332 



MOLLUSCA 



The always unpaired sexual organs in some forms (Cyclobranchs and many 

 Zygobranchs) empty into the nephridia and possibly the others utilize the rudi- 

 ments of the right kidney. The sexual opening is almost always on the right 

 side, beside the anus or in front of it on the head. Its position may be recog- 

 nized in males and in hermaphroditic species by the grooved dermal fold, the 

 penis (fig. 338, pe). Occasionally this is separated from the genital pore, 

 but is connected with it by a ciliated groove. The sexual organs are very 

 variable in structure. They show two extremes. On the one hand are com- 

 pletely dioecious species, on the other there may be complete hermaphroditism 



FIG. 339. 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; el, albumen gland; 

 /, finger-form gland;//, flagellum; /w, foot; g, cerebral ganglion; h, heart; /, liver; lu, 

 lung; m, stomach; , nephridium; n', its opening; p, penis; ps, dart sac; r, receptaculum 

 seminis; 5, pharynx; sp, salivary gland; u, uterus; 7', vagina; vd, vas deferens;' z, her- 

 maphrodite gonad. 



(many Tectibranchs, Pteropoda), in which the male and female organs are 

 united throughout their extent. Intermediate stages occur, that of the pul- 

 monates is shown in fig. 339. 



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

 the aquatic forms the eggs are laid in masses, usually gelatinous, each egg with 

 a layer of albumen and a firm shell. Occasionally there is a kind of nest, 

 as is the case with lanlhina which carries the mass of eggs, attached to the foot, 

 about with it. A few gasteropods are viviparous. 



