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 



Fig. 174. The Ship worm, t, siphons ; p, pallets ; c, collar ; s, shell ; /, foot. 

 After Verrill. 



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 

 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 

 eyes develop more, and 

 the animal alternately 

 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. 



Pig. 175._Development of the Ship-worm. 

 A." egg, with the yolk once divided; B, the 

 veiiger enclosed bytbe bivalve shells ; C, ad- 



