THE SNAILS. 



71 



wliich it is anchored to the bottom. The foot in the quo- 

 hog (Fig. 78, A, Venus mercenaria ; 78, B, Mulinia) is 

 large. 



The ship-worm (Pig. 79) belongs to this class. The body 

 is slender and worm-like. The shell is minute, the soft 

 animal living in a burrow lined with limestone. This ani- 

 mal develops like other mollusks; the young (Fig. 80, B) 

 having two equal shells inclosing the body, and swimming 

 by its ciliated velum or sail (v). After the foot (Fig. 80,/) 

 is well developed it seeks the piles of wharves and floating 



Fig. 80._Development of the Ship-worm. A, egg, with the yolk once divided; 

 B, theveUger enclosed by the bivalve shehs; C, advanced veliger with the 

 large foot (/j and velum (v). 



wood, into whicli it bores and completes its metamorphosis. 

 On the coast of New England the ship-worm lays eggs in 

 May and probably through the summer. 



Class II. — Cephalophora {Wlielks, Snails, etc.). 



General Characters of Cephalophores. — We now come to 

 Mollusks with a head bearing eyes and tentacles; but the 

 bilateral symmetry of the body, so well marked in the 

 clam, etc., is now in part lost, the animal living in a spiral 

 shell. Still the foot and head are alike on both sides of the 

 body; while the foot forms a large creeping flat disk by 

 which the snail glides over the surface of leaves, etc. 

 Moreover, these mollusks have, besides two teeth, a " lingual 



