A study of the digestive tract of the copepods Tigriopus californicus and Calanus plumchrus was carried out using techniques of light and electron microscopy. In Tigriopus californicus, the foregut contains a curved cuticle lined esophagus which extends from the ventral mouth to the junction of the anterior midgut and midgut caecum. The noncuticulized portion of the digestive tract consists of: 1. A single spherical midgut caecum located anteriorly, 2. An anterior midgut, 3. A posterior midgut. There are cuticulized anterior and posterior hindgut regions ending in a dorsal anus. In Calanus plumchrus, the foregut consists of a cuticle lined esophagus extending from the ventral mouth to the junction of the midgut and the midgut diverticulum. The noncuticulized portion of the digestive tract consists of: 1. A single midgut diverticulum, 2. A midgut that is divisible on the basis of epithelial cell type and function. There is a long abruptly narrowing cuticle lined hindgut ending in an anus. In Tigriopus californicus, four cell types could be distinguished and from such ultrastructural characteristics as the position in the digestive tract, abundance, position, and type of organelles, lipid content, presence and type of vesiculation, and electron density, functions for the cells were determined. Cell type '1' is an embryonic 'stem' cell. It functions as a replacement cell and differentiates when cells are worn away or lost in secretion. Cell type '2' is mainly a secretory cell and functions in the synthesis of proteins. It also plays a role in lipid absorption. Cell type '3' is absorptive, mainly for lipids. Cell type M1, found only in the anterior midgut is also an absorptive cell. The presence of electron dense vesicles suggests that lipid absorption is not its major function. From the abundance of cell type and from examination of the ultrastructure in the various regions of the digestive tract, the following conclusions were made: 1. The midgut caecum functions in the absorption of digested nutrients. 2. The anterior midgut plays a role in nutrient absorption but is important in secretion. 3. The posterior midgut cells are mainly absorptive. In Calanus plumchrus, five cell types could be distinguished. Cell type 'E' is an undifferentiated 'stem1 cell. Cell type 'R' found in the midgut diverticulum and posterior midgut regions, is absorptive. Its developed basal surfaces suggest a transport function between the cell and the haemo-coel . Cell type 'D' is found in the glandular region of the midgut and is absorptive. It has an ultrastructure similar to that observed for cell type 'R'. Cell type 'B' is a large vacuolated absorptive cell found in the glandular region of the midgut. Cell type 'F' functions in the synthesis and secretion of digestive enzymes. In Calanus plumchrus, the midgut diverticulum is specialized for the absorption of digested nutrients and transport of metabolites to the haemocoel . The anterior midgut regions are mainly absorptive. It includes a vacuolated glandular region specialized for pinocytotic absorption. In the middle section of the midgut, adjacent and posterior to the glandular region, is an area of epithelial cells specialized for secretion. The posterior midgut regions are mainly absorptive. In Tigriopus californicus biological markers can be used to determine cell type function in correlation with the observed ultrastructure. / Science, Faculty of / Zoology, Department of / Graduate