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The digestive tract of a harpacticoid copepod, Tiqriopus californicu: a light and electron microscope study Sullivan, Druscilla Shirley

Abstract

A study on the digestive tract of a harpacticoid copepod, 2i3lio£Ss californicus, was carried out using techniques of light and electron microscopy. It was found that a curved, cuticulized esophagus extends from the ventral mouth to the midgut. Its musculature and shape allows fairly large food particles to enter the gut. The noncuticulized portion of the digestive tract consists of; 1. A single, anterior, spherical midgut caecum, 2. An anterior midgut extending from the midgut caecum to the joint at the beginning of the urosome, 3. A posterior midgut extending almost the length of the urosome. The cuticulized hindgut can be divided, structurally, into anterior and posterior regions. It is suggested that the anterior hindgut functions in ion and water regulation as well as begins the formation of a faecal pellet. The posterior hindgut compacts the faecal pellet and retains it until defaecation. At the light and electron microscope levels four cell types could be distinguished. By studying the cell's position in the gut, electron density, amount of lipid, amount and type of vesiculation and the abundance and position of the cell*s organelles, functions for these cells were determined: 1. cell type one is an embryonic cell which will replace cells worn away or lost in secretion. 2. Cell type two functions mainly in the synthesis and secretion of proteins and also plays a role in lipid absorption. 3. Cell type three appears to function mainly in lipid absorption. 4. Cell type four also functions in lipid absorption but this cell is only found in the anterior midgut and the type of vesicles found in this cell suggest a different type of absorption is occurring than in cell type three. From the abundance of each cell type, the length of the microvilli, the development of the basal lamina and luminal projections, the following conclusions were made: 1. The midgut caecum functions mainly for absorption of digested nutrients. 2. The anterior midgut also functions for nutrient absorption but plays a more important role in merocrine and exocrine secretion. The presence of concretions in cell types two and three of the anterior midgut suggest a role in excretion, water or ion regulation. 3. The posterior midgut functions mainly in absorption, though some holocrine secretion is evident.

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