DEVELOPMENT OF GASTROPODS. 



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, Purpura lapillus and two egg- 

 capsules) of varied form attached 

 to rocks or, as in Trochus and the 

 Xudibranchs, in masses of jelly at- 

 tached to sea-weeds or stones. ' 



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

 opment of Gastropods may be cited that of Calyptrcea si~ 

 nensis, represented in our waters by Calyptrcsa striata Say 

 (Fig. 186). 



Fig. 186. rabjplrffn strwita, natural size. After Morse. 



Fig. 187. Veliger of Catyptrcea. f, foot ; v, velum ; m, mouth ; ce, ectoderm ; 'ce t 

 mesoderm. -After Salensky. 



Fig. 188. Veliger of Culi/ptraea farther advanced, m, mantle ; v, velum ; /, foot ; 

 h, larval heart ; . permanent ; k, primitive kidney ; s, crosses tlie shell and rests or. 

 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- 



