142 



IXVEETEBRATE ANIMALS. 



The first pair of legs are developed into pretty powerful nipping- 

 claws or chelee, and one of them is always mnch larger than the 

 other, and acts as a kind of plug, blocking up the entrance of the 

 shell when the animal is retracted within it. 



C. Brachyura. — The Decapod Crustaceans included in this tribe 

 are familiarly known as Crabs, and they derive their name of Brachy- 

 ura (Gr. brachux, short ; and oura, tail) from the rudimentary con- 

 dition of the abdomen. The abdomen, in fact, is not only extremely 

 short, but it is always tucked up beneath the greatly developed ceph- 

 alothorax, so that it is not visible at all, except when the animal is 

 looked at from below (fig. 89). The Crabs have very various habits, 

 but they are mostly denizens of the shore, hiding beneath stones or 

 sea-weed, in cracks of rock, or in pools near the line of low-water. 

 Some of them, however, can swim with tolerable activity, and some 



of them (the Land-crabs) even live 

 habitually inland. One group, that 

 of the " Pea-crabs," is distinguished 

 by the singular habit of living semi- 

 ■ parasitically within the shells of 

 bivalve Molluscs, such as the great 

 horse-mussel or the oyster. 



The young or larval Crab is ex- 

 ceedingly unlike the adult, and has 

 a long and well-developed abdomen, 

 thus approximating to the type of 

 structure which is permanently re- 

 tained in the Macrura. 



Order Stomapoda. 



The Stomapod Crustaceans are 

 nearly allied to the Decapods, but 

 they have six or eight pairs of legs, 

 and the gills are not placed in cham- 

 bers on the under surface of the 

 thorax, but are usually suspended 

 beneath the abdomen. They are 

 almost all marine, and the Locust- 

 Shrimp or Squilla (fig. 90) may be 

 taken as the type of the order. In 

 this Crustacean the abdomen is well developed, and its terminal 

 appendages form a broad swimming-tail. The front pairs of legs 

 are hooked, and the gills are attached to the first five pairs of ab- 

 dominal feet. 



Fig. 90. — Squilla mantis, the Lucust- 

 Shrimp. 



