In the mud-dwelling amphipod,
Corophium volutator
the foregut is lined with cuticle and consists of an oesophagus and a stomach, with the latter divided into cardiac, pyloric and funnel regions. The midgut comprises an intestine that is enlarged considerably by three pairs of diverticula: the small anterior dorsal and posterior caeca and the massive ventral caeca. Anteriorly, the intestine encompasses the funnel region and the ventral caeca open into the floor of the stomach at the posterior end of the pyloric region. The hindgut is essentially a simple tube connnecting the intestine with the anus. Particles of food pass along the oesophagus and enter the stomach through a valve. Rows of setae, or folds of cuticle, divide the stomach longitudinally into food, circulation and filtration channels. Ingested particles with a diameter greater than 2 pm are confined to the food channel and supplied with fluids and enzymes from the circulation channels. The digestive enzymes are produced primarily by the ventral caeca and are supplied to the circulation channels through a valve at the entrance of each ventral caecum. Any fine particles and soluble materials extracted from the food channel in the cardiac region are transported into the filtration channels through the first filter of a two part system. Digestible material continues to be extracted in the pyloric region where the volume of the lumen of the food channel is reduced by the intrusion of the vertex of the ventral pyloric ridge. The basis of this ridge supports the second filter which produces a filtrate with particles less than 0.06 pm in diameter. Material retained on the filter membrane is returned to the food channel by brush-like setae facing the membrane. The final filtrate is transported to the ventral caeca. A valve at the entrance to each ventral caecum prevents contamination of the filtrate by material in the food channel. All indigestible food is passed sequentially along the funnel, intestine and, finally, the hindgut from which it is voided as a faecal pellet. Most digestion and absorption occur in the ventral caeca where the epithelium is differentiated into the R /F and B cells. The R /F cells have a much thicker and denser microvillous border than the B cells. Each R /F cell also has numerous mitochondria located mainly ventral to the nucleus in the mid-region. Rough and smooth endoplasmic reticula are sited primarily in the apical and basal regions of the cell, respectively. Furthermore, most of the rough endoplasmic reticulum is confined to cells in the distal region of the caecum which probably forms the main site for the production of digestive enzymes. The proximal region of the caecum contains numerous lipid droplets and is probably involved in the absorption, transport and storage of the products of digestion. Each B cell has a single large, fluid-filled vacuole, distal to which are mitochondria and numerous smaller vacuoles of varying size forming an ‘apical complex’. The nucleus is located proximal to the vacuole together with free ribosomes and rough endoplasmic reticulum. Material from the lumen of the caecum is taken by pinocy tosis into the ‘apical complex’. The large vacuole develops at the expense of the ‘apical complex’ and the microvillous border. The vacuole is eventually liberated into the lumen of the caecum and the cell disintegrates. These discharges may supply enzymes to other regions of the gut, or they could be waste products derived from intracellular digestion. The anterior dorsal caeca and most of the intestine contain cells with a normal complement of organelles. These cells probably make a minor contribution to the processes of digestion and absorption. However, the cells of the posterior caeca and those at the posterior end of the intestine have an extensive development of smooth endoplasmic reticulum. In some cells the mitochondria have a dense matrix and there are only a few free ribosomes and cisternae of rough endoplasmic reticulum. The fine structure of the epithelium in the posterior caeca is typical of tissue that transports fluids and ions. The hindgut has a microvillous border which abuts its cuticular lining. In addition, some cells have numerous mitochondria which are often associated with infolds of the basal cell membrane. The fine structure of this tissue is similar to the ‘ion pumps’ described in the gut of insects which serve to maintain the normal ionic concentration of the blood. The posterior region of the hindgut has no structural specializations.
Ultrastructure of rhesus monkey renomedullary interstitial cells