GASTEROPODA 561 



receive a calcareous shell secreted by the epithelium of the duct. The 

 terminal portion of the duct is the thick-walled muscular vagina, into 

 which open the mucous glands , the dart sac and the spermathecal duct. 

 The sperm, on the other hand, passes down a male duct which is at 

 first only partly separate from the female duct, the cavity of both ducts 

 being in communication until the male duct leaves the company of 

 the female duct altogether, slips under a muscle, and joins the penis 

 at its junction with the slender flagellum. In this latter the sperma- 

 tozoa are compacted together and enclosed in its secretion to form 

 spermatophores . The penis is muscular and has a special retractor penis 

 muscle also attached to it. Both vagina and penis open into a common 

 genital atrium, with an opening to the exterior far forward on the 

 right side. 



Cross-fertilization is the rule in nearly all species of Helix but cases 

 of self-fertilization have been known. Usually, however, there is re- 

 ciprocal fertilization, preceded by a remarkable preparatory event in 

 which two snails approach each other and evert the genital atrium so 

 that the male and female apertures appear externally. The dart sac 

 mentioned above contains a calcareous sculptured weapon, the dart, 

 which can be secreted anew very quickly by the epithelium of the sac. 

 This is propelled by the muscles of the sac out of the female aperture 

 when the other snail is almost in contact — in fact the two darts are 

 launched almost simultaneously, with such force that they pierce the 

 body wall, traverse the cavity and are found imbedded in various 

 internal organs. Some time after this drastic stimulation, the two 

 snails approach each other again and reciprocal fertilization takes 

 place, the penis of each individual being inserted in the vagina of the 

 other (Fig. 385 B). The following account of further events has been 

 given and shows, as in the earthworm, the remarkable complexity of 

 the arrangements which are made to prevent self-fertilization in such 

 common hermaphrodites. The foreign spermatophores find their way 

 up the spermathecal duct to the terminal spermatheca, where the 

 chitinous covering of the spermatophore is dissolved, and the sper- 

 matozoa set free. These now retrace their path to the junction with the 

 female duct and then move up that duct to the fertilization pouch. Ferti- 

 lization takes place in May or June but the eggs are not laid till July. 

 It is said that the foreign sperm remains in the pouch during this 

 time, and that immediately before ovulation the sperm produced by 

 the individual itself degenerates within the hermaphrodite duct so 

 that the eggs pass down the duct without any danger of being self- 

 fertilized and meet the foreign sperm at the end. After fertilization, 

 the egg cell passes down the oviduct where it is enveloped with such 

 quantities of albumen that the diameter of the albumen envelope is 

 20-30 times that of the egg cell itself. In the outer layer of albumen 



