Anatomy of finfish and shellfish

Unit 12 - Arthropoda

12.2.2. Digestive System
In both true lobsters and spiny lobsters, the gastric mill is more highly developed than in the white shrimp. The gastric mill of the lobster is largely restricted to the region of the foregut in which the large, thin-walled anterior chamber gives way to-.the much smaller, thick-walled posterior chamber. At the constriction between the two chambers, three movable teeth, one median and two lateral, are attached to small, hard skeletal plates known as ossicles. These teeth chew the food, which arrives in the anterior chamber as long, stretched, but unchewed pieces. A well-developed gastric mill is a useful device enabling a decapod crustacean, when safely hidden from its enemies, to chew s food at leisure, after having swallowed it in large pieces. The gastric mill is least developed in such decapod crustaceans as the shrimps, in which the mouthparts chew the food quite thoroughly before the food enters the esophagus.
In the walls of the anterior chamber, a lobster has many ossicles in addition to those of the gastric mill. These addi¬tional ossicles serve as a place of attachment for muscles that move the foregut and thereby enable the ossicles of the gastric mill to grind the food. Once the food has been ground thoroughly, it passes through a setose filter that prevents all but the finest particles from entering the mid gut glands through ducts that open into the unlined caudad portion of the posterior chamber.
The midgut of the American lobster is long, extending back to the last abdominal segment, where it connects with the chum-lined hindgut, which has become modified as an en¬larged rectum. A posterior midgut diverticulum, or cecum, arises just in front of the junction of midgut and rectum. Undigested wastes are egested from the rectum through the anus. In spiny lobsters, the midgut is very short, while the hindgut is long and contains many longitudinal folds. No enlarged rectum is present, the terminal portion of the hind- gut being narrow and very muscular. By their contraction the muscles of the hindgut force undigested (fecal) material out through the anus.

In the foregut, midgut, and midgut glands of the American lobster, digestion of food takes place through the action of digestive enzymes that are secreted by the midgut glands. These glands are also the principal site for absorption of digested food and for storage of reserve food materials. Chefs call the midgut glands of the lobster the tomally; accumulated food reserves make the tomally rich and flavorful when cooked. The tomally can easily be recognized, for it is soft, large, and many-lobed, and, in color, i-1 green, bright yellow, yellow-green, or yellow-brown.