a p r e l i m i n a r y mould o f the eye was made w i t h "Ophthalmic M o l d i t e " ( O b r i g Lab., I n c . ) , a s o f t , f a s t - h a r d e n i n g m i x t u r e w h i c h hardens t o t h e c o n s i s t e n c y o f a h a r d - b o i l e d egg w h i t e . A p o s i t i v e mould o f c o e c a l d e n t a l s t o n e , a v e r y f i n e p l a s t e r , was then made from the " M o l d i t e " 11 mould as soon as i t was removed from the c o r n e a . T h i s was ' n e c e s s a r y as t h e " M o l d i t e " mould soon undergoes s h r i n k a g e and d i s t o r t i o n as t h e water i n i t e v a p o r a t e s . From t h e p l a s t e r mould, a crude c o n t a c t l e n s was l a t e r made and measured by the P l a s t i c C o n t a c t Lens Co. (Canada) L t d . F u n d u s c o p i c e x a m i n a t i o n s were a l s o made on two o f the s e a l s examined as shown i n F i g . 2 t o d e t e r m i n e i f r e t i n a l f o l d s were v i s i b l e i n the l i v i n g eye. 3. V i s u a l D i s c r i m i n a t i o n s : B e h a v i o u r a l e x p e r i m e n t s were c a r r i e d o u t i n t h e r e s e a r c h compound o f the Vancouver P u b l i c Aquarium i n S t a n l e y Park. S a l t w ater was found e s s e n t i a l t o m a i n t a i n t h e eyes i n p e r f e c t c o n d i t i o n as many o f the a n i m a l s m a i n t a i n e d i n f r e s h w a t e r sooner o r l a t e r showed a w h i t e n i n g o f the c o r n e a . T h i s c o n d i t i o n was found t o be u s u a l l y due t o an i n v a s i o n o f l e u c o c y t e s i n t o the c o r n e a as a r e s u l t o f c o r n e a l u l c e r s . Why s e a l s m a i n t a i n e d i n f r e s h w a t e r seem t o be more s u s c e p t a b l e t o t h e s e u l c e r s i s n o t p r e s e n t l y c l e a r . A t o t a l o f seven s e a l s were i n i t i a l l y s e l e c t e d , w i t h Scar, a two-year o l d male, and B l o n d i e , a f o u r - y e a r o l d female, b e i n g chosen as t h e most manageable. Each s e a l was r e q u i r e d to d i s c r i m i n a t e between a two-bar p a t t e r n and a one-bar p a t t e r n as t h e g a p . i n the two-bar p a t t e r n was g r a d u a l l y r e d u c e d i n s i z e , w i t h the p r e s e n c e o f a gap b e i n g the c r i t i c a l p a rameter. A l l t e s t s were u n d e r t a k e n i n a plywood tank 1.22 m by 2.44 m f i l l e d w i t h seawater t o a de p t h o f 51 cm. The s t i m u l u s 12 c a r d s were p r e s e n t e d i n a s m a l l e r tank 0.86 m by 0.90 m a d j a c e n t t o the t e s t i n g tank ( F i g . 3 and 4 ) . P l e x i g l a s s windows, 1.27 cm by 25.4 cm by 55.9 cm, were b u i l t i n t o b o t h t a n k s a t the same l e v e l p r o v i d i n g a c l e a r view between the two t a n k s . Two t a n k s were used, a l o n g w i t h c o n t i n u o u s f l u s h i n g o f t h e seawater, to e n s u r e t h a t suspended d e b r i s i n t h e l a r g e r t a n k would n o t i n t e r f e r e w i t h the view ; t h i s d e b r i s c o n s i s t e d p r i m a r i l y o f f i s h s c a l e s shed from the r e w a r d s . The t a n k s were e n c l o s e d i n a hut t o remove any e x t e r n a l s t i m u l i and t o a l l o w c o n t r o l o f l i g h t i n g o f the s t i m u l u s c a r d s . The w a l l s o f the t e s t i n g tank were a l i g h t g l o s s b l u e . The l i g h t i n t e n s i t y o f the w h i t e p a r t s o f t h e s t i m u l u s c a r d s 2 was 284-369 candela/m . The w a l l s o f the s m a l l e r tank were a f l a t b l a c k to remove the p o s s i b i l i t y o f g l a r e . B o t h p a t t e r n s were i l l u m i n a t e d i n d e p e n d e n t l y w i t h 75 w f l o o d l a m p s and the i n t e r i o r o f t h e h u t was i l l u m i n a t e d w i t h t h r e e , f o u r - f o o t , 40 w, c o o l w h i t e f l o u r e s c e n t t u b e s . Measurements were made w i t h a " P h o t o v o l t U n i v e r s a l Photometer, Mod. 200", and the i n t e n s i t y o f l i g h t d i r e c t e d onto the s t i m u l u s c a r d s was c o n t r o l l e d by means o f a dimmer s w i t c h . T h i s range o f i l l u m i n a t i o n was n o t c o n s i d e r e d t o a f f e c t the r e s u l t s , s i n c e i n a l l c a s e s , the a n i m a l s were adap t e d t o an i l l u m i n a t i o n o f t h i s magnitude b e f o r e commencement o f the day's t e s t i n g . The e x p e r i m e n t e r s t o o d b e h i n d the s m a l l e r tank and p r e s e n t e d b o t h s t i m u l u s c a r d s s i m u l t a n e o u s l y ( F i g . 5 ) . The s t i m u l u s c a r d s were w h i t e p l e x i g l a s s , 0.32 cm by 30.5 cm by 13 F i g u r e 3. Photo o f t h e t a n k s a t t h e Vancouver P u b l i c Aquarium where the b e h a v i o u r a l a s p e c t s o f t h i s s t u d y were done. 14 o s t o s 1 m. P w. w. T H F i g . 4 . A p p a r a t u s u s e d i n t e s t i n g t h e h a r b o u r s e a l ' s u n d e r w a t e r d i s c r i m i n a t i o n o f o n e - a n d t w o - b a r p a t t e r n s p r e s e n t e d s i m u l t a n e o u s l y . T h e r e l a t i o n s h i p s o f t h e t a n k s a r e s h o w n i n b o t h d o r s a l (A) a n d l a t e r a l (B) v i e w s . F o r t e s t i n g i n a i r , t h e w a t e r was d r a i n e d f r o m t h e s t i m u l u s p r e s e n t a t i o n t a n k (P) a n d t h e s t i m u l u s c a r d s w e r e p r e s e n t e d a t t h e l e v e l o f t h e u p p e r s e t o f w i n d o w s (l^L,); t h e w a t e r l e v e l i n t h e t e s t i n g t a n k (T) r e m a i n e d t h e same b u t t h e s e a l s w e r e t r a i n e d t o s w i m a t t h e s u r f a c e . One s e a l a t a t i m e w a s l e t f r o m t h e h o l d i n g t a n k (H) i n t o t h e t e s t i n g t a n k (T) b y means o f s l i d i n g p a r t i t i o n s . E , e x p e r i m e n t e r ; S , s t i m u l u s c a r d s ; W^, l o w e r s e t o f w i n d o w s ( f o r t e s t i n g u n d e r w a t e r ) ; P D , p o i n t o f d i s c r i m i n a t i o n f o r t h e s e a l . F i g u r e s 5.a-5.b-5.c. Photos showing d e t a i l s o f the b e h a v i o u r a l d i s c r i m i n a t i o n t a n k s and t h e p r e s e n t a t i o n o f a t y p i c a l problem. A. Photo o f t h e s t i m u l u s c a r d s , w i t h the p o s i t i v e c a r d a t t h e r e a d e r ' s r i g h t . In a n o r m al t r i a l , the c a r d s would n o t be h e l d as h i g h , so t h a t t h e s e a l would be u n a b l e t o see the c a r d s . B. A c o r r e c t r e s p o n s e by t h e s e a l , as i t chose the r i g h t - hand l e v e r . The s e a l ' s approach was made underwater, w i t h the c a r d s viewed t h r o u g h the lower window. C. Photo showing d e t a i l s o f t h e s t i m u l u s p r e s e n t a t i o n tank. The s t i m u l u s c a r d s were p r e s e n t e d j u s t t o the f o r e o f the b u c k e t , w h i c h c o n t a i n e d the h e r r i n g r e wards.  16 61.0 cm, w i t h e i t h e r one or two b l a c k b a r s . The b a r s were 20.3 cm l o n g and were o f 2.54 cm ACS tape. In the case o f the one-bar ( n e g a t i v e ) p a t t e r n , t h e w i d t h o f the b a r was 5.08 cm p l u s t h e w i d t h o f the gap on the c o r r e s p o n d i n g two- b a r ( p o s i t i v e ) p a t t e r n . The d i s t a n c e between the c e n t e r s o f any p a i r o f p a t t e r n s was 21 cm. By the use o f v e r t i c a l p a r t i t i o n s s e p e r a t i n g t h e two t a r g e t s In b o t h t a n k s , t h e s e a l s were f o r c e d t o make t h e i r c h o i c e o f t a r g e t s a t a d i s t a n c e o f 1.7 m ( F i g . 4 ) . Any change o f a l l e y w a y s a f t e r e n t r a n c e i n t o one was c o n s i d e r e d a wrong r e s p o n s e . A t r i a l c o n s i s t e d o f the s e a l w a i t i n g a t t h e f a r end o f t h e t e s t i n g tank from th e e x p e r i m e n t e r w h i l e t h e c a r d s , a two-bar c a r d w i t h i t s c o r r e s p o n d i n g one-bar c a r d , were p l a c e d i n p o s i t i o n i n t h e s m a l l e r t a n k . On t h e r i n g i n g o f an e l e c t r i c b e l l , t he s e a l was t r a i n e d t o swim f o r w a r d and a f t e r making a c h o i c e o f a l l e y w a y s , r a i s e a l e v e r w i t h i t s nose. A c o r r e c t r e s p o n s e was announced by a 5 second f l a s h o f l i g h t i n t o the t e s t i n g tank and was rewarded w i t h 1/3 o f a h e r r i n g (Clupea p a l l a s i i ) . An i n c o r r e c t r e s p o n s e was f o l l o w e d by a two minute "time- o u t " p e r i o d d u r i n g w h i c h the s e a l was r e q u i r e d t o w a i t two m i n u t e s b e f o r e t h e n e x t t r i a l was p r e s e n t e d . B oth s e a l s were f i r s t t r a i n e d t o push the l e v e r s a f t e r making an underwater approach. S i x t y t r i a l s were p r e s e n t e d d a i l y and t h e amount o f f o o d f e d t o each s e a l d a i l y was 2.7 kg o f h e r r i n g ( i n c l u d e d i n t h i s a r e the rewards f o r c o r r e c t r e s p o n s e s ) . T h i s amount o f f o o d was such t h a t b o t h s e a l s were 17 a b l e t o m a i n t a i n t h e i r a p p e t i t e and remain i n a g e n e r a l l y h e a l t h y c o n d i t i o n . The p o s i t i v e c a r d was randomly a l t e r n a t e d between the r i g h t and l e f t s i d e s i n o r d e r t o p r e v e n t l e v e r p r e f e r e n c e . The f o l l o w i n g i n i t i a l s e r i e s o f gap s i z e s were used ( i n cm): 2.54, 1.27, 0.63, 0.32, 0.25, 0.21, 0.13, 0.09 and 0.05. Each day's t r i a l s c o n s i s t e d o f from two t o t h r e e gap s i z e s randomly p r e s e n t e d w i t h t h e r a t i o b i a s e d i n f a v o u r o f t h e l a r g e r gap s i z e s . T h i s e n s u r e d t h a t a m a j o r i t y o f t h e t r i a l s were such t h a t t h e s e a l s had no d i f f i c u l t y i n making a c o r r e c t r e sponse- A s m a l l e r gap s i z e was o n l y i n t r o d u c e d when the d a t a (Table 1) showed e v i d e n c e t h a t t h e s e a l was a b l e t o see the s m a l l e s t gap s i z e b e i n g p r e s e n t l y used. P i n a l measurements o f t h e i r d i s c r i m i n a t o r y a b i l i t i e s i n b o t h a i r and wa t e r were t a k e n i n th e f i r s t h a l f o f 1969 and were based on a p p r o x i m a t e l y 600 t r i a l s w i t h gap s i z e s between 0.05 - 0.3 cm (Table 1 ) . For t e s t i n g i n a i r , the wa t e r was d r a i n e d from the s m a l l e r tank and t h e s t i m u l u s c a r d s were p r e s e n t e d a t the l e v e l o f the upper s e t o f windows (VI^ i n F i g . 4) . The water l e v e l i n th e t e s t i n g tank remained the same and the s e a l s were t r a i n e d t o swim on the wa t e r s u r f a c e . I t w i l l be n o t i c e d t h a t i n T a b l e 1, some o f t h e gap s i z e s mentioned a r e n o t i n c l u d e d i n t h e i n i t i a l s e r i e s mentioned above. T h i s i s a r e s u l t o f s h r i n k a g e o f the ACS tape due to the f r e q u e n t immersions o f t h e s t i m u l u s c a r d s i n w a t e r . I t was t h e r e f o r e found n e c e s s a r y t o remeasure the gaps a f t e r e v e r y 18 30 t r i a l s and n o t e any changes i n t h e i r s i z e s w h i c h may have o c c u r e d . H i g h p e r i o d s o f h u m i d i t y were a l s o found t o a f f e c t t h e tape, making d a i l y measurements d u r i n g a e r i a l t e s t i n g a l s o e s s e n t i a l . A l l measurements were made by eye w i t h the t h s use o f a r u l e r c a l i b r a t e d i n 1/50 o f an i n c h . Each o f t h e s e measurements was f u r t h e r d i v i d e d on the b a s i s o f where the b o r d e r o f the gap f e l l and a l t h o u g h i t i s r e a l i z e d t h a t a few e r r o r s were no doubt made, i t i s f e l t t h a t o v er 600 t r i a l s t h e y would c a n c e l o u t . F u r t h e r m o r e , a d e f i n i t e v a l u e below w h i c h the s e a l s c o u l d n o t see and above w h i c h they c o u l d i s n o t c l a i m e d , b u t r a t h e r s i m p l y t h a t t h e i r lower t h r e s h o l d l e v e l i s w i t h i n a c e r t a i n range o f v a l u e s . 19 R e s u l t s 1. Morphology and H i s t o l o g y The b a s i c m o r p h o l o g i c a l f e a t u r e s o f the h a r b o u r s e a l eye a r e shown i n F i g . 