1 . The testes of the Hermaphrodites. 2 l 



passage through the colleteric glands, as solid and chitinous; it is in reality at first glutinous 

 and permeable by the spermatozoa, and only subsequently hardens. 



Self-fertilization then in Sacculina and Peltogaster is the invariable rule, while the Cypris 

 larvae that fix themselves round the mantle are no longer functional. It is however just 

 possible that in Duplorbis, the parasite of Calathura, functional males may be present in the 

 mantle cavity, the parasite itself being female. This may also be the case in Sylon which 

 is structurally female and not hermaphrodite, but reproduction here may be parthenogenetic 

 as there is no evidence of the occurrence of males. 



We now turn to the detailed proof of these conclusions and at the end I will attempt 

 to give an explanation of the state of affairs, especially of the nature of the degenerate males. 

 The subject will be divided under the following headings — 



1. The testes of the Hermaphrodites. 



2. The moulting of the mantle-cavity and of the vasa deferentia and the passage of 

 the ova into the mantle-cavity. 



3. The maturation and fertilization of the ova. 



4. The complemental males. 



5. The sexual nature of Sylon. 



6. General discussion. 



1. The testes of the Hermaphrodites. 



(Piate 2 figs. 6—14 and Piate 8 figs. 7 and 8.) 



In our general sketch of the morphology in Chapter 1, we dealt with the position of 

 the paired testes in the various genera: it will therefore suffice here to recali the fact that 

 the testes are always situated at the junction of visceral mass and mesentery, and that their 

 paired openings into the mantle cavity are on the mesentery in the morphologically posterior 

 region of the body. In Clistosaccus there is only one testis and it differs considerable in 

 structure from that of the other forms; in Sylon there are no testes present, so that this genus 

 and Duplorbis, whose sexual nature is problematical, will be reserved for a special section. 



We will consider first the testes of Sacculina, Peltogaster and the allied genera, and 

 then that of Clistosaccus. 



The structure of the testes and the process of spermatogenesis was described by 

 Delage (9), but he wrote at a time when these matters were but little understood: it 

 remained therefore for Duboscq (13) to give a correct account of the matter, with which I ani 

 in complete agreement. 



We are not concerned here with the first appearance of the testes in organogeny, 

 which is described on p. 51, but with their evolution during the adolescence of the parasites. 



