Fig. 15.12. Representative decapod crustaceans. A, the spiny lobster, Pamilirus argus, in 

 its natural tropical marine habitat; in this lars;e decapod the first walking legs are not modified 

 into powerful chelae like those of the American lobster, but the antennae are tremendously en- 

 larged and elongated. B, the mantis shrimp, Squilla empusa: note the short carapace and the 

 peculiar modification of the first walking legs for grasping prey. C, spider crabs, Libinia 

 emaroinala: in these non-swimming crabs the last segment of the fifth walking leg is a simple, 

 pointed spine, not a broad paddle as in the swimming crabs (see Fig. 15.13). (.-1, underwater 

 photograph bv John F. Storr; 5 and C, photographs by Eugene S. Clark, Jr.) 



ment of appendages and has some specializations for a drifting planktonic 

 life, resembles in its general features a small crayfish or lobster. The 

 megalops larva, however, begins to show a slightly crab-like form, with a 

 somewhat broadened cephalothorax and a reduced abdomen. In the final 

 transformation from larva to adult, the definitive form is assumed, with a 

 relatively small abdomen folded forward under the greatly broadened and 

 flattened cephalothorax. The blue crab is adapted for swimming by modifi- 

 cation of the fifth pereiopods into paddle-like appendages;' in many other 

 crabs which do not swim, such as the spider crab, Libima (Fig. 15.12), these 

 appendages are simply pointed, as in the crayfish and lobster. 



THE CLASS INSECTA 



In numbers of known species, as well as in numbers of individuals, the 

 Insecta are the dominant form of animal life upon the land. They are 

 primarily and predominantly terrestrial animals; very few pass any part of 

 their lives in the sea, and those that live in fresh water as immature forms 

 or as adults are clearly typical insects especially modified for aquatic life. 

 The most easily recognizable external features of insect structure are the 

 division of the body into head, thorax, and abdomen; the two pairs of wings 



441 



