CLASS OF CRUSTACEA. 479 



565. Most Crustacea live on animal substances ; but 

 they offer great differences in their regime ; some live only 

 on liquid matters, others on solid food, and differences are 

 observable in the construction of the mouth, corresponding 

 to their varied circumstances. In the masticating Crustacea 

 there is before the mouth a short transverse lip, followed by 

 a pair of mandibles, a lower lip, one or two pairs of jaws pro- 

 perly so called, and in general one or three pairs of auxiliaries 

 or lirnb-jaws, serving chiefly for the prehension of the food 

 (Fig. 144). In the Crustacea which live by suction, we find, 

 on the contrary, the mouth prolonged into a kind of beak or 

 proboscis, resembling those insects whose habits are analogous. 

 In the interior of this tube are slender pointed appendages, 

 performing the functions of small lancets, and on either side 

 we find generally organs analogous to the auxiliary jaws of 

 the grinding Crustacea, but which are formed to enable the 

 animal to fix on its prey. 



Fig. 511.* 



566. The digestive canal extends from the head to the 

 posterior extremity of the abdomen, and is composed of a very 

 short gullet, a large stomach (e, Fig. 513), armed in general 

 internally with powerful teeth, a slender intestine, and a 

 rectum. In some Crustacea, the bile is secreted by biliary 

 vessels sufficiently resembling those of insects ; but in general 

 there exists a very voluminous liver (/o), divided into several 

 lobes, and composed of a number of small tubes, terminating 

 in a cul de sac, and grouped around a ramified excretory 



