DEVELOPMENT OF OASTROT'ODS. 



243 



tie on the left side of tlie 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 oyary 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 

 ISTudibranchs, 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 Calyptrcea si- 

 nensis, represented in our waters by Calyptrma striata Say 

 (Fig. 186). 



Fig. 1S5. — Purpura and its egg 

 psules, the '-^'■*- - -' "■ 

 After Morse. 



capsules, the latter enlarged. — 

 ■.fte: " 



Ko. 186, 



Fio. 188. 



Fig. 1S&. —Calypfran striata, natnral size.— After Morse. 



Fig. 187. — Veliger of CalypircBa. /, f cot ; v, velum ; 7ft, moutli ; ce^ ectoderm ; 't'e, 

 "mesoderm. — After Salensky. 



Fig. 188. — ^Veliger of Calyptrcea farther advanced, m, mantle ; v, velum ; /, foot ; 

 A, 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- 



