10 MOLLUSCA. 



mechanical organ for the attrition of food ; gustatory villi 

 have, however, been detected on the tongue of the Cepha- 

 lopods. In the bivalves the mouth is supplied with two 

 soft membranous palps, and in the Brachiopods with two 

 ciliated arms. The mouth of the Cephalopods is armed 

 with horny jaws, which act vertically like the mandibles of 

 a bird. In the Helicida there is only an upper jaw, and 

 in the Limnc^idcE there are two accessory lateral jaws. 



The tongue is a membrane covered with hooks or prickles, 

 which are arranged differently in the different families. 

 It usually forms a triple band, of which the central part 

 is called the rachis and the lateral tracts pleuree. The 

 teeth on the middle part are termed central ; those on 

 the pleurse are named uncini or laterals ; it is sometimes 

 broader than long, as in Tritonia and Doris, or it is ellipti- 

 cal and spoon-shaped, as in the Helicidfe. In others it 

 is riband-shaped and much longer than the body, in which 

 case it is reversed along the gullet and coiled spirally in 

 the stomach. In the carnivorous families the tongue is 

 forked and fleshy, armed with sharp curved teeth and 

 placed at the end of the muscular proboscis, while in the 

 phytophagous tribes it is very long and spiny, for filing 

 their food to pieces. There is no tongue in the Tunicata, 

 while in the bivalves it appears in the form of a gastric 

 dart, a cartilaginous stiliform body enclosed in a sheath 

 with a tricuspid free extremity and attached to the sides 

 of the stomach. 



In the Doris and Aplysia there are no jaws, and the 

 cartilaginous surface of the tongue is covered with curved 

 spines. In the Patella the tongue is of great length, and 

 is covered with transverse rows of spiny teeth. In the 

 Cephalopods it is short and muscular, and covered with 

 rows of spines. M. Lov6n has demonstrated that im- 



