Anatomy of finfish and shellfish

Unit 12 - Arthropoda

12.1.1.Shrimps - External Anatomy
External Anatomy
As a decapods crustacean, the white shrimp, Penaeus (Litopenacus) sctiferus, is rather primitive. It has been selected for use here because it illustrates not only the structure of a shrimp, but also the generalized body plan of a decapod crustacean.

A large portion of the white shrimp, as of any other shrimp, consists of muscle and shell, or exoskeleton. In fact, the largest of the three natural divisions of the body; namely, the abdo¬men or "tail," consists of little other than mus¬cle and shell. Two main masses of muscle are the "meat" of the shrimp's tail: (1) the rela¬tively small dorsal abdominal muscles, which lie above the intestinal tract, or gut, and above the dorsal abdominal artery, both of which are removed in preparation for eating, and (2) the large ventral abdominal muscles, which extend from either side of the intestinal tract and dorsal abdominal artery ventrally to both sides of the ventral abdominal nerve cord.

For swimming quietly, the white shrimp uses its five pairs of abdominal appendages, known as pleopods. But when the shrimp moves rapidly, it does so by contracting its ventral abdominal muscles and curving forward its tail fan, which is composed of a centrally situated telson and a pair of lateral appendages known as uropods. The powerful thrust exerted by tail and tail fan upon the water propels the shrimp back¬ward with extraordinary speed. The tail of the shrimp returns to its normal, more or less elongated, position by the contrac¬tion of the dorsal abdominal muscles, which act as extensors. The tail's flexibility results from deep folds of thin, soft chitin that link the six segments of the tail to one another.
Within the cephalothorax of the white shrimp are large portions of the digestive, circulatory, nervous, and reproduc¬tive systems. The long digestive tract, or gut, has three main subdivisions known, respectively, as foregut, midgut and hindgut. Food particles picked up by the mouth parts are ground by the mandibles and swallowed, whereupon they enter the narrow, tubular, muscular esophagus, which is the initial portion of the foregut. Lined with chitin, the esophagus nonetheless can accommodate large amounts of food since it has one anterior and two lateral folds loosely filled with connective tissue. When these folds become unfolded, the esophagus can distend greatly.