6. B r i e f l y , the eyes a r e l a r g e , b o t h i n a b s o l u t e s i z e and i n r e l a t i v e s i z e t o the body, w i t h a g l o b e d i a m e t e r of 39-40 mm i n t h e a d u l t . The c o r n ea i s r o u g h l y c i r c u l a r and f l a t t e n e d , b e i n g o n l y s l i g h t l y a r c h e d over th e a n t e r i o r chamber w h i c h has a d e p t h o f 2-2.5 mm. The l e n s i s s p h e r i c a l and a l t h o u g h l a r g e , i s n o t out o f p r o p o r t i o n to the r e s t o f the eye; i t has an e q u a t o r i a l d i a m e t e r 37% t h a t o f t h e v e r t i c a l d i a m e t e r o f the eye. The i r i s i s v e r y m uscular and h e a v i l y v a s c u l a r i z e d , and i s anchored t o t h e i n n e r a s p e c t o f t h e c o r n e a by means o f a v e r y p r o f u s e f i b e r network ( F i g . 7 ) . The s c l e r a i s t h i c k e n e d s l i g h t l y i n the p o s t e r i o r p o r t i o n o f t h e eye b u t i s a l s o n o t out o f p r o p o r t i o n t o t h e o t h e r component l a y e r s when compared t o a t y p i c a l t e r r e s t r i a l mammalian eye. The c h o r o i d i s h e a v i l y v a s c u l a r i z e d and pigmented and i n a d d i t i o n , c o n t a i n s an e x t e n s i v e tapetum w h i c h l i n e s v i r t u a l l y t h e whole o f the v i t r e o u s c a v i t y . No f o v e a nor a r e a c e n t r a l i s c o u l d be d i s t i n g u i s h e d . P u p i l s i z e and shape i n a i r appear t o be p r i m a r i l y i n f l u e n c e d by t h e amount o f l i g h t p r e s e n t , c l o s i n g down t o a p i n h o l e a t the uppermost end o f a v e r t i c a l l y c o n s t r i c t e d s l i t under h i g h l i g h t i n t e n s i t i e s and o p e n i n g up t o an o v a l or e l l i p t i c a l shape under low l i g h t i n t e n s i t i e s ( F i g . 8 ) . No F i g u r e s 6.a-6.b-6.c-6.d. Photos o f h i s t o l o g i c a l s e c t i o n s o f p i n n i p e d eyes. S c a l e - 1 cm. A. Harbour s e a l . (Phoca v i t u l i n a ) " B. N o r t h e r n f u r s e a l . ( C a l l o r h i n u s u r s i n u s ) C. Harp s e a l . ( P a g o p h i l u s g r o e n l a n d i c u s ) D. K i l l e r whale. (Grampus orca) 2 0 F i g u r e 7 . Close-up o f the e x t e n s i v e f i b e r network c h a r a c t e r i s t i c around the p e r i p h e r y o f the harbour s e a l i r i s (I) . Arrow p o i n t s t o p u p i l . F i g u r e 8.a-8.b-8.c. - P u p i l shapes under d i f f e r e n t l i g h t i n t e n s i t i e s . A. Very h i g h l i g h t i n t e n s i t y . T h i s i s as s m a l l as the p u p i l may be made. B. High l i g h t i n t e n s i t y . The p u p i l i s more t e a r - shaped than i n A. C. E f f e c t o f a t r o p i n e . T h i s shows the maximum s i z e o b t a i n a b l e , and r e p r e s e n t s the p u p i l s i z e f ound under v e r y low l i g h t i n t e n s i t i e s .  23 p u p i l s l i t t h a t i s f u n c t i o n a l a l o n g i t s e n t i r e l e n g t h i s formed i n b r i g h t l i g h t , such as would e x i s t on a sunny day. In w a t e r , th e p u p i l i s the same-' s i z e as i n a i r , w i t h p u p i l s i z e and hence shape i n water a p p e a r i n g t o be a l s o p r i m a r i l y i n f l u e n c e d by l i g h t i n t e n s i t y . The g r o s s h i s t o l o g i c a l f e a t u r e s o f the h a rbour s e a l r e t i n a a r e seen i n F i g . 9. Deep t o t h e n e r v e f i b e r l a y e r i s a s i n g l e l a y e r o f g a n g l i o n c e l l s ( F i g . 10) i n t e r s p e r s e d around the v e r t i c a l l y t r a n s v e r s i n g r a d i a l f i b e r s o f M u l l e r . The g a n g l i o n c e l l s v a r y l i t t l e i n s i z e e x c e p t f o r t h e o c c a s i o n a l g i a n t g a n g l i o n c e l l . These l a t t e r c e l l s may be e a s i l y i d e n t i f i e d by t h e i r huge b o d i e s ( F i g . 11) and e x t e n s i v e d e n d r i t i c b r a n c h i n g . The i n n e r n u c l e a r l a y e r i s d i s t i n g u i s h e d by the absence o f any d i s t i n c t i v e z o n a t i o n and by t h e v e r y l a r g e h o r i z o n t a l c e l l s w h i c h e x t e n d t h r o u g h o u t t h i s l a y e r ( F i g . 12). I n t e r s p e r s e d around t h e s e c e l l s and t h e i r e x t e n s i v e a x i s c y l i n d e r s a r e the b i p o l a r , amacrine and n e u r o g l i a l c e l l s . The b i p o l a r c e l l s a r e c h a r a c t e r i z e d by p r o c e s s e s w h i c h e x t e n d outward i n t o the o u t e r p l e x i f o r m l a y e r , where t h e y synapse w i t h the t e r m i n a l p e d i c l e s o f t h e p h o t o r e c e p t o r c e l l s , and inward i n t o t h e i n n e r p l e x i f o r m l a y e r , where they synapse w i t h the g a n g l i o n c e l l s . The b i p o l a r s a r e l o c a t e d p r e d o m i n a t e l y i n the o u t e r r e g i o n s o f t h e i n n e r n u c l e a r l a y e r and may be i d e n t i f i e d i n F i g . 12 b y ' t h e i r outward e x t e n d i n g p r o c e s s e s and o v a l n u c l e i . Other o c c a s i o n a l c e l l s h e r e i d e n t i f i e d as amacrine c e l l s , w i t h o n l y b r o a d , s i n g l e p r o c e s s e s e x t e n d i n g i n t o t h e i n n e r p l e x i f o r m E x p l a n a t i o n o f F i g u r e s 9-21 A b b r e v i a t i o n s used i n F i g u r e s : n f o p t i c n e r v e f i b e r l a y e r g g a n g l i o n c e l l l a y e r i p i n n e r p l e x i f o r m l a y e r i n i n n e r n u c l e a r l a y e r op o u t e r p l e x i f o r m l a y e r on , o u t e r n u c l e a r l a y e r r l a y e r o f p h o t o r e c e p t o r s r i s r e c e p t o r i n n e r segments r o s r e c e p t o r o u t e r segments pe pigment e p i t h e l i u m t tapetum b b i p o l a r c e l l h h o r i z o n t a l c e l l Mf r a d i a l f i b e r s o f M u l l e r a amacrine c e l l n n e u r o g l i a l c e l l bv b l o o d v e s s e l p p o l y s y n a p t i c p e d i c l e o o l i g o s y n a p t a c p e d i c l e V e r t i c a l c r o s s s e c t i o n o f the c e n t r a l a r e a o f the r e t i n a . T o l u i d i n e b l u e . V e r t i c a l s e c t i o n t h r o u g h the o p t i c n e r v e f i b e r and g a n g l i o n l a y e r s . L a r g e r a d i a l f i b e r s o f M u l l e r can be seen v e r t i c a l l y t r a n s v e r s i n g t h e s e l a y e r s . Arrows p o i n t t o g a n g l i o n c e l l s and a s m a l l b l o o d v e s s e l . T o l u i d i n e b l u e . V e r t i c a l s e c t i o n t h r o u g h a g i a n t g a n g l i o n c e l l . H a e m o t o x y l i n and e o s i n . V e r t i c a l s e c t i o n t h r o u g h the i n n e r n u c l e a r l a y e r showing l a r g e h o r i z o n t a l c e l l s (h) , amacrine c e l l s ( a ) , b i p o l a r c e l l s (b) and n e u r o g l i a l c e l l s (n) . H o r i z o n t a l c e l l a x i s c y l i n d e r s a r e marked by arrows. T o l u i d i n e b l u e . V e r t i c a l s e c t i o n t h r o u g h t h e i n n e r n u c l e a r l a y e r showing t h e synapses between b i p o l a r c e l l s (b) and a . p o l y s y n a p t i c p e d i c l e ( a r r o w ) . T o l u i d i n e b l u e .  25 l a y e r , were o b s e r v e d on the i n n e r a s p e c t s o f the i n n e r n u c l e a r l a y e r . The s m a l l , d a r k l y - s t a i n e d n u c l e i s u r r ounded by l i t t l e c y t o p l a s m i n t h e i n n e r r e g i o n s o f t h i s l a y e r ( F i g . 13) a r e t h e n u c l e i o f n e u r o g l i a l c e l l s . Two v a r i e t i e s o f p h o t o r e c e p t o r p e d i c l e can be d i s t i n g u i s h e d i n t he o u t e r p l e x i f o r m l a y e r , r o d - l i k e o l i g o s y n a p t i c p e d i c l e s and c o n e - l i k e p o l y s y n a p t i c p e d i c l e s ( F i g . 14). The o l i g o s y n a p t i c p e d i c l e s a r e most numerous, w i t h a r a t i o o f a p p r o x i m a t e l y 23:1. They a r e a l s o much s m a l l e r than the p o l y s y n a p t i c p e d i c l e s and c o n t a i n o n l y one b a s a l f i l a m e n t i n c o n t r a s t t o the many s h o r t , c o n v e r g i n g f i l a m e n t s found i n p o l y s y n a p t i c p e d i c l e s . The p o l y s y n a p t i c p e d i c l e s a r e a l s o s i t u a t e d s l i g h t l y c l o s e r t o the i n n e r . n u c l e a r l a y e r than a r e t h e o l i g o s y n a p t i c p e d i c l e s . There i s no o u t e r f i b r e l a y e r o f Henle. The o u t e r n u c l e a r l a y e r i s the t h i c k e s t o f a l l the r e t i n a l l a y e r s , b e i n g 10-12 c e l l s deep. R e g i o n a l v a r i a t i o n i n the t h i c k n e s s o f t h i s l a y e r was o n l y o b s e r v e d i n t h e o u t e r p e r i p h e r a l a r e a s where i t g r a d u a l l y t h i n n e d o u t . The p h o t o r e c e p t o r n u c l e i appear t o be o f two t y p e s , w i t h most o f t h e n u c l e i c o n t a i n i n g two r e l a t i v e l y l a r g e clumps o f c h r o m a t i c m a t e r i a l and l o c a t e d i n from the o u t e r l i m i t i n g membrane. A few, however, c o n t a i n many s m a l l e r clumps o f c h r o m a t i n and a r e d i r e c t l y a d j a c e n t to. t h i s membrane ( F i g . 14 and 15) . These l a t t e r cone- type n u c l e i a r e a l s o o f t e n a s s o c i a t e d w i t h a l a r g e gap i n the o u t e r l i m i t i n g membrane. F i g . 14 V e r t i c a l s e c t i o n t h r o u g h the o u t e r p l e x i f o r m and o u t e r n u c l e a r l a y e r s showing o l i g o s y n a p t i c (o) and p o l y s y n a p t i c (p) t e r m i n a l p e d i c l e s . Arrow p o i n t s t o cone-type p h o t o r e c e p t o r n u c l e u s . T o l u i d i n e b l u e . F i g . 15 V e r t i c a l s e c t i o n t h r o u g h t h e o u t e r l i m i t i n g membrane and l a y e r o f r o d s and cones, showing p h o t o r e c e p t o r i n n e r ( r i s ) and o u t e r (ros) segments. Arrow p o i n t s t o cone-type p h o t o r e c e p t o r n u c l e u s . T o l u i d i n e b l u e . F i g . 16 V e r t i c a l s e c t i o n t h r o u g h pigment e p i t h e l i u m . Arrows p o i n t t o f i n e c y t o p l a s m i c p r o c e s s e s e x t e n d i n g towards t h e p h o t o r e c e p t o r o u t e r segments. T o l u i d i n e b l u e . F i g . 17 H o r i z o n t a l s e c t i o n ( s l i g h t l y o b l i q u e ) t h r o u g h r e g i o n o f o u t e r l i m i t i n g membrane ( p o i n t s ) . Arrows p o i n t t o p h o t o r e c e p t o r n u c l e i . Azur I I . F i g . 18 V e r t i c a l s e c t i o n t h r o u g h pigment e p i t h e l i u m and tapetum. Arrows p o i n t t o c h o r i o - c a p i l l a r i e s t r a n s v e r s i n g t h e tapetum. T o l u i d i n e b l u e .  27 The p h o t o r e c e p t o r l a y e r shows two d i s t i n c t zones when s t a i n e d ( F i g . 15) , a l i g h t l y s t a i n i n g i n n e r segment l a y e r and a d a r k e r s t a i n i n g o u t e r segment l a y e r . Only one t ype o f p h o t o r e c e p t o r can be d i s t i n g u i s h e d on the b a s i s o f i n n e r segment morphology ( F i g s . 15 and 17). C l o s e l y a s s o c i a t e d w i t h the p h o t o r e c e p t o r c e l l l a y e r i s the pigment e p i t h e l i u m ( F i g . 16), a u n i c e l l u l a r l a y e r , from the c e l l s o f w h i c h f i n e c y t o p l a s m i c p r o c e s s e s e x t e n d down between the o u t e r segments o f t h e p h o t o r e c e p t o r c e l l s . No pigment g r a n u l e s c o u l d be o b s e r v e d . A l t h o u g h n o t p a r t o f the r e t i n a , t h e r e a l s o e x i s t s a v e r y w e l l - d e v e l o p e d and e x t e n s i v e tapetum c e l l u l o s u m ( F i g . 18). I t i s composed o f 20-22 l a y e r s o f f l a t t e n e d c e l l s l y i n g i n the s u p e r f i c i a l c h o r o i d deep t o t h e pigment e p i t h e l i u m , t h i n n i n g g r a d u a l l y towards the p e r i p h e r y . T h i s l a y e r i s r e g u l a r l y p e n e t r a t e d by many s m a l l c a p i l l a r i e s a r i s i n g from th e c h o r o i d w h i c h t r a n s v e r s e l a t e r a l l y i n t h e r e g i o n between the pigment e p i t h e l i u m and tapetum. Two t y p e s o f " r e t i n a l f o l d i n g " were o b s e r v e d i n the whole s e c t i o n s embedded i n p a r a f f i n . The most common type o f f o l d i n g was a s l i g h t one ( F i g . 