352 



MOLLUSGA. 



down on all sides or, like a cowl, covers either the anterior or 

 posterior parts of the body. The mantle is of importance in two 

 ways: its outer surface is covered with epithelium which, like that 

 of the adjacent surface, has the power of secreting shell, a thick 

 cuticular layer of organic matter (conchiolin) largely impregnated 

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



Fig. 340.— Dia(2:rains of three moUuscan classes. ^. a ceplialopod (5ppia> ; i?. a gas- 

 teropod (Helix); C, an acephal iAnodonta). a, anus; c, cerebral ganglion: fu, foot; 

 ))(, mantle chamber; .sc7t, shell; ^, siphon; v, visceral ganglion. Visceral sac 

 dotted; mantle lined, shell black. 



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

 which, from its most important function, is also called the liran- 

 rhial chaml/er. Since most molluscs are aquatic, special vascular 

 processes of the body, the gills or branchiae, lie in this space; in 

 the terrestrial forms its walls serve as lungs and thus are respiratory. 

 From the foregoing it will be seen that the character of the 

 mantle must exert an influence on the shape of the shell and on 

 the respiratory organs. Paired mantle folds necessitate two valves, 

 right and loft, to the shell; a right and left branchial chamber, 

 an<I right and left gills. AVith an unpaired mantle the shell is 



