116 CLASS CRUSTACEA. 



hearing is seen, in tracing the animal kingdom from 

 the most perfect animals to those whose formation is 

 apparently less complex. 



The body of these creatures appears to be composed 

 of only two principal parts, the body and the tail ; for the 

 head is so intimately united and confounded with the 

 trunk, as to appear to he merely a portion of it. The two 

 eyes are fixed at the top of two moveable supports, and 

 are placed in a hollow prepared for their reception, on 

 each side of a projecting portion of the shell that covers 

 the head. The antenna?, which are usually four in 

 number, are placed about this spot ; they are inserted 

 beneath the stems that support the eyes. The two 

 outermost of these antennae are generally the longest. 



The branchia?, or organs by which they breathe, as- 

 sume a Torm somewhat pyramidical, arranged like a 

 series of leaves, or the web of a feather ; they are placed 

 in the interior of the shell along each side, and are so ar- 

 ranged as to adhere to the roots of the feet, so that each 

 of these feet has a hidden branchia attached to its base. 



The mouth is composed of a fleshy lip, projecting 

 between the mandibles ; of two hard triangular man- 

 dibles, more or less notched at their extremity, and each 

 having a kind of feeler inserted on the upper part; they 

 also possess a little tongue between these mandibles, at 

 the root of which is the opening to the stomach : they 

 have besides two pair of jaws, like leaves, the borders 

 of which are fringetl, and six other members, which 

 Lamarck calls foot-jaws, from tlieir bearing some re- 

 semblance to legs, or feet. From this it appears that the 

 parts of the mouth in the Crustacea form a complicated 

 apparatus, and accordingly we find the whole tribe 

 exceedingly voracious, the Crabs in particular, feeding 



