ORGANS OP SENSE OF CRUSTACEA. 237 



organs are situated on moveable footstalks (Fig. 462) ; an 



FlG. 462. PODOPHTHALMA. 



arrangement which we do 



not see in any other division of 

 Articulated animals. In many 

 Crustacea, there is an apparatus 

 for hearing, situated at the base 

 of the external antennas, and 

 composed of a small membrane, 

 under which we find a sort of 

 vestibule filled with liquid, and 

 inclosing the termination of a 



F,GM63.-Anterior part of the inferior par ti cu l a r nerve. This COUSti- 



surface of the body of a Crab (Maia) : r 



ai, internal antennae; a, external tuteS the simplest form of an 



antennae; y eyes; o, organ of hear- A ,., , . 



ing; m, feet-jaws; b, mouth ; p, base Auditory apparatus. (ANIM. 



of the anterior limbs; r entrance to P HY SIOL. 512). We know 



the respiratory cavity ; s, sternum. % , 



nothing positive concerning the 

 senses of Smell and Taste in these animals. 



775. Most Crustacea subsist on animal substances ; but they 

 present great differences in their regimen, some being only nour- 

 ished upon' liquid matters, whilst others feed upon solid aliment ; 

 and we observe corresponding differences in the formation of 

 their mouths. In masticating Crustacea there is in front of 

 this opening a short transverse lip, followed by a pair of man- 

 dibles, by an inferior lip, by one or two pairs of jaws properly so 

 called, and generally by one or three pairs of auxiliary jaws, or 

 feet-jaws, which serve principally for the prehension of food 