19 and 20) i n v o l v i n g the o u t e r n u c l e a r and p h o t o r e c e p t o r b u t n o t the i n n e r l a y e r s o f the r e t i n a . T h i s f o l d i n g was o b s e r v e d i n f o u r o f the seven h a r b o u r s e a l eyes examined and i n t h e s e eyes, was o n l y a b s e n t i n the p e r i p h e r a l r e g i o n s o f t h e r e t i n a . The second type o f f o l d i n g ( F i g . 21) , much more i n t r i c a t e and complex and i n v o l v i n g the whole r e t i n a l l a y e r , was o b s e r v e d i n t h r e e o f the seven h a r b o u r s e a l s s t u d i e d ' i F i g . 19 and 20 V e r t i c a l s e c t i o n s t h r o u g h the r e t i n a showing the s l i g h t form o f r e t i n a l f o l d i n g . Only the o u t e r n u c l e a r l a y e r and l a y e r o f r o d s and cones a r e f o l d e d . H a e m o t o x y l i n and e o s i n . F i g . 21 V e r t i c a l s e c t i o n t h r o u g h t h e r e t i n a showing t h e complex form o f r e t i n a l f o l d i n g . A l l the r e t i n a l l a y e r s a r e f o l d e d . H a e m o t o x y l i n and e o s i n . 28 29 (these t h r e e a l s o showed the f i r s t type o f r e t i n a l f o l d i n g ) . T h i s f o l d i n g i s much more l o c a l i z e d than the f i r s t t y p e o f f o l d i n g mentioned, and was o n l y observed t o o c c u r on a s m a l l p r o p o r t i o n (10% a t the most) o f t h e r e t i n a shown through s e c t i o n i n g , u s u a l l y i n t h e c e n t r a l or m i d d l e r e t i n a l r e g i o n s ; Owing t o i t s magnitude, t h i s f o l d i n g s h o u l d be c l e a r l y v i s i b l e t h r o u g h f u n d u s c o p i c e x a m i n a t i o n . . S i n c e none was obs e r v e d , i t was not p o s s i b l e t o c o n f i r m t h a t t h i s t y pe o f f o l d i n g o c c u r s as a n a t u r a l phenomenon. No f o l d i n g o f e i t h e r t y pe was o b s e r v e d i n t h o s e s e c t i o n s embedded i n Epon. I t i s not p o s s i b l e t h e r e f o r e t o d e s c r i b e i n d e t a i l the r e t i n a l o r g a n i z a t i o n about t h e s e f o l d s . The p e r i p h e r a l t e r m i n a t i o n o f the s e n s o r y r e t i n a , the o r a t e r m i n a l i s , was o b s e r v e d t o be a t the base o f the c i l i a r y m u s c l e s . There was thus no p a r s p l a i n a as o c c u r s i n the human eye. 2. R e f r a c t i o n R e f r a c t i v e measurements o f t h e f o u r h a r b o u r s e a l s examined by r e t i n o s c o p y a r e g i v e n i n T a b l e 1. I n i t i a l d i f f i c u l t i e s i n c o n t r o l l i n g the s e a l s were overcome by means o f a n a e s t h e t i c s , b u t problems were s t i l l e n c o u n t e r e d i n me a s u r i n g the o p t i c a l axes c o r r e c t l y . A l l t h r e e p e o p l e t h a t took t h e s e measurements ar e p r o f e s s i o n a l d o c t o r s and w e l l - s k i l l e d i n the measurement of r e f r a c t i o n i n humans, b u t none o f them c o u l d s t a t e w i t h c e r t a i n i t y the v a l u e s f o r t h e ha r b o u r s e a l . The e f f e c t o f the tapetum, a l a y e r n ot p r e s e n t i n man, i n r e f l e c t i n g back 30 S e a l R e f r a c t i o n A x i s V e r t i c a l ( D i o p t e r s ) R e f r a c t i o n A x i s Hor i z o n t a l ( D i o p t e r s ) A s t i g m a t i s m ( D i o p t e r s ) I n s t r u m e n t O p e r a t o r Su s i . -7.0 (B) -7.0 (B) 0.0 S l i t 1 O r p h i e -5.0 (B) -5.0 (B) 0.0 S l i t 1 Scar 1.5 (B) 2.0 (B) 0.5 S l i t 1 -3.5 -2.5 (R) (L) 3.0 4.5. (R) (L) 6.5 7.0 Round 2 3.0 (L) 5.5 (L) 2.5 S l i t 2 8.0 (B) 1. 0 (B) 7.0 S l i t 3 B l o n d i e -11.5 (B) 1.5 (B) 13. 0 Round 2 -11.5 (B) * 1.5 (B)* 13.0* Round 2 Or p h i e (Water) 4. 0 (B) 4.0 (B) 0.0 S l i t 1 - -13. 0 -4. 0 9. 0 - Johnson Mean o f 12 eyes (harp s -12. 5 , V t o e a l ) (-16 -11 .5 -8.0 .5) (11.5 t o 6.0 (12 t o -3.5) 5) P i g g i n s M f n ° f 5.25 12 eyes (harp seal)- (Water) (9 3. t o 4.70 75) (9 t o 3. 75) 0.0 (2 t o 0) P i g g i n s T a b l e l . : R e f r a c t i v e measurements o f the h a r b o u r s e a l as o b t a i n e d i n t h i s s t u d y and by Johnson (1893). Harp s e a l measurements were ta k e n by P i g g i n s (1970). These v a l u e s were o b t a i n e d i n a i r by means o f r e t i n o s c o p y , w i t h t h o s e v a l u e s o b t a i n e d i n wa t e r n o t e d . B = i n b o t h eyes, R = r i g h t eye, L = l e f t eye, * = w i t h o u t a n a e s t h e s i a . 3 1 the r e t i n o s c o p y l i g h t , combined w i t h the l a r g e eye and i t s upward t i l t , made a l l the measurements q u e s t i o n a b l e ; i t was no t p o s s i b l e t o d e f i n i t e l y s t a t e t h a t measurements o b t a i n e d were a l o n g the o p t i c a x i s . B o t h a t t e m p t s a t o b t a i n i n g a c c u r a t e measurements by the use o f c o n t a c t l e n s e s f a i l e d , f o r when the l e n s e s were l a t e r s t u d i e d , i t was noted t h a t d i m p l i n g o f the cornea had o c c u r e d . T h i s was presumably as a r e s u l t o f a drop i n b l o o d p r e s s u r e w i t h i n t h e eye owing t o d e a t h . F u r t h e r m o r e , the l e n s e s were a l s o found t o be too f l a t t o be measured by c o n v e n t i o n a l c o m m e r c i a l u l t r a s o n i c equipment, w h i c h i s c a l i b r a t e d f o r human eye measurement. Owing t o a s h o r t a g e o f s e a l s , e f f o r t s i n m e a suring c o r n e a l c u r v a t u r e by t h i s t e c h n i q u e were not pur s u e d . C o r n e a l a s t i g m a t i s m was o b s e r v e d t o o c c u r , however, by means o f a P l a c i d o ' s d i s c . T h i s a s t i g m a t i s m had i t s d i s t o r t i o n e f f e c t i n the v e r t i c a l , w h i c h i n d i c a t e s t h e r a d i u s o f l e a s t c o r n e a l c u r v a t u r e i n the h o r i z o n t a l . No q u a n t i t a t i v e v a l u e s a r e o b t a i n a b l e w i t h t h i s t e c h n i q u e however. 3 . V i s u a l d i s c r i m i n a t i o n s : . T h e g r e a t e s t p r o blem e n c o u n t e r e d i n t h i s phase o f the s t u d y was the l o n g p e r i o d o f time r e q u i r e d . t o t r a i n t he s e a l s t o r e s p o n d t o t h e c o r r e c t s t i m u l u s c a r d ( F i g . 22). As mentioned e a r l i e r , B l o n d i e and Scar were t h e two s e a l s chosen as b e i n g the most manageable from seven i n i t i a l l y s e l e c t e d s e a l s . However, even t h e s e two were v e r y slow l e a r n e r s . F i g u r e s 22.A-22.B. Performances o f the two h a r b o u r s e a l s d u r i n g c o n d i t i o n i n g . Each day's t r i a l s were brok e n down i n t o b l o c k s , o f twenty t r i a l s each to b e t t e r d e s c r i b e p e r formance b e h a v i o u r . A. The performance o f the f o u r - y e a r o l d fema l e , B l o n d i e B. The performance o f t h e two-year o l d male,. Scar. D o t t e d l i n e = s t a g e a t i n t r o d u c t i o n o f t h e b a f f l e s i n t o t h e t a n k ; S t a r = s i m p l i f i c a t i o n o f the problem from a two-bar ( p o s i t i v e ) : one-bar ( n e g a t i v e ) one , t c a one c a r d one (onl y t h e two-bar ( p o s i t i v e ) c a r d was p r e s e n t e d ) .  33 B l o n d i e , the female, was t r a i n e d f i r s t and r e q u i r e d 1540 t r i a l s b e f o r e she was a b l e t o r e s p o n d c o r r e c t l y t o a l l o f one b l o c k ' s t r i a l s (the day's 60 t r i a l s were s u b d i v i d e d i n t o t h r e e b l o c k s o f 20 t r i a l s e a c h ) , the c r i t e r i o n f o r s o l v i n g the problem. The p r o blem w h i c h she was r e q u i r e d t o s o l v e was a t h r e e c a r d one, c o n s i s t i n g o f one p o s i t i v e s t i m u l u s c a r d , a two-bar p a t t e r n , and two n e g a t i v e s t i m u l u s c a r d s , a b l a n k w h i t e c a r d and a b l a n k b l a c k c a r d . T h i s c o m b i n a t i o n was used as some g e n e r a l i z a t i o n e x p e r i m e n t s were i n i t i a l l y p l a n n e d , b u t owing t o t h e l o n g p e r i o d o f time r e q u i r e d i n t r a i n i n g the s e a l s , t h i s a s p e c t was l a t e r dropped. The sudden improvement i r i B l o n d i e ' s r e s p o n s e s a f t e r 940 t r i a l s o c c u r r e d a f t e r i n c r e a s i n g the " t i m e - o u t " p e r i o d f o l l o w i n g an i n c o r r e c t r e s p o n s e from 30 seconds t o 3 m i n u t e s . T h i s s u g g e s t s t h a t perhaps one o f the r e a s o n s f o r the l a r g e number o f t r i a l s r e q u i r e d t o s o l v e t h e p r o blem was i n e x p e r i e n c e i n t r a i n i n g on my p a r t . However, Scar was t r a i n e d a f t e r B l o n d i e and y e t s t i l l r e q u i r e d a comparable number o f t r i a l s . Schusterman (1968) a l s o e n c o u n t e r e d s i m i l a r d i f f i c u l t i e s i n h i s t r a i n i n g o f the h a r b o u r s e a l . S c ar, t h e male, r e q u i r e d a p p r o x i m a t e l y the same number o f t r i a l s ( F i g . 22) t o l e a r n the two-bar ( p o s i t i v e ) : one-bar ( n e g a t i v e ) problem as B l o n d i e r e q u i r e d t o l e a r n t h e t h r e e c a r d one, even when the problem was s i m p l i f i e d a f t e r 600 t r i a l s t o a one c a r d one ( o n l y t h e -two-bar ( p o s i t i v e ) c a r d was p r e s e n t e d , w i t h t h e o n l y r e q u i r e m e n t b e i n g t o push the l e v e r on t h e same s i d e as the c a r d ) . Once the problem was l e a r n t , however, b o t h s e a l s showed c o n s i d e r a b l e . r e t e n t i o n o f 34 the problem and would p e r f o r m c o r r e c t l y even a f t e r a two month l a p s e i n t e s t i n g . The r e s p o n s e s o f B l o n d i e and Scar i n b o t h a i r and water f o r the f i n a l s e t o f measurements, i e . f o r gap s i z e s between 0.05--3.0 cm, i s g i v e n i n T a b l e 2 and F i g s . 23 and 24 r e s p e c t i v e l y . Weighted r e g r e s s i o n s were performed on the d a t a f o r b o t h a i r and w a t e r , and when the s t a n d a r d p s y c h o p h y s i c a l c r i t e r i o n i s a p p l i e d t o t h e d a t a (the d i f f e r e n c e t h r e s h o l d b e i n g d e f i n e d as t h e gap s i z e c o r r e s p o n d i n g t o the 75% perf o r m a n c e l e v e l ) , i t i s found t h a t b o t h s e a l s c o u l d r e s o l v e a v e r t i c a l gap between two b l a c k b a r s as s m a l l as 1 mm when a t a d i s t a n c e o f 1.7 m. There i s no s i g n i f i c a n t d i f f e r e n c e (p = 0.05) between the v a l u e s i n a i r and water f o r e i t h e r s e a l and between s e a l s i n a i r . There i s a s i g n i f i c a n t d i f f e r e n c e between the' two s e a l s i n w a t e r . 3 5 • i T a b l e 2 : R e s p o n s e s o f b o t h s e a l s i n b o t h a i r a n d w a t e r f o r f i n a l t e s t i n g s . V a l u e s a r e g i v e n i n % c o r r e c t f o r e a c h g a p s i z e . G a p S i z e B l o n d i e S c a r (mm) ( m i n . o f a r c ) A i r • n W a t e r n A i r n W a t e r n 2 . 1 6 4 . 3 5 1 0 0 3 0 - - 1 0 0 2 0 - - 2 . 0 3 4 . 1 0 8 5 1 0 8 - 9 8 9 0 - - 1 . 7 9 3 . 5 7 9 0 1 0 0 9 6 2 4 1 0 0 1 0 0 - - 1 . 6 6 3 . 3 4 8 4 9 8 - - 9 6 1 0 0 - - 1 . 5 4 3 . 0 7 - 1 0 0 5 8 - - - - 1 . 4 2 2 . 8 3 • - 9 9 1 0 1 - - 8 3 5 9 1 . 2 9 2 . 5 7 - - 9 9 1 4 4 - - 9 5 1 4 1 1 . 2 2 2 . 4 3 - - - - - 9 2 2 2 1 . 1 6 2 . 3 1 8 5 6 0 9 7 1 0 5 9 5 6 0 9 / 1 3 0 . 1 . 0 8 2 . 1 5 - ' - 9 4 1 7 - - 6 9 4 8 1 . 0 2 2 . 0 5 8 3 1 1 9 ' 9 0 5 1 8 8 1 2 0 8 3 2 4 0 . 9 7 1 . 9 5 8 0 2 0 - - 7 5 2 0 - - 0 . 9 0 1 . 8 0 - - 8 1 4 7 - ' - 6 2 2 1 0 . 7 6 1 . 5 4 6 4 2 4 7 5 4 4 5 8 3 0 5 7 9 9 0 . 7 1 1 . 4 1 - - 7 4 1 9 - - - - 0 . 6 4 , 1 . 2 8 5 9 5 5 7 0 3 3 6 3 7 0 6 2 2 6 0 . 5 7 1 . 1 3 , . ; . - . 7 1 1 7 - - 6 3 3 0 0 . 5 1 1 . 