134 SINGULAR STRUCTURE OF THE MOUTH. 



or floor of the oral cavity, is provided with a small 

 tongue, of a cartilaginous texture, and having its sur- 

 face marked by transverse ridges and depressed lines, 

 so disposed as to render it effective in assisting to propel 

 the food into the gullet. In some species the tongue is 

 covered by minute horny recurved hooks, which are, 

 doubtless, of use. 



In other groups the mouth is much more simple, 

 and is merely a muscular tube, capable of being pro- 

 truded or contracted, and destitute of teeth; the lips, 

 or moveable margins of the tube are, however, equal 

 to the seizing of the food on which the animals live. 



A more singular structure of the mouth is that which 

 is found in the tritonia, one of the shell-less gastero- 

 podes, (order, Nudibranchia, Cuv.) The mouth, of an 

 oval figure, is furnished with large fleshy lips, and a 

 tongue covered with spines ; within the lips are two 

 lateral horny jaws, resembling two sharp-edged blades, 

 those, for example, of a pair of shears, and working 

 upon an elastic hinge; the edge of one blade opposes 

 and glides over the edge of the opposite, (as in shears 

 or scissors,) and both are acted upon by powerful 

 muscles, so that they are capable of biting very hard 

 substances with great facility. The spines, or papillae 

 of the tongue, are all directed backward, and serve, by a 



