246 



ZOOLOGY. 



From a definite part of the body a larger 

 or smafler fold of skin (" mantle ") grows out, whicli 

 encloses a space termed the " mantle cavity," and also, 

 from its function, the " respiratory cavity." In most 

 molluscs this mantle secretes an external or internal 

 calcareous mass. In this way a " shell " is developed 

 (Fig. 143, A, S), which is usually external. This is 

 the case, for example, with the edible, or Roman snail, 

 and the common garden snail. In other cases (cuttle- 

 fishes), the shell is formed inside the mantle, or it 



B 



Fig. 143. — Diai^rams to explain the form of the body of a Mollusc. A, a Bivalve 

 Mollusc ; B, a Snail (cross sections). In both ; K^ body ; F^ foot ; S, shell ; 

 m, mantle ; k, /j, gills. 



may be replaced by a collection of calcareous granules 

 (various slugs). In most molluscs the chief organ 

 of locomotion is the so-called "foot" (Fig. 143, F). 

 This is a very muscular region of the body, covered 

 by skin, and serving either for crawling (snails), or 

 else for digging in the sand, and even springing 

 (bivalve molluscs). Its structure varies according to 

 its function. I mention three classes, those of Cepha- 

 lopoda (Cuttlefishes), Gastropoda (Snails and Slugs), 

 and Lamellibranchiata (Bivalve Molluscs). 



