346 



MOLLUSC A. 



the common duct beside which it runs, in appearance 

 suggesting the flagellum, but expanding at its free end into 

 a globular sac— the receptaculum seminis or spermatheca 

 In Helix aspersa a long slender diverticulum is given off 

 from the duct of the receptaculum. This is also occasionally 

 seen in H. pomatia (see Fig. 150). A spermatophore from 

 another snail passes into the receptaculum, and is there dis- 

 solved after some days, liberating hundreds of spermatozoa. 

 By these spermatozoa the ova of this snail are fertilised. It 

 seems likely that the place of fertilisation is at the lower 



end of the hermaphrodite 

 ! ' duct, whither the sper- 



- Ee _ matozoa are said to find 

 their way. The second 

 structure associated with 

 the female duct is a con- 

 spicuous mucus gland, 

 formed of two sets of 

 finger-like processes. The 

 mucus secretion of this 

 gland is very abundant 

 during copulation, and as 

 it contains not a little 

 lime, it is possible that it 

 may form the calcareous 

 shells of the eggs. 



(g) Finally, between the 

 entrance of oviduct and 

 penis into the terminal aperture there lies a firm cylindrical 

 structure, larger than the penis and with muscular walls. It 

 is the Cupid's Dart Sac, and contains a pointed calcareous 

 arrow (spiculum amoris), which is jerked out previous to 

 copulation. The dart is sometimes found adhering to the 

 skin of a snail, and after copulation the sac is empty, soon, 

 however, to be refilled. 



When two snails pair, the genital apertures are dilated, the 

 protruded penis of one is inserted into the aperture of the 

 other, and the transference of a spermatophore is thus effected. 

 The eggs are laid in the earth in June and July. Each is 

 surrounded by gelatinous material acquired in the oviduct, 

 and by an elastic but calcareous shell. 



-bl. 



Fig. 151. — Diagram of larva of Palu- 

 dina. — After Erlanger. 



Ec, Ectoderm; En. t endoderm ; v. t velum, 

 with cilia ; g., gut-cavity ; S'.c, segmenta- 

 tion cavity ; c.p., ccelom pocket from gut ; 

 bl.g., blastopore groove closed; except at 

 bl., which becomes the anus. The origin 

 of the mesoderm from a gut-pocket has as 

 yet only been described in Paludina among 

 Mollusca. 



