IV. GASTEROPODA. 



377 



grooved dermal fold, the penis (fig. 370, pe). Occasionally this is 

 separated from the genital pore, but is connected with it by a cili- 

 ated groove. 



The terrestrial snails lay their large tough-shelled eggs in damp 

 earth; in the aquatic forms the eggs are laid in masses, usually 



FIG. 371. Anatomy of Helix pomatia, the roof of the pulmonary sac cut at the left 

 side and turned to the right; the pericardium and visceral sac opened and the 

 viscera separated, a, anus; c, columellar muscle; d, intestine; ei, albumen 

 gland ; /, finger-form gland; /Z, flagellum: fu, foot ; 0, cerebral ganglion ; h, heart ; 

 Z, liver; ZM, lung ; m, stomach; w, nephriaium; n', its opening; p, penis; ps, dart 

 sac; r, receptaculum seminis; s, pharynx; sp, salivary gland; M, uterus; t>, 

 vagina ; vd, vas def erens ; 2, hermaphrodite gonad. 



gelatinous, each egg with a layer of albumen and a firm shell. 

 Occasionally there is a kind of nest, as is the case with lanthina 

 which carry the mass of eggs, attached to the foot, about with 

 them. A few gasteropods are viviparous. 



In the development the great constancy with which the veliger 

 stage (figs. 342, 343) appears is noticeable. Most marine larvae 

 swim by their velum (often divided) at the surface before creeping 

 at the bottom. But in those cases where the snail leaves the egg 



