MORPHOLOGY OF GENITALIA. XV 



mon cloaca, the atrium or vestibule. It is now held that the herma- 

 phrodite condition is secondary in mollusks, the male organs being 

 superimposed or grafted upon the female individual (see Pelseneer 

 Quart. Jouru. Mic. Sci. 1894, p. 19). The proofs for this view com- 

 ing from many sides, all indicate that in the primitive mollusks the 

 sexes were separate. 



Embryological data indicate that the entire generative system 

 except atrium, penis sack and their special appendages, are of meso- 

 dermal origin. Simroth is probably right in holding that the 

 atrium and evertible penis (but not epiphallus) are ectodermal evag- 

 inations. The case of Limax primitivus which he cites to prove that 

 the penis has been " pulled out " from the atrium, is, however a case 

 of degeneration in all probability. It is very probable that the 

 penis in land mollusks is strictly homologous with that of Tecti- 

 branchs, and its union with the female organs at the atrium has 

 been brought about by the gradual moving forward of the female 

 orifice, originally posterior in position. 



It seems likely that the dart apparatus is primarily an outgrowth 

 from the atrium, although in some cases it has moved upward on 

 the vagina. It is not homologous with the dart sack of Philomyeus, 

 nor with that of certain Zonitidce. The gland or sack upon the 

 penis, called the appendix, is probably a very ancient character, and 

 is homologous with that sometimes developed upon the atrium (see 

 Helicella), but not with the blind sack found high on the vagina in 

 such forms as Panda, etc., which seems to be an independent growth 

 from the vagina, probably serving as a temporary receptacle for 

 spermatophores (packets of spermatozoa), analogous to the diverti- 

 culum of the spermatheca duct. Although both male elements 

 (spermatozoa) and female (ova) are produced in the same acini of 

 the hermaphrodite gland, the former ripen first, and passing down 

 are enclosed in a leathery or chitinous case, the spermatophore 

 (" capreolus") secreted by flagellum or epiphallus. In forms 

 lacking these the spermatophore is absent. In the female system 

 these spermatophores are stored in the spermatheca and its ap- 

 pendages, pending the ripening of eggs and their passage down- 

 ward. The dart apparatus is only a stimulating organ, the dart 

 being thrust from one individual into another during copulation. 

 Von Ihering considers the papilla in the penis also a sensory organ. 

 The function of the penis-gland is unknown. During copulation the 

 penis is everted in most Helices, but in some there are reasons for 



