DEVELOPMENT OF GASTROPODS. 



263 



tie on the left side of the body behind the head. In a large 

 Lunatia it is an inch long, with a vein at the base, the gill- 

 lobes arranged like the teeth in a comb. A smaller, much 

 narrower gill lies within and parallel to it. The ovary is 

 situated near the stomach, the ovi- 

 duct ending near the vent. 



The eggs are laid in capsules (Fig. 

 190, Purpura lapittiis and two egg- 

 capsules) of varied form attached 

 to rocks or, as in Trochus and the 

 Nudibranchs, in masses of jelly at- 

 tached to sea-weeds or stones. C aSieT a 



As a type of the mode of devel- After Morse. 

 opment of Gastropods may be cited that of Calyptraa si- 

 nensis, represented in our waters by Calyptrcea striata Say 

 (Fig. 191). 



Fia. 191. 



PIG. 193. 



Fig. 191. Oalyptrcea striata, natural size. After Morse. 



Fig. 192. Veliger of Calyptrcea. f, foot ; , velum ; m, mouth ; ce, ectoderm ; 'ce, 

 mesoderm. After Salensky. 



Fig. 193. Veliger of Calyptrcea farther advanced, m, mantle ; v, velum ; /, foot ; 

 h, larval heart ; n, permanent ; k, primitive kidney ; s, crosses the shell and rests on 

 the yolk. After Salensky. 



According to Salensky, after segmentation of the yolk 

 into eight cells the first four cells or "spheres of segmenta- 

 tion" subdivide, enclosing the yolk-mass, and constituting 

 the ectoderm or outer germ-layer, the yolk-mass forming the 

 endoderm. The cells of the outer germ-layer multiply and 

 form the blastoderm, from which the skin, mantle, and ex- 

 ternal organs, as well as the walls of the mouth, arise. The 



primitive" mouth of the gastrula is formed by the invagi- 



K 



