MOLLUSCA 



311 



into the/oo/, a muscular mass, usually used for locomotion. The mantle 

 or pallium, a dermal fold, extends downward from the body and encloses 

 a part of the body. In the Acephala (C) it has two halves, but in the 

 snails {B) and cephalopods (.4) it is unpaired, and either extends down 

 on all sides {Chiton, Patella), or, like a cowl, covers the anterior side 

 (most gasteropods), or envelops the posterior part of the body (pteropods, 

 cephalopods). The mantle is important in two ways: its outer surface 



Fig. 311. — Diagrams of three molluscan classes. A, a cephalopod (Sepia); B, a 

 gasteropod [Helix); C, an acephal (Anodonta). a, anus; c, cerebral ganglion; /w, foot; 

 m, mantle chamber; sell, shell; t, siphon; v, visceral ganglion. Visceral sac dotted; 

 mantle Uned, shell black. 



is covered with epithelium which, like that of the adjacent surface, may 

 secrete shell, a thick layer of organic matter {conchiolin) largely impregnated 

 with calcic carbonate. The inner surface of the mantle, together with 

 the outer surface of the body, bounds a space, the mantle cavity, which, 

 from its most important function, is also called the respiratory chamber. 

 Since most molluscs are aquatic, special vascular processes of the body, 

 the gills or branchiae, lie in this space; in the terrestrial forms it contains 

 air and with a richly vascular dorsal wall, serves as a lung. 