0 2 4 6 4 0 . - - 5 8 5 0 - . - 6 5 4 6 6 0 6 6 0 6 0 0 FOUR-YEAR OLD FEMALE 0.5 1.5 GAP SIZE mm. F i g . ' 2 3 .A c o m p a r i s o n o f the a b i l i t y o f the f o u r - y e a r o l d female ha.rbour s e a l t o d i s c r i m i n a t e s m a l l d i f f e r e n c e s i n the gap s i z e o f the two-bar p a t t e r n . i n both a i r and water.. A w e i g h t e d l i n e a r r e g r e s s i o n was performed t o determine the d i f f e r e n c e t h r e s h o l d w h i c h i s d e f i n e d as the gap s i z e c o r r e s p o n d i n g t o t h e 7 5 % ( 6 0 a r c s i n tt) performance l e v e l . 37 TWO-YEAR OLD MALE i 1 * i i i i i i 0.5 1 1.5 2 GAP SIZE mm. F i g . 2 h . A comp a r i s o n of the a b i l i t y o f the two-year o l d male h a r b o u r s e a l t o d i s c r i m i n a t e s m a l l d i f f e r e n c e s i n the gap s i z e of t h e two-bar p a t t e r n i n both a i r and water. A we i g h t e d l i n e a r r e g r e s s i o n was performed t o det e r m i n e the d i f f e r e n c e t h r e s h o l d w h i c h i s d e f i n e d as the gap s i z e c o r r e s p o n d i n g t o the 75%(60 a r c s i n 7%) performance l e v e l . 38 D i s c u s s i o n The g e n e r a l aim o f t h i s s t u d y i s t o d e t e r m i n e t h e n a t u r e and degree o f the h a r b o u r s e a l ' s v i s u a l a d a p t a t i o n s t o an amphibious h a b i t . I w i l l f i r s t d i s c u s s the g r o s s m o r p h o l o g i c a l and h i s t o l o g i c a l f e a t u r e s , as an adequate u n d e r s t a n d i n g o f the s t r u c t u r e o f the eye i s e s s e n t i a l b e f o r e i t s f u n c t i o n a l c a p a b i l i t i e s may be a s s e s s e d . T h i s s t u d y shows t h a t the eye o f the h a r b o u r s e a l i s m o r p h o l o g i c a l l y s i m i l a r t o t h a t o f o t h e r p i n n i p e d s and c o n f i r m s W a l l s ' (1942) d e s c r i p t i o n o f t h e p i n n i p e d eye. B r i e f l y , t h e e y e b a l l i s l a r g e , b o t h i n a b s o l u t e s i z e and i n r e l a t i v e s i z e t o the body, and i s r o u g h l y s p h e r i c a l i n shape. The cornea i s f l a t t e n e d i n i t s c e n t e r r e g i o n s , where i t i s much f l a t t e r than i n humans. I t i s n o t more convex as s t a t e d by Johnson (1893). The a n t e r i o r chamber i s r e l a t i v e l y deep, a t l e a s t when compared t o t h e a q u a t i c t e l e o s t eye, w h i c h t y p i c a l l y has the l e n s i n d i r e c t c o n t a c t w i t h t h e c o r n e a ; i t i s n o t "very deep" as a g a i n s t a t e d by Johnson. The l e n s i s e s s e n t i a l l y s p h e r i c a l i n shape and a l t h o u g h l a r g e , i s n o t out o f p r o p o r t i o n t o t h e r e s t o f t h e eye and i s n o t " n o c t u r n a l " i n s i z e . By t h i s I mean t h a t i t s e q u a t o r i a l d i a m e t e r r e l a t i v e t o t h e v e r t i c a l d i a m e t e r o f the eye i s o n l y 0.37, a v a l u e much c l o s e r t o t h a t o f t h e t w e n t y - f o u r - h o u r u n g u l a t e s (0.40) than t h a t o f t h e n o c t u r n a l c a r n i v o r e s (0.50) ( W a l l s , 1942). T h i s i n c r e a s e i n c u r v a t u r e , as W a l l s p o i n t s o u t , seems thus t o be an a d a p t a t i o n t o the ' l o s s ' o f 3 9 - t h e cornea under w a t e r and n o t t o n o c t u r n a l i t y , an h y p o t h e s i s f u r t h e r s u p p o r t e d by t h e d i u r n a l h a b i t o f p i n n i p e d s . The s c l e r a i n the h a r b o u r s e a l i s n o t out o f p r o p o r t i o n i n t h i c k n e s s t o the r e s t o f t h e eye, as i s t h e case i n c e t a c e a n s ; i t i s t h i c k e n e d s l i g h t l y i n t h e r e g i o n s u r r o u n d i n g t h e o p t i c n e r v e . T h i s would seem t o q u e s t i o n W a l l s ' (1942) c l a i m t h a t the t h i c k e n e d s c l e r a i n c e t a c e a n s f u n c t i o n s p r i m a r i l y as a s u p p o r t i n g s t r u c t u r e owing t o t h e l a r g e a b s o l u t e s i z e o f the eye, as the P a c i f i c w h i t e - s i d e d d o l p h i n , Lagenorhynchus o b l i q u i d e n s , w h i c h has a s m a l l e r a b s o l u t e e y e b a l l s i z e than t h e h a r b o u r s e a l , has a t h i c k e r s c l e r a . However, perhaps the g r e a t e r swimming speeds a t t a i n e d by c e t a c e a n s , w i t h the r e s u l t i n g i n c r e a s e i n c o r n e a l s t r e s s , n e c e s s i t a t e s t h i s d i f f e r e n c e . W i t h r e g a r d t o f i n e s t r u c t u r e , t h i s s t u d y shows t h a t the r e t i n a o f the h a r b o u r s e a l i s h i s t o l o g i c a l l y s i m i l a r t o t h a t o f o t h e r p i n n i p e d s , e s p e c i a l l y t h a t o f the h a r p s e a l as d e s c r i b e d by Nagy and R o n a l d (1970). B o t h s p e c i e s show l i t t l e z o n a t i o n i n t h e i n n e r n u c l e a r l a y e r , p o s s e s s l a r g e h o r i z o n t a l c e l l s , have the o u t e r n u c l e a r l a y e r as t h e t h i c k e s t o f a l l the r e t i n a l l a y e r s , and have s i m i l a r n u c l e a r d e n s i t i e s w i t h i n the d i f f e r e n t r e t i n a l l a y e r s t h r o u g h o u t t h e r e t i n a . The degree o f summation o f r e c e p t o r c e l l s t o b i p o l a r c e l l s to g a n g l i o n c e l l s i s a l s o q u i t e h i g h and s i m i l a r between th e two s p e c i e s , r o u g h l y 100:10:1. The e x t e n s i v e h o r i z o n t a l c e l l network a l s o s u g g e s t s t h a t a w e l l - d e v e l o p e d l a t e r a l l y c o n d u c t i n g network e x i s t s , a l t h o u g h t h e f u n c t i o n a l i m p o r t a n c e o f such a network i s s t i l l s p e c u l a t i v e . v 40 In t h e i n n e r n u c l e a r l a y e r , the b i p o l a r , amacrine and n e u r o g l i a l c e l l s were i n i t i a l l y i d e n t i f i e d and c l a s s i f i e d by the methods o f G o l g i and E h r l i c h ( P olyak, 1941) . S i n c e t h e s e methods were n o t used i n t h i s s t u d y , i d e n t i f i c a t i o n o f t h e s e elements i s u n c e r t a i n , owing t o t h e i r s i m i l a r m o r p h o l o g i c a l appearances. The shape, s i z e and p o s i t i o n o f the n u c l e i , and t h e appearances o f the c e l l u l a r p r o c e s s e s , a r e n o t s u r e c r i t e r i a . In c o n t r a s t t o Nagy and Rona l d (1970), however, i t i s f e l t t h a t cone-type r e c e p t o r s a r e p r e s e n t , a l t h o u g h n o t perhaps i n t h e c l a s s i c a l c o n t e x t as d e s c r i b e d by P o l y a k (1941) P e d l e r (1969) has r e c e n t l y p r o p o s e d a new m o r p h o l o g i c a l c l a s s i f i c a t i o n o f p h o t o r e c e p t o r s i n v o l v i n g t h r e e b a s i c v a r i e t i e s : Types A, B and C. The p r e s e n c e o f type A, r e c e p t o r s r e l a t i v e l y i n s e n s i t i v e , p o l y s y n a p t i c c e l l s r e p r e s e n t a t i v e o f P o l y a k ' s (1941) "cones", was n o t c o n c l u s i v e l y d emonstrated, as i t was i m p o s s i b l e t o d i s c e r n any d e f i n i t e m o r p h o l o g i c a l d i f f e r e n c e s between the i n n e r segments o f t h e p h o t o r e c e p t o r s i n v e r t i c a l ( F i g . 15) or h o r i z o n t a l ( F i g . 17) s e c t i o n . However, t h e r e i s some e v i d e n c e i n d i c a t i n g t h a t t h i s c e l l t y p e may i n f a c t be p r e s e n t on t h e b a s i s o f the l a r g e gaps i n the o u t e r l i m i t i n g membrane w h i c h a r e c h a r a c t e r i s t i c a l l y a s s o c i a t e d w i t h cone-type n u c l e i (Fig.. 15) In l i e u o f the i n a b i l i t y t o c o n f i r m the p r e s e n c e o f type A r e c e p t o r s , t y p e B r e c e p t o r s , r e l a t i v e l y s e n s i t i v e , p o l y s y n a p t i c c e l l s , a r e su g g e s t e d as t h e c o n e - l i k e type o b s e r v e d i n the r e t i n a 4 1 owing t o the p r e s e n c e o f p o l y s y n a p t i c p e d i c l e s . Those c e l l s p o s s e s s i n g o l i g o s y n a p t i c p e d i c l e s a r e type C r e c e p t o r s , s e n s i t i v e c e l l s o f the t y p e termed " r o d s " by P o l y a k ( 1 9 4 1 ) . These l a t t e r r e c e p t o r s c o m p r i s e the m a j o r i t y o f t h e p h o t o r e c e p t o r s found i n t h e h a r b o u r s e a l r e t i n a . Type B and C p h o t o r e c e p t o r s would a l s o appear t o be p r e s e n t i n the h a r p s e a l on the b a s i s o f the p r e s e n c e o f b o t h r o d - l i k e and c o n e - l i k e r e c e p t o r t e r m i n a l s (Nagy and Rona l d , 1 9 7 0 ) . They a l s o r e p o r t e d o n l y one m o r p h o l o g i c a l d i s t i n c t i n n e r segment v a r i e t y . L a c k i n g n e u r o p h y s i o l o g i c a l d a t a , t h e i m p l i c a t i o n s o f such a p h o t o r e c e p t o r c o m p o s i t i o n i n the p h o c i d r e t i n a a r e n o t c l e a r . The p r e s e n c e o f ty p e B r e c e p t o r s does n o t n e c e s s a r i l y g i v e them b e t t e r v i s u a l a c u i t y than an a l l r o d eye, owing t o the s p a r s e numbers t h a t a r e p r e s e n t . The e x t e n s i v e summation undergone by the s e n s i t i v e r e c e p t o r s t h r o u g h the b i p o l a r and g a n g l i o n c e l l s l i k e l y c r e a t e s h i g h l y s e n s i t i v e r e c e p t o r groups t o d e a l w i t h the r e l a t i v e l y low l i g h t i n t e n s i t i e s such as would e x i s t a t g r e a t d e p t h s , beneath snow-covered i c e , o r a t n i g h t . Those r e c e p t o r s p o s s e s s i n g p o l y s y n a p t i c p e d i c l e s may i n t u r n be c o n n e c t e d t o h o r i z o n t a l c e l l s , c o n n e c t i o n s w h i c h might i m p a r t a h i g h i n f o r m a t i o n p r o c e s s i n g c a p a b i l i t y t o the r e t i n a ( P e d l e r , 1 9 6 9 ) as t h e s e c e l l s conduct i m p u l s e s i n a l a t e r a l d i r e c t i o n . Such a system would c e r t a i n l y be advantageous t o a p r e d a t o r r e q u i r e d t o hunt a f a s t - m o v i n g p r e y i n a t h r e e - d i m e n s i o n a l environment under low l i g h t i n t e n s i t i e s . R e t i n a l f o l d i n g has been r e p o r t e d i n the c e t a c e a n eye by 42 1 Mann (1946) on t h e b a s i s o f h i s o b s e r v a t i o n s on the sperm 1 whale ( P h y s t e r macrocephalus) and t h e f i n whale ( B a l a e n o p t e r a p h y s a l u s ) , and most r e c e n t l y , by P i l l e r i (1967) i n the b e l u g a ( D e l p h i n a p t e r u s l e u c a s ) , the hippopotamus (H. amphibus), and the o t t e r ( L u t r a l u t r a ) . The r e t i n a l f o l d i n g r e p o r t e d by Mann (1946) i n v o l v e d o n l y the o u t e r n u c l e a r l a y e r and l a y e r o f r o d s and cones and was s p e c u l a t e d t o f u n c t i o n i n movement-detection. He t h e o r i z e d t h a t s i n c e movements a r e b e t t e r seen i n the p e r i p h e r a l r e g i o n s o f the r e t i n a than c e n t r a l l y , t h e l e s s the r e t i n a i s adapted f o r a c u t e v i s i o n , t h e b e t t e r - w i l l be i t s a b i l i t y t o d e t e c t movement. Thus, the i n c r e a s e d summation r e s u l t i n g from u n d u l a t i o n s o f the p h o t o r e c e p t o r l a y e r , w h i c h i n c r e a s e s the number o f r e c e p t o r s per u n i t a r e a o f r e t i n a l s u r f a c e w i t h o u t a c o r r e s p o n d i n g i n c r e a s e i n the number o f n e r v e f i b e r s i n the n e r v e f i b e r l a y e r , would, d e c r e a s e v i s u a l a c u i t y and so a i d i n movement- d e t e c t i o n . T h i s s i t u a t i o n w i t h r e g a r d t o p h y s i o l o g i c a l f a c t o r s i s n o t borne o u t , however, a l t h o u g h a c c o r d i n g t o W a l l s (1942), p s y c h i c f a c t o r s s u p p o r t b e t t e r movement-detection i n the p e r i p h e r y , where movements have a s a l i e n c y and a t t e n t i o n - v a l u e q u i t e o ut o f p r o p o r t i o n t o the c l a r i t y w i t h w h i c h they a r e a c t u a l l y d i s c r i m i n a t e d . I t seems u n l i k e l y , t h e r e f o r e , t h a t r e t i n a l f o l d i n g f u n c t i o n s i n movement-detection as Mann s p e c u l a t e s . • The f o l d i n g s r e p o r t