SECT. xi. TONGUES OF MOLLUSC A. 239 



shell, which is sunk and very strongly glued into it, 

 yet the Limpet dissolves the glue with a liquid secretion, 

 roams in quest of food, and returns again to its home : 

 both fluids are secreted by a multitude of glands in the 

 foot, which is the instrument of adhesion. 



The tongue of the carnivorous Gastropods is a very 

 formidable weapon, used for boring holes in the hardest 

 shells. The round holes in dead shells frequently met 

 with on our coasts show that their inhabitants had 

 fallen a prey to some of these zoophagous Mollusks. The 

 tongue of these predatory Gastropods is a narrow me- 

 chanical file, sometimes twice or even three times the 

 length of the whole animal, and when not in use it is 

 curled up near the foot. It is spined in various micro- 

 scopic patterns according to the genus, and is supported 

 by two firm parts from whence the muscles spring that 

 work the rasp. 



The Periwinkles have a ribbon- shaped tongue, rough 

 with hooked teeth ; the Scalariae have also predatory 

 tongues, but of all the Gastropod mollusks, the Whelk 

 and its numerous allies are the most predacious. The 

 Purpura or Dog Whelk especially is the most ravenous 

 of mollusks. Its long tongue is armed with hooked and 

 spined teeth, placed three in a row ; with this weapon 

 and a proboscis capable of boring, they have been 

 known to exterminate a whole bank of Mussels. 



The Common Whelk is represented in fig. 1 74. When 

 in the act of crawling, its head, with two tentacles, is at 

 one extremity, its foot at the other, sometimes used as 

 an organ of prehension ; and it has a siphon for carrying 

 water to the gills at the end of the shell. 



All the families of the naked mollusks or Sea Slugs, 

 furnish beautiful objects for the microscope. The two 

 sexes are united in the same individual, and in their 

 embryonic state they have a shell, which is cast off long 

 before they come to maturity. The gills placed on the 



