10 



as long as the sexual oi*{;an contains ripe sexual product:^. 

 X.ylotrya fimbriata I have observed but rarely extruding 

 its sexual products: in this manner, out why ihurc is a dif- 

 ference in this habit I have not determined. 



In accordance with thoir free development in the 

 water, the eggs of the "Ship-worm" are verj' small and very 

 numerous. Wiile they vary somewhat in size, they have an 

 average diameter of somewhat less than 1-20 ram. Very 

 large specimens may lay p;reat nvimbers of eggs at one time; 

 in one case I estimated the number laid by a large female 

 of T.norvegica at one hundred million. The sperms are 

 very minute, and mucih more numerous than the eggs. The 

 eggs of both species that lay their eggs free may be fer- 

 tilized artificially, and pass the early stages of devel- 

 opment with great liniformity and rapidity in aquaria. The 

 eggs when first laid ai-e of irregular shapes. They soon 

 become spherical, and if fertilized, the polar bodies are 

 soon extruded and segmentation begins. Development is 

 ver;^ rapid and on v/arm days, the embryos become free-swim- 

 ming within three hours after the eggs are laid. Within 

 a day the shell has been formed and the typical Lanolli- 

 branch veliger stage i ^- i-eached. ^r-'-ond tliis stage they 



