HERMAPHRODITISM IN MOLLUSC A. 83 



Nematodes (Pelodytes^ Rhabdonema nigrovenosa), 

 Myzostomidse, parasitic Isopods^ Ascidians and Salps, My xine, 

 Chrysophris, &c. 



IV. Ontogenetic Evolution of the Hermaphrodite Gland 



IN MOLLUSCA. 



According to the theory of the sexuality of the embryonic 

 layers, the female reproductive elements should be of endo- 

 dermic, and the male elements of ectodermic origin. As 

 regards the Mollusca this view has been maintained by Fol 

 {" Pteropoda" Thecosomata, 42). 



But this theory has not been confirmed for any other 

 mollusc, even for any of the nearest allies of the " Pteropods," 

 viz. the Tectibranchs (e.g. Aplysia). 



I have not had an opportunity of studying the fresh larvae 

 of Thecosomatous " Pteropods ;" but in sections of various pre- 

 served larvae I have been able to make out that the " corps 

 pyriforme" (which, according to Fol, is the testicular part 

 of the future hermaphrodite gland) neither has the structure 

 of a testicle nor contributes at all to the formation of the 

 genital gland. The latter is unique from its origin, as it is in 

 its final condition. The same is the case in the other molluscs 

 hitherto studied. 



However, according to Trinchese (43), in Bosellia(a near 

 ally of Elysia) the ova are not produced in the same part of 

 the body as the spermatozoa, although this author is unable to 

 explain how the male and female parts of the hermaphrodite 

 acini eventually combine with one another. Now in Elysia, 

 specimens of which I have examined at every age from this 

 point of view, I find that the first ova arise in the same acini 

 in which spermatozoa alone were present in the stage imme- 

 diately preceding. So that it is impossible in this case to hold 

 that the hermaphrodite gland results from the fusion of two 

 male and female parts of independent origin. The same fact 

 may be observed equally well in the "Pteropod'' Clione 

 (vide supra). 



VOL. 37, PART 1. NEW SEE. C 



