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, Pur pur a lapillus and two egg- 

 capsules) of varied form attached 

 to rocks or, as in Trochns and the 

 Nudibranchs, in masses of jelly at- 



rn PI wporla nv ernnp< 

 DneS - 



Fi g- 185. Purpura and its egg 

 capsules, the latter enlarged.-^ 



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

 opment of Gastropods may be cited that of Cahjptrcea si~ 

 nensis, represented in our waters by Calyptrcea striata Say 

 (Fig. 186). 



FIQ 186. 



FIG. 188. 



Fig. V&. rab/ptr&astriata, natural size. After Morse. 



Fig. 187. Veliger of Cali/ptrcea. f, foot ; v, velum ; in, mouth ; ce, ectoderm ; ';, 

 mesoderm. After Salensky. 



Fig. 188. Veliger of Calyptrwa further advanced, m, mantle ; r, velum ; f, 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- 



