ANNULOSA : CRUSTACEA. 249 



the carapace. They consist each of a central stem supporting 

 numerous laminae, and they are richly supplied with blood, 

 but are not ciliated. The water which occupies the gill-cham- 

 bers is renovated partly by the movements of the legs, and 

 partly by the expanded epipodite of the second pair of 

 maxillae, which constantly spoons out the water from the front 

 of the branchial chamber, and thus causes an entry of fresh 

 water by the posterior aperture of the cavity. 



The nervous system is of the normal " homogangliate " type, 

 consisting of a longitudinal series of ganglia of different sizes, 

 united by commissural cords, and placed along the ventral 

 surface of the body. The organs of sense consist of the two 

 compound eyes, the two pairs of antennae, and two auditory 

 sacs. 



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 Macrura, nor so rudi- 

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

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

 development in the Anomura appears invariably to take place 

 through Zoea-forms. 



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 abdominal appendages, however, are mostly 

 unsymmetrical. The carapace is not strong, but the claws 

 are well developed, one being always larger than the 

 other. Other forms of the Anomura are the Sponge-crabs 

 (Dromia\ the Crab-lobsters (Porcellance), and the Tree-crabs 

 (Birgus). 



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



