DEVELOPMENT OF THE SHIP-WORM. 



segmentation of the yolk (Fig. 180 A] passes through a 

 veliger stage, the shell begins to grow, and when five days 



Pig. 179. The Ship worm, t, siphons ; p, pallets ; c, collar ; a, shell ; /, foot- 

 After VerriJl. 



and a half old the germ appears as in Fig. 178, 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 



size of a grain of millet, 



it becomes spherical, as 



in Fig. 180, 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 



PVPQ rlpvplrm Triorp nnrl 

 e)et> ueve. te, ail 



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. 



p,-g. ISO. Development of the Ship-worm. 



A ' eggl with the yolk once divided ; B ' the 

 veliger enclosed by the bivalve shells; C, ad- 