e d by P i l l e r i (1967) i n v o l v e d a l l the l a y e r s o f the r e t i n a w i t h the e x c e p t i o n o f the pigment e p i t h e l i u m 43 As t h r e e mammalian o r d e r s were i n v o l v e d , he c o n c l u d e d t h a t f o l d i n g was an a d a p t a t i o n t o a q u a t i c l i f e , and suggested t h a t i t f u n c t i o n e d somehow i n d i v i n g , t h i s perhaps b e i n g an a d a p t a t i o n t o changes i n water p r e s s u r e t o a l l o w f o r f l u c t u a t i n eye volumes. T h i s i s u n l i k e l y a l s o - t h e eye c o n t a i n s no a i r p o c k e t s whose compr e s s i o n would cause d i s t o r t i o n . T h i s r a i s e s t h e q u e s t i o n o f the e x i s t e n c e o f r e t i n a l f o l d i n g as a " r e a l " phenonom i n the mammalian eye, b e . i t a d apted t o an a q u a t i c , amphibious or t e r r e s t r i a l mode o f l i f e as i t i s h a r d t o see how such a f e a t u r e might f u n c t i o n . I f i t i s " r e a l " , i t must have some f u n c t i o n a l s i g n i f i c a n c e ( C ain, 1964). I s t h e r e , t h e r e f o r e , a p o s s i b i l i t y t h a t i t might be an a r t i f a c t ? The appearance o f the f o l d s on h i s t o l o g i c a l e x a m i n a t i o n s u p p o r t s t h e i r " r e a i " s t a t u s , as l i t t l e s h e a r i n g o f the r e t i n a a ppears t o have o c c u r r e d . O f t e n , the p h o t o r e c e p t o r c e l l s between f o l d s a r e s t i l l c o n n e c t e d t o t h e pigment e p i t h e l i u m . T h i s i s e s p e c i a l l y e v i d e n t i n the f i r s t type o f f o l d i n g , w h i c h j u s t i n v o l v e s the o u t e r n u c l e a r and p h o t o r e c e p t o r l a y e r s . N e v e r t h e l e s s , t h i s o b s e r v a t i o n s h o u l d be q u a l i f i e d . The second type o f f o l d i n g , i n v o l v i n g a l l t h e r e t i n a l l a y e r s , i s a l m o s t c e r t a i n l y an a r t i f a c t , owing t o our i n a b i l i t y t o o b s e r v e any f o l d i n g t h r o u g h f u n d u s c o p i c e x a m i n a t i o n o f the l i v i n g s e a l eye. T h i s second t y p e , because o f i t s magnitude, s h o u l d be c l e a r l y v i s i b l e i f p r e s e n t . 4 4 That f o l d i n g o f any n a t u r e was p r e v e n t e d i n t h e l i v i n g a n i m a l and was an a r t i f a c t i n the p r e p a r e d s e c t i o n s seems l i k e l y f o r the f o l l o w i n g r e a s o n s . The p r i m a r y f u n c t i o n o f the o u t e r l i m i t i n g membrane i s t o o r i e n t the p h o t o r e c e p t o r c e l l s i n t h e same m e r i d i a n as t h e l i g h t p a s s i n g t h r o u g h t h e r e t i n a . T h i s i s n e c e s s a r y i n o r d e r t o p r o v i d e a r e a s o n a b l e degree o f v i s u a l a c u i t y , f o r i f they were p e r p e n d i c u l a r or even o b l i q u e l y o r i e n t e d t o the l i g h t p a s s i n g t h r o u g h the r e t i n a , then the c e l l s c o u l d be s t i m u l a t e d over a r e l a t i v e l y l a r g e r e t i n a l a r e a as opposed t o j u s t t h e d i a m e t e r o f the segment p o r t i o n o f the p h o t o r e c e p t o r c e l l . A l s o the e f f e c t o f h a v i n g o n l y p a r t s o f the r e t i n a w i t h r e c e p t o r s i n the same m e r i d i a n as the l i g h t p a t h i s the f o r m a t i o n o f a l t e r n a t i n g r e g i o n s o f a c u t e and f u z z y v i s i o n , a s i t u a t i o n w h i c h would n o t seem advantageous i n a p r e d a t o r . I t i s even more d i f f i c u l t t o s u ggest a f u n c t i o n f o r the second type o f f o l d i n g (gross f o l d i n g ) , assuming t h a t i t i s n o t an a r t i f a c t . An e q u a l l y i m p o r t a n t c o n s i d e r a t i o n i s the pr o b l e m o f r e t i n a l n u t r i t i o n c r e a t e d by the f o l d i n g . The r e g i o n o f the r e t i n a s u p e r f i c i a l t o and i n c l u d i n g the o u t e r p l e x i f o r m l a y e r i s t r a n s v e r s e d by b l o o d v e s s e l s o r i g i n a t i n g from t h e o p t i c n e r v e r e g i o n o f the r e t i n a and r e c e i v e s i t s n u t r i t i o n from t h e s e v e s s e l s . The o u t e r n u c l e a r and p h o t o r e c e p t o r l a y e r s however, r e c e i v e t h e i r n u t r i t i o n t h r o u g h d i f f u s i o n from the c h o r o i d a l b l o o d v e s s e l s t r a n s v e r s i n g between the pigment e p i t h e l i u m and tapetum ( F i g . 18). D i s t a n c e from t h e s e 45 v e s s e l s i s t h e r e f o r e a c r i t i c a l f a c t o r . The d i s t a n c e c r e a t e d by f o l d i n g , e s p e c i a l l y o f the second t y p e , would seem j u s t t o o g r e a t t o ensure adequate n u t r i t i o n (McCormick, 1970). However, t h e s e o b j e c t i o n s t o f o l d i n g as a " r e a l " phemenon are based p r i m a r i l y on an i n a b i l i t y to a s s o c i a t e any s p e c i f i c f u n c t i o n t o i t , w h i c h as C a i n (1964) p o i n t s out, i s n o t s u f f i c i e n t c r i t e r i o n . Too o f t e n i n the p a s t such c o n c l u s i o n s have proven f a l s e . F u r t h e r s t u d y i s t h e r e f o r e r e q u i r e d b e f o r e t h i s q u e s t i o n may be f u l l y r e s o l v e d . C o n f i r m a t i o n o f i t s p r e s e n c e c o u l d be o b t a i n e d i f f o l d s a r e o b s e r v e d i n s e c t i o n s embedded i n Epon, f o r t h e e x a c t c e l l u l a r r e l a t i o n s h i p s c o u l d then be o b s e r v e d t o see i f l o c a l i z e d s h e a r i n g has o c c u r e d . U n f o r t u n a t e l y , my Epon-embedded s e c t i o n s showed no f o l d s . T h e i r absence h e r e does n o t n e c e s s a r i l y r u l e out t h e i r e x i s t e n c e , however, as t h e d i s t a n c e between f o l d s was o f t e n g r e a t e r than the w i d t h o f t h e r e t i n a l s e c t i o n s c u t w i t h t h i s t e c h n i q u e - - The tapetum c e l l u l o s u m i n the h a r b o u r s e a l i s as e x t e n s i v e as i n d i c a t e d by W a l l s (1942) and l i n e s t h e whole v i t r e o u s c a v i t y , t e r m i n a t i n g j u s t . b e f o r e t h e r e t i n a does. A c c o r d i n g t o W a l l s (1942), t h i s g i v e s p i n n i p e d s the most e x t e n s i v e tapetum among mammals. The mechanism o f t a p e t a l r e f l e c t i o n i s s t i l l u n c e r t a i n , a l t h o u g h s e v e r a l have been s u g g e s t e d . W a l l s has s u g g e s t e d i n t e r f e r e n c e e f f e c t s b u t P e d l a r (1963), a l t h o u g h showing t h a t t h i s i s l i k e l y , shows t h a t i n t e r f e r e n c e as a s o l e mechanism i s u n l i k e l y . There may i n a d d i t i o n be a pigment, 46 a l t h o u g h none has been i s o l a t e d t o d a t e . W e i t z e l ejt a l . (1956) have i s o l a t e d z i n c c y s t e i n e from th e s e a l tapetum, b u t no c o n n e c t i o n s have been shown t o e x i s t between t h e c o l o u r o f t h i s m e t a l and t h e c o l o u r o f t h e tapetum. H e l l e r (1967) has shown t h a t t h i s compound p o s s e s s e s p h o t o e l e c t r i c p r o p e r t i e s , however, and s t a t e s t h a t t h i s i s r e l a t e d t o the i n c r e a s e d v i s u a l power o f c a r n i v o r e s i n t h e d a r k . Nagy and R o n a l d (1970) p o i n t o u t , however, t h a t t h i s would seem u n l i k e l y , when one c o n s i d e r s t h e e x t e n s i v e knowledge r e l a t i n g t o p h o t o r e c e p t o r e x c i t a t i o n as a c c l o m p l i s h e d by the p h o t o c h e m i c a l c o n v e r s i o n o f l i g h t energy i n t o n e u r a l i m p u l s e s . I f a pigment i s indeed p r e s e n t , i t may s u b s e r v e the same f u n c t i o n as s u g g e s t e d f o r r i b o f l a v i n i n the lemur (Galago e r a s s i c a u d a t u s ) r e t i n a ( P i r i e , 1959) . The f l u o r e s e n c e maximum o f r i b o f l a v i n i s 520 mu a t n e u t r a l i t y , and i s m a x i m a l l y a c t i v a t e d by l i g h t o f 370 and 445 mu. These w a v e l e n g t h s w i l l p e n e t r a t e t h e eye, be absorbed by the tapetum and then be r e - e m i t t e d as l i g h t o f 520 mu, owing t o the f l u o r e s c e n s e o f t h e r i b o f l a v i n . Thus, i f a pigment i s p r e s e n t , i t would be advantageous f o r i t t o r e f l e c t p a r t i c u l a r w a v e l e n g t h s o f l i g h t w h i c h a r e more s u i t a b l e t o t h e optimum s e n s i t i v i t y o f t h e v i s u a l c e l l s , f o r i t c o u l d t h e r e f o r e i n c r e a s e the s t i m u l u s r e c e i v e d by t h e p h o t o r e c e p t o r c e l l s . R e f r a c t i o n : jyxy r e f r a c t i v e r e s u l t s (Table 1) agree b a s i c a l l y w i t h t h o s e r e p o r t e d by Johnson (1893) and P i g g i n s (1970) f o r the 4 7 h a r b o u r s e a l and h a r p s e a l r e s p e c t i v e l y ; by means o f r e t i n o s c o p y , b o t h n o t e d c o n s i d e r a b l e myopia and a s t i g m a t i s m i n a i r , and i n wa t e r , P i g g i n s found moderate h y p e r m e t r o p i a and n e g l i g i b l e a s t i g m a t i s m . H y p e r m e t r o p i a , o r f a r - s i g h t e d n e s s , r e s u l t s i f the e y e b a l l i s t o o s h o r t , so t h a t the accommodation p r o c e s s i s in a d e q u a t e t o p u l l t h e image f o r w a r d o n t o t h e r e t i n a and t h e sharp image l i e s b e h i n d t h e eye. Myopia, or n e a r - s i g h t e d n e s s , i s t h e o p p o s i t e , where the e y e b a l l i s too e l o n g a t e d and t h e image l i e s t oo f a r f o r w a r d , i e . i n t h e v i t r e o u s c a v i t y . The t h i r d and l a s t r e f r a c t i v e e r r o r , a s t i g m a t i s m , i s when t h e r e t i n a l image o f a p o i n t i s n o t a p o i n t b u t a l i n e , owing t o one o f t h e r e f r a c t i n g s u r f a c e s b e i n g p a r t l y c y l i n d r i c a l as w e l l as s p h e r i c a l i n i t s c u r v a t u r e . T h i s i s almo s t always i n the cornea i n humans, b u t i t i s p o s s i b l e t o have l e n t i c u l a r a s t i g m a t i s m o r a s t i g m a t i s m r e s u l t i n g from t h e back o f the eye i t s e l f . The l o c a t i o n o f t h e a s t i g m a t i s m i n s e a l s i s e s p e c i a l l y c r i t i c a l , as o n l y i f i t i s c o r n e a l w i l l i t be removed under w a t e r , owing t o the s i m i l a r i t y i n r e f r a c t i v e i n d i c e s between the cornea and w a t e r . T h i s i s why c o n s i d e r a b l e e f f o r t was made t o q u a n t i f y the c o r n e a l a s t i g m a t i s m i n d e p e n d e n t l y o f any w h i c h might be i n h e r e n t i n the o t h e r eye components. R e t i n o s c o p y measures the r e f r a c t i o n o f t h e eye as a whole, and cannot be used t o d e t e r m i n e the r e f r a c t i o n o f any one r e f r a c t i n g s u r f a c e . W i t h r e s p e c t t o Johnson's (1893) and P i g g i n s ' (1970) r e s u l t s , owing t o the d i f f i c u l t i e s e n c o u n t e r e d by v e r y competent 4 8 d o c t o r s i n the t a k i n g o f my measurements, the a c c u r a c y o f t h e i r r e s u l t s seems q u e s t i o n a b l e , as n e i t h e r has had h i s r e s u l t s c o n f i r m e d . I found c o n s i d e r a b l e v a r i a t i o n i n v a l u e s t a k e n by d i f f e r e n t d o c t o r s on the same s e a l on the same day and even by the same d o c t o r w i t h d i f f e r e n t i n s t r u m e n t s (a round r e t i n o s c o p e v e r s u s a s l i t r e t i n o s c o p e ) on t h e same s e a l on t h e same day (see T a b l e 1 , Scar) . The r e a s o n s f o r t h e s e d i f f i c u l t i e s a r e n o t c e r t a i n , b u t I f e e l t he major problems were t h e p r e s e n c e o f a tapetum, the l a r g e eye, and the upward and h o r i z o n t a l l y (15°) d i r e c t e d o p t i c a x i s , a l l f e a t u r e s n o t c h a r a c t e r i s t i c o f the human eye w i t h which they were most e x p e r i e n c e d . These f e a t u r e s combined made i t v e r y d i f f i c u l t t o be s u r e t h a t c o r r e c