578 EVOLUTION OF ANIMALS Part V 



General External Structure. The exoskeleton of crustaceans is a secretion 

 of the epidermis in which lime is gradually deposited (Fig. 29.9). Exoskeleton 

 not only covers the outside of the body but lines the digestive tube except for 

 the midgut. It will not stretch except while it is soft: neither is it enlarged by 

 additions like the shells of clams. Periodically the crayfish sheds the old skeleton 

 for a new one and this introduces a crisis in its life such as all arthropods share. 



Body Regions. Arthropods have fewer and far less regular segments than 

 their annelid relatives, the clamworm and earthworm. The differences be- 

 tween them are most striking at the front end, far more exciting in a crayfish 

 than a worm. The body of the crayfish is divided into a fused head and thorax, 

 the cephalothorax, and a jointed abdomen. Each part is composed of seg- 

 ments. In the cephalothorax, covered by the jacketlike carapace, the seg- 

 ments are indicated by appendages, the mouth parts and legs; in the abdomen 

 by the obvious segmentation of the body and the swimmerets. 



Appendages. Crayfishes and lobsters can use their variously specialized 

 appendages in numerous ways: as feelers for exploring; as jaws for tearing 

 and grinding; as food handlers; as bailers for dipping water; as pincer claws 

 for seizing prey; as paddles for swimming; in the male for transferring sperm 

 cells; and in the female for carrying eggs and young ones (Table 29.1). The 



Fig. 29.7. Stages in the development of the shrimp, Penaeus. Like other animals, 

 the higher crustaceans go through developmental stages that are in some respects 

 similar to the adults of simpler ones. A, nauplius has three pairs of two-branched 

 appendages as in certain simpler crustaceans; B, protozoea stage with six pairs of 

 appendages; C, the zoea stage, with a distinct head and abdomen; D, mysis stage 

 with more appendages on the cephalothorax; E, adult shrimp, (Courtesy, Pauli: 

 The World of Life. Boston, Houghton Mifflin Co., 1949.) 



