Anatomy of finfish and shellfish

Unit 12 - Arthropoda

12.2.1.External Anatomy
In general body plan a lobster does not differ greatly from a shrimp. A lobster has the same type of muscular abdomen, or tail, which undergoes sudden flexion by contraction of the large ventral abdominal muscles and more leisurely extension by contraction of the smaller dorsal abdominal muscles. As in shrimp, the tail of a lobster provides the animal with its surest means of escape- jetlike propulsion backward.

In lobster as in shrimp, a carapace covers the head and the thorax and, except for the presence of the cervical groove, obscures the boundary between these two regions. In lob¬sters, the cephalothorax is commonly called the "body," while in shrimp this same region is known as the "head."
Despite similarities in body plan, it is quite easy to distin¬guish a lobster from a shrimp. Even as an adult, a shrimp is relatively small and its shell is somewhat fragile. An adult lobster, on the other hand, may reach very large size and acquire an extremely hard shell. Furthermore, a lobster is compressed dorsoventrally (from top to bottom), not laterally (from side to side), as is a shrimp. True lobsters have yet another distinguishing characteristic: their first pair, of thoracic legs is modified as large claws, or chelipeds. In some species, such as the American lobster, Homarus americanus, one large claw, the crusher, is much heavier than the other claw, known as the pincer, or the biting, cutter, or ripper claw. The crusher of the American lobster occurs about as frequently on the right side of the body as on the left. These are large & claws are lacking in the spiny or rock, lobsters.

As in a shrimp, many vital organs of a lobster are situated under the carapace within the cephalothorax. Here are the chitin-lined foregut, at least a portion of the midgut, and the midgut glands. Here also lie the brain, heart, gills, excretory organs, and a large part of the male and female reproductive organs.