t measurements a l o n g t h e o p t i c a l a x i s were t a k e n . A n a e s t h e s i a appeared t o have no e f f e c t on eye r e f r a c t i o n , as we were a b l e t o t a k e the r e f r a c t i o n o f one o f the s e a l s ( B l o n d i e ) w i t h o u t i t ; t h e s e v a l u e s were s i m i l a r t o l a t e r ones t a k e n w h i l e she was a n a e s t h One f u r t h e r s p e c u l a t i o n as t o why we had d i f f i c u l t y i n d e t e r m i n i n g t h e h a r b o u r s e a l ' s r e f r a c t i o n i s the i n f l u e n c e o f the a n i m a l ' s n e r v o u s s t a t e . I t has been c o n c l u s i v e l y d e m o n s t r a t e d t h a t among humans, e s p e c i a l l y c h i l d r e n , nervous t e n s i o n can and does change the r e f r a c t i v e s t a t e o f t h e eye s i g n i f i c a n t l y , o f t e n up t o 3 d i o p t e r s ( R a d l e r , 1955). T h i s change i s t y p i c a l l y one o f i n c r e a s e d myopia, and appears t o be r e l a t e d t o the d i f f i c u l t y o f the t a s k p r e s e n t e d . There i s no r e a s o n t o suppose t h a t a s i m i l a r r e a c t i o n t o s t r e s s 49 c o u l d n o t o c c u r i n o t h e r a n i m a l s , and i f such changes took p l a c e w h i l e the s e a l s were b e i n g t e s t e d , t h i s c o u l d perhaps a c c o u n t f o r a p a r t o f t h e v a r i a t i o n o b s e r v e d . R e f r a c t i o n i n v o l v e d t y i n g the a n i m a l down and. s h i n i n g a l i g h t i n t o i t s eyes and was d e f i n i t e l y bothersome t o a t l e a s t some o f the s e a l s . W i t h a P l a c i d o ' s d i s c , I was, however, a b l e t o c o n f i r m Johnson's (1893) and P i g g i n s 1 (1970) o b s e r v a t i o n o f t h e a x i s o f l e a s t c o r n e a l c u r v a t u r e i n the h o r i z o n t a l . T h i s o b s e r v a t i o n t h e r e f o r e , q u e s t i o n s some o f the r e t i n o s c o p y measurements, as some showed no a s t i g m a t i s m (Table 1, O p e r a t o r 1, S u s i e and Orphie) and a s t i g m a t i s m w i t h t h i s a x i s i n t h e v e r t i c a l (Table 1, O p e r a t o r 3, S c a r ) . Only th o s e measurements showing the a x i s o f l e a s t c o r n e a l c u r v a t u r e i n the h o r i z o n t a l a r e t h e r e f o r e c o n s i s t e n t w i t h my o b s e r v a t i o n s , as a l l o t h e r e v i d e n c e t o d a t e s u g g e s t s t h a t the l e n s and p o s t e r i o r c u r v a t u r e o f the eye a r e n o t s i g n i f i c a n t l y a s t i g m a t i c . I t i s f e l t t h a t t h e s e v a l u e s are m e a n i n g f u l , f o r when combined w i t h Johnson's measurements f o r the ha r b o u r s e a l , t h e y s h o u l d p r o v i d e a t l e a s t an i n d i c a t i v e r e f r a c t i v e v a l u e f o r t h i s s p e c i e s . I f t h i s i s indeed the cas e , and o n l y f u r t h e r measurements w i l l c o n f i r m i t , then W a l l s ' (1942) t h e o r i z i n g seems p a r t l y c o n f i r m e d i n t h a t the a s t i g m a t i s m w i l l be e l i m i n a t e d by t h e v e r t i c a l l y c o n s t r i c t e d p u p i l . The power o f a c y l i n d e r i s d i r e c t e d a t 90° t o i t s a x i s o f l e a s t c u r v a t u r e i e . t h e h o r i z o n t a l . However, t h e f u n c t i o n o f a s t i g m a t i s m 5 0 i n t h e d i o p t r i c m e c h a n i s m i s s t i l l n o t e x p l a i n e d . A r e f r a c t i v s t u d y o f t h e b e a r d e d s e a l , E r i g n a t h u s b a r b a t u s , w o u l d n o w b e i n t e r e s t i n g , a s i t s p u p i l s l i t i s a l m o s t h o r i z o n t a l i n c o n t r a s t t o t h a t o f o t h e r p i n n i p e d s ( W a l l s , 1 9 4 2 ) . I t s a x i s o f l e a s t c o r n e a l c u r v a t u r e w o u l d t h u s b e e x p e c t e d t o b e v e r t i c a l , i f i n d e e d a s t i g m a t i s m i s p r e s e n t a t a l l . T h i s r e m o v a l o f t h e a s t i g m a t i s m , h o w e v e r , r e q u i r e s a s t e n o p e i c a p e r a t u r e , s o t h a t a n a p p r o a c h t o t h e p e r f o r m a n c e o f a p i n h o l e a p e r t u r e r e s u l t s . O n l y u n d e r v e r y b r i g h t c o n d i t i o n s t h o u g h , s u c h a s o c c u r o n a s u n n y d a y , d o e s t h e p u p i l e v e r f u l l y a p p r o a c h t h i s c o n d i t i o n a n d o f t e n , a s i s t h e c a s e o n d u l l d a y s a n d u n d e r t h e c o n d i t i o n s o f m y b e h a v i o u r a l e x p e r i m e n t s , t h e p u p i l a d o p t s a r a t h e r m o r e o v a l o r e l l i p t i c a l s h a p e , d e p e n d i n g o n t h e a c t u a l l i g h t i n t e n s i t y . V i s i o n w o u l d t h u s l i k e l y b e p o o r e r u n d e r l o w l i g h t i n t e n s i t i e s t h a n u n d e r h i g h l i g h t i n t e n s i t i e s , a s t h e e f f e c t s o f t h e c o r n e a l a s t i g m a t i s m w o u l d n o t b e e n t i r e l y e l i m i n a t e d b y t h e n o w w i d e n e d p u p i l . I f t h e m e a n v a l u e o f t h e d e g r e e o f c o r n e a l a s t i g m a t i s m f o r t h e h a r b o u r s e a l i s c o r r e c t ( 0 . 6 d i o p t e r o f h y p e r m e t r o p i a i n t h e v e r t i c a l m e r i d i a n a n d 8 . 5 d i o p t e r s o f m y o p i a i n t h e h o r i z o n t a l , r e s u l t i n g i n 9 d i o p t e r s , o f a s t i g m a t i s m ) t h e n t h i s c a n n o t b e c o n s i d e r e d l i g h t l y , a n d a n y d e v i a t i o n f r o m a s t e n o p e i c a p e r t u r e w o u l d l i k e l y h a v e a p r o n o u n c e d e f f e c t . . I n w a t e r , t h e r e f r a c t i v e r e s u l t s a r e q u e s t i o n a b l e a s t h e - o p e r a t o r t a k i n g t h e m w a s u n a b l e t o d e t e c t a s t i g m a t i s m 51 when t e s t i n g i n a i r , a s t i g m a t i s m d e f i n i t e l y shown t o e x i s t by use o f a P l a c i d o ' s d i s c . However, they i m p l y moderate h y p e r m e t r o p i a w i t h no a s t i g m a t i s m , r e s u l t s w h i c h seem r e a s o n a b l e when compared t o t h o s e o f the harp s e a l ( P i g g i n s , 1970) i n T a b l e 1. Bo t h methods o f measurement a r e comparable i n t e c h n i q u e , a l t h o u g h a g a i n , i n d i v i d u a l v a r i a t i o n s h o u l d be e x p e c t e d . As t h e s e a r e t h e o n l y underwater r e f r a c t i o n s c a r r i e d o u t t o d a t e on p i n n i p e d s , i t would appear t h a t t h e s e two s p e c i e s o f p h o c i d s a t l e a s t a r e s i m i l a r i n r e f r a c t i o n underwater. These f i n d i n g s a l s o agree w i t h the i n f e r e n c e s o f Johnson (1893) and W a l l s (1942) t h a t the a s t i g m a t i s m i n p h o c i d s i s r e s i d e n t i n the cornea, f o r i t i s l a r g e l y removed i n w a t e r ; underwater, c o r n e a l a s t i g m a t i s m s h o u l d be l o s t w i t h t h e r e d u c t i o n i n e f f e c t i v e n e s s o f t h e cornea (assuming t h a t i t s r e f r a c t i v e i n d e x i s the same as o t h e r mammals i n b e i n g s i m i l a r t o t h a t o f w a t e r ) . V i s i o n under low l i g h t i n t e n s i t i e s would thus l i k e l y be p o o r e r i n a i r than water f o r t h e same r e a s o n s mentioned e a r l i e r , as the a s t i g m a t i s m would o n l y be e f f e c t i v e and d e t r i m e n t a l i n a i r . The q u e s t i o n i s now r a i s e d as t o why s e a l s p o s s e s s a s t i g m a t i s m a t a l l and how th e y p u t up w i t h such an extreme degree as 9 d i o p t e r s . Among the c a r n i v o r e s and h i g h e r mammals, no s i g n i f i c a n t a s t i g m a t i s m i s t h e r u l e (Johnson, 1901), a l t h o u g h i t i s found t o a s m a l l degree, i e . between 0.5 - 1.0 D., t h r o u g h o u t the o r d e r s . Only among the p i n n i p e d s and c e t a c e a n s i s i t p r e s e n t t o such an extreme degree. M a t t h i e s e n 52 (Johnson, 1901), who has r e f r a c t e d s e v e r a l A t l a n t i c cetaceans' eyes r e p o r t s a c o r n e a l a s t i g m a t i s m o f between 4 - 4 . 5 d i o p t e r s among a l l t h e s p e c i e s he observed.. Presumably, however, i t would be o r i e n t e d d i f f e r e n t l y from t h a t o f the p i n n i p e d s , as t h e i r p u p i l i s o f a c o n s i d e r a b l y d i f f e r e n t shape. Johnson's o b s e r v a t i o n s (1901) a l s o show t h a t among mammals, e x c l u d i n g the d o m e s t i c s p e c i e s , i n w h i c h r e f r a c t i o n v a r i e s c o n s i d e r a b l y i n a l l d i r e c t i o n s o f e r r o r , v i s i o n i s c h a r a c t e r i s t i c a l l y h y p e r m e t r o p i c . A s l i g h t degree o f h y p e r m e t r o p i a (under one d i o p t e r ) i s t y p i c a l o f the h i g h e r mammals, w h i l e h i g h e r h y p e r m e t r o p i a ( 2-5 d i o p t e r s ) i s found i n w i l d s p e c i e s o f r o d e n t s , t h e e d e n t a t e s and the m a r s u p i a l s . P i n n i p e d s a r e t h e r e f o r e u n i q u e , a l o n g w i t h t h e p r i m a t e s , i n p o s s e s s i n g myopia as a common and p e r s i s t e n t c h a r a c t e r . U n f o r t u n a t e l y , no a c c e p t a b l e e x p l a n a t i o n f o r t h e s e i r r e g u l a r i t i e s i s p r e s e n t l y a v a i l a b l e , f o r a l t h o u g h t h i s t h e s i s c o n f i r m s t h e i r p r e s e n c e , i t i s i m p o s s i b l e t o draw c o n c l u s i o n s on the f u n c t i o n a l s i g n i f i c a n c e o f t h e s e c h a r a c t e r s w i t h o u t f u r t h e r s t u d y . A s t e n o p a i c p u p i l w i l l f u n c t i o n a l l y c o r r e c t the d e t r i m e n t a l e f f e c t s imposed by a s t i g m a t i s m and myopia, b u t t h e s e e r r o r s a r e s t i l l n o t r e q u i r e d . In f a c t , the p r i m a r y f u n c t i o n o f the s l i t p u p i l i s presumably s i m p l y t o c u t down the i n t e n s e l i g h t e n c o u n t e r e d by t h e r o d - r i c h r e t i n a under p h o t o p i c c o n d i t i o n s . I t i s under low l i g h t i n t e n s i t i e s , when the p u p i l e n l a r g e s and ceases t o be f u n c t i o n a l l y s t e n o p a i c , t h a t t h e s e e r r o r s become s e r i o u s . V i s u a l D i s c r i m i n a t i o n s : When the s t a n d a r d p s y c h o p h y s i c a l c r i t e r i o n i s a p p l i e d t o t h e d a t a (the d i f f e r e n c e t h r e s h o l d b e i n g d e f i n e d as the v i s u a l a n g l e c o r r e s p o n d i n g t o t h e 75% p erformance l e v e l ) , i t i s found t h a t b o t h s e a l s c o u l d r e s o l v e a v e r t i c a l gap between two b l a c k l i n e s as s m a l l as 1 mm a t a d i s t a n c e o f 1.7 m. F u r t h e r , f o r each s e a l the v i s u a l d i s c r i m i n a t i o n was t h e same i n a i r and i n w ater ( F i g . 