ms: SNAILS, 
71 
which it is anchored to the bottom. The. foot in the quo- 
hog (Fig. 78, Ay Venus mercenaria ; 78, B, Muliiiia) is 
large. 
The ship-worm (Fig. 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; 
thevehger enclosed by the bivalve shells; C, advanced veliger with the 
large foot (/) and velum {v). 
wood, into which 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 11. — Cephalophoea {Whelks, 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 
