DEVELOPMENT OF THE SHIP-WORM. 235 
segmentation of the yolk (Fig. 175 A) passes through a 
veliger stage, the shell begins to grow, and when five days 
ae Feria Ship-worm. ¢, siphons; 7, pallets; ¢, collar; s, shell ; 7, foot.— 
and a half old the germ appears as in Fig. 173, B, the shell 
almost covering the larva. Soon after this the velum 
becomes larger, and then decreases, the gills arise, the audi- 
tory sacs develop, the foot grows, though not reaching to the 
edge of the shell, and the larva can still swim about free in 
the water. When of the A C 
size of a grain of millet, 
it becomes spherical, as 
in Fig. 175, C, brown 
and opaque. The long 
and slender foot projects 5 ll 
far out of the shell, and 
the velum assumes the 
form of a swollen ring on 
which is a double crown 
of cilia. The ears and Fig. 175.—Development of the Ship-worm. 
A, egg, with the yolk once divided; B, the 
eyes develop more, and wetiege enclosed by the bivalve shells; C, ad- 
the animal alternately ogee foot (f) and velum 
swims with its velum, or 
walks by means of the foot. At this stage Quatre- 
fages thinks it seeks the piles of wharves and floating 
wood, into which it bores and completes its metamor- 
phosis. On the coast of New England the ship-worm 
lays eggs in May and probably through the summer. 
