Anatomy of finfish and shellfish

Unit 12 - Arthropoda

12.1.2. Digestive system
From the esophagus, food particles enter the anterior chamber, the second portion of the chitin-lined foregut. Many authors have called this the cardiac stomach. The an¬terior chamber has lateral longitudinal folds that permit it to expand when filling with food. The anterior chamber also has ventrally situated longitudinal ridges that lead back to the openings of the midgut glands in the caudad part of the posterior chamber, frequently called the pyloric stomach.
Walls of the anterior chamber contain a triangular structure consisting of a median tooth and a row of tooth-like denticles along each side. When food enters the anterior chamber, the muscles that insert on the chamber alternately contract and relax, thereby causing the median tooth to move against the denticles and lateral ridges. In so doing, this grinding apparatus, termed the gastric mill, breaks down the food into very fine particles.
While food is within the anterior chamber, it is mixed with digestive juices that flow forward ventrally from the posterior chamber. The juices enter the caudad part of the posterior chamber viaducts that originate in the lumen, or cavity, of many-branched tubules constituting the paired midgut glands. Thus, the lumen of the tubules is continuous with the lumen of the gut.
Digestion of food takes place partly in the anterior chamber, partly in the posterior chamber, and partly in the tubules of the midgut glands. In the posterior chamber, there is a filter formed by two lateral ridges and one ventral median ridge densely covered with hair-like setae. Owing to this filter, only fluid and minutely divided food particles can pass from the posterior chamber into ducts leading to the midgut glands and thence into its branching tubules for further digestion. From the midgut glands, end products of digestion are readily absorbed into the hemolymph.
Fine indigestible material within the midgut glands is forced back into the posterior chamber and then into the straight, unlined, tubular portion of the midgut. Here end- products of digestion enter the hemolymph via the many small blood vessels connecting the tubular portion of the midgut with the dorsal abdominal artery just above. Here also the fine indigestible material is mixed with larger indigestible particles that had been filtered away from the openings of the midgut glands and had passed directly from the posterior chamber into the tubular part of the midgut.
Within the midgut, indigestible material is packaged into long fecal pellets and enclosed within a membrane, the pen¬trophic membrane (from the Greek, pen, around; troplio, feed), which is secreted by epithelial cells of the midgut and is mucoid in nature. Strong peristaltic contractions of the mid- gut push-the fecal pellets along to the chitin-lined hindgut, which is enlarged as a rectum. A series of rapid contractions by the rectum then forces the fecal pellets out of the body by way of the anus.
At the junction of midgut and hindgut in the sixth abdomi¬nal segment, the midgut gives rise to a diverticulum, called the posterior midgut cecum by some authors and the hindgut or rectal gland by others. The function of this organ is not known, but its cells appear to be secretory.
Presumably the midgut would be distinguishable from the foregut and hindgut by its lack of chitinous lining. Thus, the esophagus, anterior chamber, and cephalad portion of the posterior chamber are lined with chitin and are clearly foregut. The caudad portion of the posterior chamber also is lined, although incompletely, with chitin; yet this is midgut. Dorsally and laterally, the chitinous lining of the foregut in the white shrimp (and in many other decapod crustaceans) extends into the midgut well past the openings Of the midgut glands; ventrally a caudad extension of the chitinous lining separates the openings of the midgut glands and also covers the epithelium of more posterior portions of the midgut for some distance. These caudad extensions of the chitinous lin¬ing probably direct sand and other indigestible particles to the peritrophic membrane for packaging without damage, en route, to the delicate area around the openings of the midgut glands.