GASTEROPODA 487 



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 



o.d. sph. sjH.d. 



Fig. 361. Helix pomatia. A, Reproductive organs. B, Section through the 

 copulatory organs of two mating snails at the moment of the transference of 

 the spermatophores. After Meisenheimer. The organs of the two individuals 

 are indicated by shading sloping in different directions, al.gl. albumen gland ; 

 d.s. dart sac; fl. flagellum; m.gl. mucous glands; o.d. oviduct; p. penis; 

 rec.sem. receptaculum seminis ; retp. retractor muscle of the penis; sph. 

 spermatophore ; spt. spermatheca ; spt.d. spermathecal duct ; sp.d. sperm duct ; 

 ^ gl. hermaphrodite gland (ovotestis), and ^ d. duct. 



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. 361 B). The following account of further events has been given and 

 shows, as in the earthworm, the remarkable complexity of the arrange- 

 ments 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 



