DEVELOPMENT OF GAJSTSOPODS. 



343 



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. 

 185, Purpitra lapillus and two egg- 

 capsules) of varied form attached 

 to rocks or, as in TrocMis and the 

 Nudibranchs, in masses of jelly at- 

 tached to sea-weeds or stones. 



As a type of the mode of devel- 

 opment of Gastropods may be cited that of Calyptrma si- 

 nensis, represented in our waters by Calyptrma striata Say 

 (Fig. 186). 



Pig. \^.— Purpura and its egg^ 

 capsules, the latter enlarged.^ 

 After Morse. 



Kg. 183. 



Fig. 186. — CalijT>trce,a striata, natural size.— After Morse. 



Fig. 187. — Veliger of Calyptr(ea. /, foot ; v, velum ; in, mouth ; ce, ectoderm ; 'ce^ 

 mesodeiin. — After Salensky. 



Fig. 188. — ^Veliger of CuZyptr(Ea farther advanced, m. mantle ; d, velum ; /, font ; 

 h, larval heart ; n, permanent ; k, primitive kidney ; 8, crosses the shell and rests oil 

 the yolk. — After Salensky. 



According to Salensky, after segmentation of tlie 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- 