23 and 24) a l t h o u g h t h e r e was a s i g n i f i c a n t d i f f e r e n c e between the two s e a l s d i s c r i m i n a t i o n s i n w a t e r . T h i s d e v i a t i o n i s f e l t t o r e p r e s e n t i n d i v i d u a l b e h a v i o u r d i f f e r e n c e s , however, r a t h e r than m o r p h o l o g i c d i f f e r e n c e s , owing t o the s e a l s p e r s o n a l i t y d i f f e r e n c e s . Owing t o t h e w ide v a r i e t y o f s t i m u l u s p a t t e r n s used, i n v i s u a l d i s c r i m i n a t i o n s t u d i e s , and t h e i r d i f f e r e n t c h a r a c t e r i s t i c s , i t i s always n e c e s s a r y t o ask what p h y s i o l o g i c a l c a p a c i t y has been measured w i t h the p a t t e r n chosen. L i t (1968) has summarized the t y p e s o f e x p e r i m e n t s p o s s i b l e f o r m e a s u r i n g v i s u a l a c u i t y and s t a t e s t h a t "when the s u b j e c t i s r e q u i r e d t o d i s c r i m i n a t e o n l y the p r e s e n c e o r absence o f the t e s t s t i m u l u s . . . , the t a s k o f d e t e c t i n g an i l l u m i n a t e d t a r g e t a g a i n s t a b l a c k background becomes e q u i v a l e n t t o t h a t o f m e a s u r i n g th e a b s o l u t e s e n s i t i v i t y o f the eye t o l i g h t . " I f b o t h s e a l s i n t h e e x p e r i m e n t r e p o r t e d on h e r e i n were t h e r e f o r e o r i e n t e d t o the d e t e c t i o n o f an i l l u m i n a t e d gap a g a i n s t a b l a c k background r a t h e r than t o t h e d i s c r i m i n a t i o n o f a s e p a r a t i o n between the b a r s , t a r g e t s i z e would n o t be 54 c r i t i c a l , as t h e r e s o l u t i o n o f t a r g e t d e t a i l was n o t i n v o l v e d . The s m a l l n e s s o f an o b j e c t t h a t can be d e t e c t e d i s n o t an adequate measure o f v i s u a l a c u i t y . T h e r e f o r e , i f v i s u a l a c u i t y i s d e f i n e d as the c a p a c i t y o f v i s u a l systems t o d i s c r i m i n a t e f i n e d e t a i l s o f o b j e c t s l o c a t e d i n the f i e l d o f niew, the a b s o l u t e s e n s i t i v i t y o f t h e eye t o l i g h t r a t h e r t han v i s u a l a c u i t y i n the s t r i c t sense would seem t o have been measured. In t h e e v a l u a t i o n o f the h a r b o u r s e a l eye's a b s o l u t e s e n s i t i v i t y t o l i g h t i n b o t h a i r and w a t e r , the e f f e c t s o f t h e d i f f e r e n t p h y s i c a l p r o p e r t i e s between the two media cannot be i g n o r e d . For example, the a b s o r p t i o n and s c a t t e r i n g o f l i g h t a r e g r e a t e r i n w a t e r . As a r e s u l t , t h e f u r t h e r the a n i m a l i s away from th e t e s t s t i m u l u s , t h e g r e a t e r w i l l be t h e l o s s o f o b j e c t l i g h t i n water r e l a t i v e t o a i r . S i n c e t h e s e a l s i n t h i s s t u d y were d i s c r i m i n a t i n g a t a presumably t h r e s h o l d l e v e l a t a s t a n d a r d d i s t a n c e i n b o t h a i r and w a t e r , the a b s o l u t e s e n s i t i v i t y o f the eye would t h e r e f o r e be e x p e c t e d t o be l e s s i n w a t e r ; i n t h i s medium the l i g h t i n t e n s i t y per u n i t a r e a would d e c r e a s e a t a g r e a t e r r a t e than the s q u are o f the d i s t a n c e owing t o t h e e x a g g e r a t e d p r o p e r t i e s o f w a t e r . However, t h i s was n o t t h e case - the r e s u l t s i n d i c a t e s i m i l a r v a l u e s i n b o t h media. There a r e two p o s s i b l e 2 e x p l a n a t i o n s : e i t h e r the range o f i l l u m i n a t i o n , 85 candela/m , was g r e a t enough t o h i d e any d i f f e r e n c e w h i c h might e x i s t , w i t h 1.2 m b e i n g too s h o r t a d i s t a n c e t o s i g n i f i c a n t l y 55 i n f l u e n c e t h e r e s u l t s , or t h e r e was an i n c r e a s e i n p u p i l s i z e , t h e r e b y c o u n t e r - a c t i n g any s c a t t e r i n g e f f e c t s . Johnson (1893) has s u g g e s t e d t h a t the l a t t e r o c c u r s , s t a t i n g t h a t t h e p u p i l opens t o the f u l l , i . e . , a complete c i r c l e , i n w a t e r , as a c o r r e c t i o n f o r a s t i g m a t i s m i s n o t needed i n t h i s medium. However, no s i g n i f i c a n t change i n p u p i l s i z e o f t h e s e a l eye i n water has been o b s e r v e d . The f i r s t p o s s i b i l i t y , t h a t t h i s t e c h n i q u e was n o t d e s i g n e d t o d e t e c t t h i s v a r i a t i o n , would thus seem t o be the case. I t s h o u l d be n o t e d , however, t h a t w i t h i n c r e a s e d t e s t i n g d i s t a n c e s t h i s p r o p e r t y d i f f e r e n c e between the two media c o u l d become s i g n i f i c a n t . A wide v a r i e t y o f t e s t o b j e c t s a r e a v a i l a b l e f o r m e a s u r i n g v i s u a l a c u i t y , b u t b o t h R i g g s (1965) and L i t (1968) have d i s c u s s e d v i s u a l a c u i t y i n terms o f a c l a s s i f i c a t i o n system w h i c h i d e n t i f i e s f o u r d i f f e r e n t t a s k s : d e t e c t i o n , r e c o g n i t i o n , r e s o l u t i o n , and l o c a l i z a t i o n . In each case, v i s u a l a c u i t y i s s p e c i f i e d i n terms o f the r e c i p r o c a l v a l u e o f t h e v i s u a l a n g l e formed by some d e t a i l o f the t e s t o b j e c t w h i c h i s d i s c r i m i n a t e d . As d i s c u s s e d above, the t a s k i n t h i s s t u d y appears t o be one o f d e t e c t i o n , where o n l y t h e p r e s e n c e o r absence o f t h e o b j e c t i s asked b u t owing t o the b r o a d d e f i n i t i o n o f v i s u a l a c u i t y , i t i s p o s s i b l e t o c o n s i d e r t h i s t a s k as one o f r e s o l u t i o n , where d i s c r i m i n a t i o n o f a s e p a r a t i o n between elements o f the p a t t e r n i s r e q u i r e d . W i l c o x and Purdy (1933) have d i s c u s s e d t h e a m b i g u i t y a s s o c i a t e d w i t h 5 6 i t h i s d o u b l e l i n e t a r g e t i n d e t a i l , and d i s t i n g u i s h two a s p e c t s r e l a t i n g t o the s e p a r a t i o n t h r e s h o l d , the "minimum v i s i b l e t h r e s h o l d " i s t h e l e a s t p e r c e p t i b l e d i s t a n c e between two s t i m u l u s l i n e s o f i n f i n i t e s i m a l w i d t h , whereas t h e "minimum v i o i b l c t h r e s h o l d " i s t h a t r e s o l u t i o n t h r e s h o l d where the b r e a d t h o f the l i n e s i s so g r e a t t h a t a f u r t h e r i n c r e a s e i n t h e i r w i d t h would n o t improve th e d i s c r i m i n a t i o n o f a gap between them. W i t h t h e s e d e f i n i t i o n s , my measure was o f t h e l a t t e r , b u t i t i s s t i l l p o s s i b l e t o c o n s i d e r t h e s e v a l u e s as a measure of v i s u a l a c u i t y as t h i s d i f f e r e n c e i s n o t d i s t i n g u i s h e d i n the b r o a d d e f i n i t i o n o f v i s u a l a c u i t y . T h e r e f o r e , i f one d e f i n e s t h e r e s o l u t i o n t h r e s h o l d as the "minimum s e p a r a b l e " , i . e . t h e m i n i m a l a n g u l a r d i s t a n c e between two o b j e c t s i n o r d e r f o r them t o be seen as s e p a r a t e , i t i s p o s s i b l e t o c a l c u l a t e v a l u e s f o r t h i s p r o p e r t y f o r the h a r b o u r s e a l i n b o t h media (see T a b l e 3 ) . In c o n s i d e r i n g t h e v i s u a l a c u i t y o f the h a r b o u r s e a l i n a i r , i t has been shown t h a t t h e eye i s a s t i g m a t i c w i t h t h e a x i s o f most c u r v a t u r e b e i n g v e r t i c a l , l i k e i t s s l i t p u p i l (Johnson, 1893); an approach t o the c h a r a c t e r i s t i c s o f a p i n h o l e a p e r t u r e i s thus t h o u g h t t o r e s u l t . However, i t has been o b s e r v e d t h a t o n l y under h i g h l i g h t i n t e n s i t i e s , such as o c c u r on a b r i g h t day, does the p u p i l e v er f u l l y a pproach t h i s narrow s l i t c o n d i t i o n and t h a t under low l i g h t i n t e n s i t i e s , such as i n the e x p e r i m e n t s , the p u p i l a d o p ts a r a t h e r more o v a l or e l l i p t i c a l shape. I f Johnson's (1893) measurements 5 7 T a b l e I I I : E s t i m a t e d r e s o l u t i o n t h r e s h o l d s f o r b o t h s e a l s i n a i r a n d w a t e r . V a l u e s i n p a r e n t h e s e s r e f e r t o t h e 9 5 % c o n f i d e n c e l i m i t s . R e s o l u t i o n T h r e s h o l d s t , M n j . . , ^ d f ( m i n . o f a r c ) A i r W a t e r T w o - y e a r o l d m a l e 1 . 6 4 ( 3 . 3 5 - 0 . 0 6 2 . 1 3 • ( 2 . 9 6 - 1 . 3 0 ) 0 . 6 5 S e a l s F o u r - y e a r o l d f e m a l e 2 . 1 3 ( 4 . 1 1 - 0 . 1 5 ) 1 . 3 2 ( 2 . 4 8 - 0 . 1 6 ) 0 . 6 9 t<^> d f 0 . 4 4 2 . 2 0 t . . 1 . 9 6 0 P = . 0 5 « » - d f 5 8 o f the degree o f c o r n e a l a s t i g m a t i s m (9 d i o p t e r s ) a r e c o r r e c t f o r the h a r b o u r s e a l then t h i s cannot be c o n s i d e r e d l i g h t l y , and any w i d e n i n g o f the p u p i l would have a pronounced e f f e c t i n a i r , even a t t h e s h o r t d i s t a n c e o f 1.7 m. T h e r e f o r e , on t h e b a s i s o f c u r r e n t t h e o r i e s , v i s u a l a c u i t y under the t e s t c o n d i t i o n s would l i k e l y be p o o r e r i n a i r than i n w a t e r , as t h e e f f e c t s o f t h e c o r n e a l a s t i g m a t i s m would n o t be e n t i r e l y e l i m i n a t e d by the now widened p u p i l . S i n c e my r e s u l t s showed no d i f f e r e n c e i n a i r and w ater d i s c r i m i n a t i o n s , and t h e p u p i l i n a i r was r a t h e r o v a l (and t h e eye hence a s t i g m a t i c ) , i t seems most l i k e l y t h a t t h e a b s o l u t e s e n s i t i v i t y o f t h e eye t o l i g h t , n o t v i s u a l a c u i t y , was measured. Thus, my e x p e r i m e n t s do n o t n e c e s s a r i l y i m p l y t h a t the v i s u a l a c u i t y o f t h e h a r b o u r s e a l i s t h e same i n a i r as i n w a t e r . I n s t e a d , I s u ggest the o p p o s i t e , t h a t under low l i g h t i n t e n s i t i e s and moderate d i s t a n c e s , v i s u a l a c u i t y would be b e t t e r i n w a t e r . F u r t h e r s u p p o r t t h a t v i s u a l a c u i t y per se was n o t measured i n t h i s s t u d y i s p r o v i d e d by Schusterman and B a l l i e t ( 1 9 7 0 a ) . T h e i r p a t t e r n s were g r a t i n g s c o n s i s t i n g o f l i n e s v a r y i n g i n w i d t h , t h e r e b y c o r r e c t i n g the problems i n h e r e n t i n my p a t t e r n s . T h r e s h o l d v a l u e s o f . 8 . 5 ' and.8.1' were o b t a i n e d f o r Phoca v i t u l i n a i n w a t e r , v a l u e s c o n s i d e r a b l y d i f f e r e n t from t h o s e o b t a i n e d i n t h i s t h e s i s ( 2 ' ) . I t s h o u l d be n o t e d , however, t h a t s i n c e a l l t h e s e measurements were made on o n l y a few i n d i v i d u a l s , i t i s n o t p o s s i b l e t o d e c i d e the degree t o w h i c h 59 t h e s e v a l u e s a r e c h a r a c t e r i s t i c o f t h e s p e c i e s . These v a l u e s , t h e r e f o r e , a r e n o t as s u i t a b l e as a measure o f v i s u a l a c u i t y might have been i n the e s t i m a t i o n o f the e f f e c t i v e n e s s o f t h e d i o p t r i c mechanism i n b o t h media. However, t h e y do s u p p o r t my h i s t o l o g i c a l o b s e r v a t i o n s and i n d i c a t e t h a t t h e har b o u r s e a l p o s s e s s e s a v e r y s e n s i t i v e v i s u a l system; i t i s comparable t o t h a t o f many n o c t u r n a l s p e c i e s (Table 4 ) . I t must be borne i n mind, however, t h a t t h e s e v a l u e s a r e n o t e a s i l y comparable w i t h o t h e r v a l u e s p u b l i s h e d i n the l i t e r a t u r e . D i f f e r e n t w o r k e r s may use d i f f e r e n t d e f i n i t i o n s , as p o i n t e d out by L i t (1968) , and o f t e n d i f f e r e n t e x p e r i m e n t a l methods (Table 4 ) . For example, W a l l s (1942) p r e s e n t s a t a b l e o f t h e v i s u a , l a c u i t i e s o f a few a n i m a l s f o r p a r a l l e l l i n e s (from v a r i o u s s o u r c e s ) , b u t as he f a i l s t o s p e c i f y the p a t t e r n s , d i s t a n c e s and l i g h t i n g used i n each case, t h e v a l u e s a r e m e a n i n g l e s s f o r a c c u r a t e c o m p a r a t i v e p u r p o s e s . Indeed, v i s u a l a c u i t y i n the s t r i c t sense may n o t have been measured, as i n t h i s t h e s i s . The amphibious p i n n i p e d eye as r e p r e s e n t e d by the har b o u r s e a l i s s i g n i f i c a n t l y d i f f e r e n t from the t e r r e s t r i a l mammalian eye. I t p o s s e s s e s a s p h e r i c a l l e n s f o r v i s i o n i n water, w i t h a complex d i o p t r i c mechanism f o r a i r . The most u n i q u e f e a t u r e s a r e h i g h myopia and a s t i g m a t i s m , c h a r a c t e r s n o t commonly found i n t e r r e s t r i a l mammals. I t s h o u l d be n o t e d t h a t a l t h o u g h the eyes o f many s m a l l n o c t u r n a l mammals a l s o p o s s e s s s p h e r i c a l l e n s e s , t h i s i s an a d a p t a t i o n t o n o c t u r n a l i t y . 