LIMPET'S TONGUE. 139 



passage into the stomach; and in others the tongue is 

 armed with spinous processes, obviously intended to 

 assist in the preparation of the food. The tongue of 

 the Common Limpet, shown in the preceding figure, is 

 a curious piece of mechanism. It is from two to three 

 inches long, and half a line in diameter, flat, between 

 horny and membranous, with a spoonlike extremity, 

 and when at rest, retracted into the stomach. Its 

 whole extent is armed with transverse rows of sharp, 

 hooked teeth, four in each row; and between the 

 rows are placed two trifid, rather obliquely-set teeth, 

 one at each side of the strap. Our figure shows the 

 general form of the whole tongue, and a small portion 

 magnified. 



So large a class as the Gasteropoda necessarily in- 

 cludes animals of very different aspect and variously- 

 modified structure, which it becomes necessary to clas- 

 sify on some principle derived from their organization. 

 The classification usually adopted is founded on differ- 

 ences in the shape and position of the gills, or breath- 

 ing apparatus. The Common Land Snails, as well as 

 the Fresh-water Snails, breathe air, which is received 

 into a cavity lined with a delicate network, analo- 

 gous to the lungs of air-breathing animals; and the 

 fresh-water kinds are obliged to rise to the surface every 

 time they require to take in fresh air. These constitute 

 the first group, or Pulmoni branchiata. Such a mode 

 of aerating the blood would obviously be unsuited to 

 marine Gasteropoda; consequently, all the remaining 

 orders are furnished with gills, variously placed. There 

 are eight of these orders; and I shall mention five, as 



