WOLLUSCA. 423 



probably assists in the contraction or enlargement of the 

 aperture. 



The animals of the cephalopodous class, besides contain- 

 ing complicated eyes, are likewise furnished with ears. 

 These are situate in the annular cartilage which supports 

 the arms. In this cartilage, there are two cavities, in each 

 of which there is a bag filled with a gelatinous, transparent 

 fluid, and containing a calcareous substance, differing in its 

 consistence according to the species, from the brittleness 

 of starch to the hardness of bone. The auditory nerve pe- 

 netrates the walls of this labyrinth, and ramifies on the 

 membranous bag which it contains. There is no external 

 opening, nor any. apparent alteration in the thickness of the 

 investing integuments. 



The digestive system of the cephalopoda exhibits several 

 appearances by which it may be distinguished. The arms 

 which surround the mouth, seize the animals which are to 

 serve as food, and bring them to the mouth. The mouth 

 is situated in the centre of the disc, round which the ten- 

 tacula are arranged. It is surrounded with a slight fold 

 of the skin, which may be compared to lips, and which is 

 rough on the central aspect. Within these are the two 

 mandibles, of a deep brown colour, hard, horny consistence, 

 and in form resembling the beaks of a parrot. Where 

 free, they are conico-tubular, but where covered, they are 

 open at the central side. The under beak, unlike the same 

 organ in birds, is the largest, the most crooked, and em- 

 braces the upper, or the one on the dorsal margin of the 

 mouth. These jaws are merely able to open and shut, as 

 .they possess no lateral motion. They are supported by 

 the muscular bed of the mouth, which serves as a mould 

 to fill the cavity towards the point. The tongue is situate 

 between the beaks, and is armed with reflected teeth. 

 These teeth, in consequence of the undulatory motion of 