60 T a b l e 4. A comparison o f v i s u a l a c u i t y t h r e s h o l d s (V.A.T.) and r e t i n a l s t r u c t u r e f o r a number o f a n i m a l s from v a r i o u s s o u r c e s . ( " W a l l s , 1942; 2=McCormick, 1970; 3 = 0 r d y & S a m o r a j s k i , 1968; 4 5 6 =Schusterman & B a l l i e t , 1970a; =Nagy & Ronald, 1970; ^ P i l l e r i , 7 1961; =Spong, 1968 ( b e h a v i o u r a l a s p e c t s o n l y ) ) . D = d i u r n a l ; N = n o c t u r n a l ; D/N = 24 h r . h a b i t ; P.L. = p a r a l l e l l i n e s S p e c i e s # t e s t e d P a t t e r n V.A.T. Outer Rows Gang- (min.) n u c l e i o f l i o n Inner c e l l s n u c l e i Domestic p i g e o n (Columba l i v i a ) Human, a d u l t 1, 2 P.L. P.L. S t r i a e S t r i a e S.A. owl monkey 4 (Aotus t r i v i r g a t u s 3 Tree shrew 4 (Tupaia g l i s ) 3 Bush baby 4 S t r i a e (Gagalo c r a s s i c a u d a t u s ) 3 Lemur 4 S t r i a e (Lemur c a t t a ) Harbour s e a l i n 1 S t r i a e w a t e r 4 (Phoca v i t u l i n a ) Harbour s e a l i n a i r 2 2 P.L. and water (Phoca v i t u l i n a ) ' 5 Harp s e a l - - ( P a g o p h i l u s g r o e n l a d i c u s ) 6 Beaver - ( C a s t o r c a n a d e n s i s ) K i l l e r whale l (Grampus orca) 2 P.L. P a c i f i c w h i t e - s i d e d 1 d o l p h i n ' 2 P.L. (Lagenorhynchus o b l i q u i d e n s ) 2.7 0.44- 0. 83 4.0 1. 0 4.0 0.5 8.3 6.0 6.0 4.5 10 13 8 10 2.0 10 10 7.5 15 3.4 4.5 2.5 2.5 4.5 1 D D N D N D D/N D/N D/N N D/N D/N 61 In t h e s e a n i m a l s , the l e n s i s always v e r y l a r g e i n p r o p o r t i o n t o eye s i z e , t h e r e b y p l a c i n g the o p t i c a l c e n t e r o f t h e eye deep w i t h i n t h e eye. T h i s i s n o t the case i n p i n n i p e d s . The n e t r e s u l t i n n o c t u r n a l mammals i s a s m a l l b u t b r i g h t image e q u a l l y good i n t h e p e r i p h e r y o f t h e r e t i n a as i n the c e n t e r . T h i s f e a t u r e i n s m a l l mammals i s a l s o t y p i c a l l y a s s o c i a t e d w i t h a b r o a d c o r n e a i n o r d e r t o m a i n t a i n a w i d e - a n g l e d v i s u a l f i e l d ( W a l l s , 1942) . I t i s d i f f i c u l t t o compare the amphibious p i n n i p e d eye t o the a q u a t i c mammalian eye, f o r as i n amphibious mammals, r e l a t i v e l y l i t t l e i s known about t h e i r v i s u a l systems and c a p a c i t i e s . Gross eye morphology i s well-documented ( P u t t e r , 1903; W a l l s , 1942) b u t l i t t l e i s known about the s t r u c t u r a l o r g a n i z a t i o n o f t h e eye. The c e t a c e a n eye, however, i s r e p o r t e d t o c o n t a i n many r o d s w h i c h a r e b i g g e r than t h o s e o f t e r r e s t r i a l mammals ( S l i j p e r , 1962), as w e l l as cones, a tapetum l u c i d u m , and a w e l l - d e v e l o p e d h o r i z o n t a l l y c o n d u c t i n g network w i t h i n the r e t i n a (Mann, 1946). M o r p h o l o g i c a l l y , t h e c i l i a r y m uscles a r e w e l l - d e v e l o p e d i n o d o n t o c e t e s b u t a r e a l m o s t n o n - e x i s t a n t i n m y s t i c e t e s . The l e n s i s s p h e r i c a l and has a g r e a t e r r e f r a c t i v e i n d e x than t h a t o f t e r r e s t r i a l mammals, w h i l e the co r n e a , aqueous humor, and p r o b a b l y the a i t r e o u s humor have the same r e f r a c t i v e i n d e x as sea water ( S l i j p e r , ' 1962). A c c o r d i n g t o Mann (1946), however, the main f e a t u r e o f the c e t a c e a n eye i s t h a t the d i s t a n c e between the l e n s and r e t i n a i s g r e a t e r i n t h e upper p o r t i o n o f the eye and s m a l l e r i n t h e lower p o r t i o n o f the eye. The eye i s thus 6 2 d i v i d e d i n t o two p a r t s , w i t h a i r images r e c e i v e d a t the bottom o f t h e eye and w ater images r e c e i v e d on the upper h a l f o f the eye, the l o n g - a x i s . T h i s s p e c i a l method o f s e e i n g i n a i r and w a t e r he terms "amphyophthalmia". T h i s s t r u c t u r a l o r g a n i z a t i o n has n o t been c o n f i r m e d , however, and owing t o the v i s u a l h a b i t s o f c e t a c e a n s , i t i s h a r d t o see how such a v i s u a l a p p a r a t u s would be r e q u i r e d . N e v e r t h e l e s s , i t would seem from t h e s e f r a g m e n t a r y o b s e r v a t i o n s t h a t the c e t a c e a n eye i s indeed a q u a t i c a l l y adapted and t h a t the p i n n i p e d eye has many b a s i c s i m i l a r i t i e s . I t would thus seem t h a t t h e p i n n i p e d eye has passed th e "halfway p o i n t " i n the t r a n s i t i o n between a t e r r e s t r i a l and an a q u a t i c eye, i n t h a t i t s v i s u a l system i s most " e f f i c i e n t " i n w a t e r . However, such a s t a t e i s d i f f i c u l t to a s s e s s , as e x a c t l y what s t a g e s ' a r e i n v o l v e d i n t h e t r a n s i t i o n a r e u n c e r t a i n . The p r e s e n c e o f a s p h e r i c a l l e n s , as opposed t o w r i n g i n g t h e l e n s (the method employed by t h e t u r t l e , the cormorant, and t h e o t t e r ) , would seem a major achievement, though. F u r t h e r s t u d i e s on t h e e x t e n t o f t h i s a d a p t a t i o n a r e now r e q u i r e d , u t i l i z i n g e l e c t r o n m i c r o s c o p y and p h y s i o l o g i c a l t e c h n i q u e s . The e x a c t n a t u r e o f the v i s u a l pigments and whether or n o t a pigment i s p r e s e n t i n t h e tapetum a r e s u g g e s t e d s u b j e c t s f o r f u r t h e r s t u d y . 6 3 SUMMARY ' T h i s s t u d y shows t h a t the h a r b o u r s e a l eye i s m o r p h o l o g i c a l d i f f e r e n t from t h a t o f t e r r e s t r i a l mammals and a q u a t i c mammals, b e i n g adapted f o r v i s i o n i n b o t h a i r and w a t e r . I t p o s s e s s e s a s p h e r i c a l l e n s t o compensate f o r the l o s s i n e f f e c t i v e n e s s o f t h e co r n e a under w a t e r . In a i r , a unique d i o p t r i c mechanism i n c o r p o r a t i n g the r e t i n a and p u p i l p r e v e n t the need f o r e x c e s s i v e accommodation. T h i s mechanism a l s o c o r r e c t s t h e r e f r a c t i v e e r r o r s under h i g h l i g h t i n t e n s i t i e s , b u t l o s e s e f f e c t i v e n e s s as t h e p u p i l e n l a r g e s w i t h d e c r e a s i n g l i g h t i n t e n s i t y . , A d e t a i l e d s t u d y o f r e t i n a l o r g a n i z a t i o n was a l s o completed w i t h t ype B p h o t o r e c e p t o r c e l l s b e i n g p o s i t i v e l y i d e n t i f i e d f o r the f i r s t t i m e i n a p i n n i p e d . A v e r y e x t e n s i v e network o f l a t e r a l l y c o n d u c t i n g elements was a l s o o b s e r v e d , i m p l i n g perhaps a h i g h i n f o r m a t i o n p r o c e s s i n g a b i l i t y w i t h i n the r e t i n a . Owing t o the v e r y l a r g e number o f t y p e C p h o t o r e c e p t o r c e l l s a l s o p r e s e n t , the r e t i n a may be assumed t o be a l s o v e r y l i g h t s e n s i t i v e , a c o n d i t i o n w h i c h meets the r e q u i r e m e n t s o f the d i o p t r i c mechanism; a s e n s i t i v e r e t i n a i s r e q u i r e d f o r p h o t o p i c v i s i o n owing t o t h e e f f e c t o f the s t e n o p e i c a p e r t u r e , as w e l l as f o r n o c t u r n a l v i s i o n . R e t i n a l f o l d s were a l s o o b s e r v e d , b u t t h e i r e x i s t e n c e i n the normal l i v i n g a n i m a l i s c o n s i d e r e d u n l i k e l y on the b a s i s o f p r e s e n t knowledge. The o r g a n i z a t i o n o f the ha r b o u r s e a l r e t i n a i s thus b a s i c a l l y 64 s i m i l a r t o t h a t r e p o r t e d f o r t h e h a r p s e a l (Nagy and Ronald, 1970) . R e f r a c t i v e o b s e r v a t i o n s have shown t h a t the h a r b o u r s e a l eye i s v e r y myopic and a s t i g m a t i c i n a i r , and h y p e r m e t r o p i c w i t h no d i s c e r n a b l e a s t i g m a t i s m i n w a t e r . The a x i s o f l e a s t c o r n e a l c u r v a t u r e i s h o r i z o n t a l , c o n f i r m i n g J o h n s o n 1 s (1893) o b s e r v a t i o n s and h i s and W a l l s ' (1942) t h e o r i z i n g as t o i t s f u n c t i o n i n a i r . T h i s l a t t e r o b s e r v a t i o n i s a l s o i n agreement w i t h t h a t r e p o r t e d by P i g g i n s (1970) f o r the h a r p s e a l . B e h a v i o u r a l e x p e r i m e n t s a l s o c o n f i r m e d t h e r e l a t i v e l y h i g h s e n s i t i v i t y o f t h e h a r b o u r eye t o l i g h t , as s u g g e s t e d by the h i s t o l o g i c a l o b s e r v a t i o n s . B o t h s e a l s t e s t e d were c a p a b l e o f d i s c r i m i n a t i n g a 1 mm gap a t a d i s t a n c e o f 1.7 m i n b o t h a i r and w a t e r . The s i m i l a r i t y o f t h e s e r e s u l t s i n b o t h media s u p p o r t s t h e c o n t e n t i o n t h a t v i s u a l s e n s i t i v i t y and n o t v i s u a l a c u i t y i n t h e s t r i c t sense was measured. 6 5 G l o s s a r y o f Terms c y c l o p l e g i a - p a r a l y s i s o f the c i l i a r y m uscle; p a r a l y s i s o f accommodation. d i o p t e r - t h e r e f r a c t i v e power o f a l e n s w i t h a f o c a l d i s t a n c e o f one meter (assumed as a u n i t o f measurement f o r r e f r a c t i v e power). d i o p t r i c mechanism - p e r t a i n i n g t o t h e eye s t r u c t u r e s i n v o l v e d i n r e f r a c t i o n f u n d u s c o p i c e x a m i n a t i o n - e x a m i n a t i o n o f t h e fundus o f the eye k e r a t o m e t e r - an i n s t r u m e n t f o r m e a s u r i n g the c u r v e s o f the c o r n e a P l a c i d o ' s d i s c - a d i s c w i t h c o n c e n t r i c c i r c l e s marked on i t used i n e x a m i n i n g the c o r n e a . s t e n o p e i c - h a v i n g a narrow s l i t o r o p e n i n g . r e t i n o s c o p y - an o b j e c t i v e method f o r i n v e s t i g a t i n g , d i a g n o s i n g , and e v a l u a t i n g r e f r a c t i v e e r r o r s o f the eye, by p r o j e c t i o n o f a beam o f l i g h t i n t o the eye and o b s e r v a t i o n o f t h e movement o f the i l l u m i n a t e d a r e a on the r e t i n a l s u r f a c e and o f the r e f r a c t i o n by the eye o f the emergent r a y s . 66 REFERENCES Aberc r o m b i e , M., 1946, E s t i m a t i o n o f n u c l e a r p o p u l a t i o n s from microtome s e c t i o n s . Anat. Rec. 94: 239-247. C a i n , A. J . , 1964, The p e r f e c t i o n o f a n i m a l s . V i e w p o i n t s i n B i o l , 3: 36-63 C h i e v i t z , J . H., 1889, Untersuchungen uber d i e A r e a c e n t r a l i s r e t i n a e . A r c h . f_. Anat. S u p p l . -Band: 139-196 C u l l i n g , C.F.A., 1963, Handbook of H i s t o p a t h o l o g i c a l T e c h n i q u e s . 2^- ed. B u t t e r w o r t h s , London. 553 pp. F e i n s t e i n , S.H. & R i c e , C.E., 1966, D i s c r i m i n a t i o n o f a r e a d i f f e r e n c e s by t h e h a r b o u r s e a l . Psychon. S c i . 4(11) : 379-380 H e l l e r , S., 1967, Zur F u n c t i o n des Z i n k - c y s t e i n s im Tapetum l u c i d u m . Hoppe-Seyler ' s Z. f_. P h y s i o l . Chem. 348: 1211-1212 Johnson, G.L., 1893, O b s e r v a t i o n s on t h e r e f r a c t i o n and v i s i o n o f t h e s e a l ' s eye. P r o c . Z o o l . Soc. 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Graham, C.H.; W i l e y , N.Y. : 321-349 Romer, A.S., 1949, The V e r t e b r a t e Body, Saunders, P h i l a d e l p h i a S a b a t i n i , D.D. , B n s c h , K., and B a r r n e t t , R.J., 1963, The p r e s e r v a t i o n o f c e l l u l a r u l t r a s t r u c t u r e and e n z y m a t i c a c t i v i t y by a l d e h y d e f i x a t i o n . Jj_ C e l l B i o 1. 17: 19-58. Schusterman, R.J., K e l l o g g , W.N. , and R i c e , C.E., 1965, Underwater v i s u a l d i s c r i m i n a t i o n by t h e C a l i f o r n i a sea l i o n . S c i e n c e 147: 1594-1596. Schusterman, R.J., G e n t ry, R.L. and Schmook, J . , 1967, Underwater sound p r o d u c t i o n by c a p t i v e C a l i f o r n i a sea l i o n s . Z o o l o g i c a 52: 21-24 Schusterman, R.J., 1968, P e r s o n a l communication Schusterman, R.J. and B a l l i e t , R.F., 1970a, V i s u a l a c u i t y o f the h a r b o u r s e a l and the S t e l l e r sea l i o n u n d e r w a t e r . N a t u r e 226: 563-564. 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