MOLLUSCA GASTEROPODA. 



ica 



143. Skeleton. The Gasteropoda are provided with a 

 mantle which encloses the body like a sac. This mantle, 

 in most cases, secretes a shell, which is generally univalve, 

 or composed of a single piece. A few have multivalve 

 shells, which are composed of eight pieces, placed one 

 behind another. None possess a bivalve shell. Some. 



.0. 



t... 



FIG. 43. ANATOMY OF THE SXAIL. 



to, ocular tentacles; oe, oesophagus; sn, cerebral ganglion; j, gizzard ; 

 e, stomach; t, liver;/, biliary ducts; i, intestine; a, anus; r, kidney; p, 

 breathing organ ; c, heart. 



as the common slugs, are destitute of shells. The univalve 

 shell is either a simple cone, as in the limpet; or "spiral," 

 which is its most usual form The various coils of the 



