The digestive system of the cephalocarid Hutchinsoniella macracantha (Crustacea)

The digestive tract of the cephalocarid Hutchinsoniella macracantha begins with an atrium oris, posterior to the mouth. The esophagus loops up through the head and ends with a valve to the midgut. The epithelial cells and cuticle of both these structures are connected with long, winding, apically distended microvilli. The midgut is a straight tube with a pair of diverticula anteriorly. Midgut epithelial cells have microvilli, light vesicles, and a peculiar endoplasmic reticulum that is produced into a palisade of extensions toward the apical surface. Outside the basal lamina are muscle and peri-intestinal cells which send fingerlike projections into the basal portion of the midgut epithelial cells. The rectum joins the midgut near the end of the eighteenth segment and consists of unspecialized epithelial cells. In spite of the simple gross morphology the digestive tract has a complicated musculature. Circular muscles follow the whole tract including the diverticula. Radial muscles attach to the esophagus and rectum. Many longitudinal muscles are found inside the circular muscles in the anterior part of the midgut but only a few outside posteriorly. Gland cells occur in the labrum and diverticula. Absorption of nutrients seems to be limited to the midgut epithelial cells. Metabolites are transported via their endoplasmic reticulum to that of the peri-intestinal cells