264 



MOLLUSCA. 



like mass of teeth (fig. 137) which can be drawn in and 

 out, called the radula, and which is used by snails and slugs 

 in devouring the vegetation. The only- 

 group with which we need deal here is that 

 of the Gasteropoda, as they alone are of 

 any agricultural importance. The others, 

 however, may be briefly referred to, as com- 

 pleting the series of the animal kingdom. 



Lamellibranchiata. — These are all bi- 

 valve headless MoUusca, whose mantle con- 

 sists of two distinct flaps, each of which 

 secretes one of the valves. They respire 

 by gills placed in the cavity formed by the 

 two halves of this mantle. The cavity so formed communicates 

 (in one division) by a siphon with the exterior (Siphonata). 

 Bivalve shells with the line marked on the inside of the shell 

 — the pallial line, indented (fig. 138, c)- — can at once be dis- 



FlG. 137.— Two TEANS- 

 VEESE SEEIES OF TEETH 

 FEOM nADUr,A OF LIM- 

 PET. Much enlaj^gad. 

 (Brit. Mus. Guide.) 



Fig. 138. — Shells of Lamellibbanchs. 



1, Cyclas, with two muscle scars (h b) and entire pallial line (a) ; 2, Tapes, with in- 

 dented pallial line (c); 3, Pema, with one muscle scar and complete pallial line (a). 

 (Nicholson.) 



tinguished as siphonate forms. Those that have no pallial 

 sinus are asiphonate forms (fig. 138, 3). The foot is generally 

 a large, wedge-shaped, muscular body. Most are marine, but 

 some — such as Unio and Anodonta, Fresh-water Mussels — live 

 in our rivers and ponds. To this class Oysters (Ostrea), Mussels 

 {Mytilus), &c., belong. 



