412 INVERTEBRATE MORPHOLOGY. 



oped, the anterior ones becoming chelate. In the Crayfish, 

 Cambariis, and the Lobster, Homarus, the first pereiopod is an 

 exceedingly strong chela, and the same arrangement is found 

 in Alpheus, while in the Shrimp, Pcdcemonetes, the second pe- 

 reiopod is somewhat longer than the first. 



The branchiae are usually numerous and are for the most 

 part bunches of cylindrical processes, but in Pcdcemonetes and 

 the shrimps and prawns in general, which form the family Ca- 

 rididse, and in the Hermit-crabs they are lamellate. In Lucifer 

 branchiae are entirely wanting. The'Macrura are essentially 

 marine, a few forms, such as Cambarus and some species of 

 Pcdcemon, occurring in fresh water. The genus Birgus, one of 

 the Hermit-crabs, commonly known as the robber-crab, is 

 almost entirely terrestrial, living in holes in the ground and 

 climbing cocoa-nut palms for the sake of the nuts, on which 

 it lives. In harmony with its terrestrial life the inner surface 

 of the branchial chamber is thrown into folds richly supplied 

 with blood-lacunae, a luuglike structure, recalling the lungs of 

 the Pulmonate Gasteropods, being thus developed. 



2. Suborder Bracliyura. 



In the Brachyura the body is exceedingly compact, the 

 abdomen being very much reduced in size and usually desti- 

 tute of a tail-fin, and in addition 

 is bent up so as to lie in a groove 

 upon the ventral surface of the 

 cephalothorax. In some cases 

 the cephalothorax is almost glo- 

 bular, though prolonged anterior- 

 ly into a strong rostral spine, as 

 in Libinia, the spider-crab ; while 

 in other cases it is more flattened 

 and triangular in shape and lacks 



a distinct rostrum, as in the 

 FIG. 188. PanopcKus depressus -,., -, , /-, 77 - 



(after EMKBTON from VKBRA^ edlble crab > CaUmectes, the lady- 



crab, Platyonychus, and the com- 

 mon crab, Cancer, and in others again is more or less 

 quadrangular and thicker, as in Pinnotheres, the oj^ster-crab, 

 Ocypoda, the sand-crab, and Gelasimus, the tiddler-crab. The 



