ANNULOSA: CRUSTACEA. 217 



The sexes are invariably distinct, and the generative pro- 

 ducts are conveyed to the exterior by efferent ducts, which 

 open at the base of one of the pairs of thoracic legs. The 

 ovum is " meroblastic," a portion only of the vitellus under- 

 going segmentation. The neural side of the body, that is to 

 say, the ventral surface, appears on the surface of the ovum, 

 so that the embryo is built up from below, and the umbilicus 

 is situated posteriorly. 



TRIBE B. ANOMURA. The Decapods which belong to this 

 tribe are distinguished by the condition of the abdomen, which 

 is neither so well developed as in the Macntra, nor so rudi- 

 mentary as in Crabs. Further, the abdomen does not termi- 

 nate posteriorly in a caudal fin, as in the Lobster. 



The most familiar of the Anomura are the Hermit-crabs 

 (Pagurida). In the common Hermit-crab (Pagurus Bernhar- 

 dus) the abdomen is quite soft, and is merely enclosed in a 

 membrane, so that the animal is compelled to protect itself by 

 adopting the empty shell of some Mollusc, such as the common 

 Whelk, which it changes at will, when too small. The Hermit 

 is provided with a terminal caudal sucker, and with two or 

 three pairs of rudimentary feet developed upon the abdomen, 

 by means of which he retains his position within his borrowed 

 dwelling. The carapace is not strong, but the claws are well 

 developed, one being always larger than the other. 



TRIBE C. BRACHYURA. The " short-tailed " Decapods, or 

 Crabs, are distinguished from the two preceding tribes by the 

 rudimentary condition of the abdomen, which is very short, 

 and is tucked up beneath the cephalothorax, the latter being 

 disproportionately large. The extremity of the abdomen is 

 not provided with any appendage, and it is merely employed 

 by the female to carry the ova. The Crabs (fig. 72) are mostly 

 furnished with ambulatory limbs, and are not formed for 

 swimming, most of them being littoral in their habits, and 

 some few even living inland. 



In all the essential points of their anatomy the Crabs do not 

 differ from the Lobster and the other Macrura j but they are 

 decidedly higher in their organisation. This is especially seen 

 in the disposition of the nervous system, the ventral ganglia 

 in the Crab being concentrated into a single large ganglion, 

 from which nervous filaments are sent to all parts of the body. 



Reproduction in the Crabs is the same as in the Macrura^ 

 but the larva is exceedingly unlike the adult, and approximates 

 closely to the type of the Macrura, another proof that the 

 Brachyura stand higher in the Crustacean scale. The larval 

 Crab was originally described as a distinct animal, under the 



