DSlYBLOPMENT 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 lung lies within and parallel to it. Tlie ovary is 

 situated near the stomach, the ovi- 

 duct ending near the vent. 



The eggs are laid in capsules (Fig. 

 190, Purpura lapillus and two egg- 

 capsules) of varied form attached 

 to rocks or, as in Trochus and the 

 I^udibranchs, 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. 191). 



Pig. 'iSO.—Furpnra, and Its egg 

 capsules, the latter enlarged. — 

 After Morse. 





Fis. 191. 



>A ». r 



Fig. 198. 



Fig. yi\.—Calyptroia striata, natural size.— After Morse. 



Fig. 193.— Veliger of Calyptrma. /, foot ; ti, velum ; m, mouth ; ce, ectoderm ; '«, 

 mesoderm. — After Salensky. 



Fig. 19.3.— Veliger of Calyptrma farther advanced, m, mantle ; v, velum ; /, foot ; 

 h, larval heart ; n, permanent ; *, 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- 



