6. General discussion. 23 



the presence or absence of a testis. I gather that Dr. Hoek studied two specimens of Sylon 

 by means of sections, and in only one of them could he detect anything that "might be inter- 

 preted as a testis", so that from his account I suppose that he himself is not confident of the 

 presence of this organ. His figure of the body which he interprets as perhaps a testis does not 

 very closely resemble a section through the testis of any other Rhizocephala. In the four 

 specimens studied by me no trace of a testis was to be seen; and since the four specimens 

 were at different stages of maturity, and in one the eggs were nearly ready for fertilization, 

 it is almost impossible that I should have entirely missed this organ. 



I am therefore convinced that Sylon is structurally female and not hermaphrodite, and 

 its method of reproduction must be either parthenogenetic or else by means of dwarf males. 

 Since no one has ever seen a trace of these males, we may expect that parthenogenesis 

 will be found to occur in this forni. 



In the gregarious parasite Thylacoplethus, found on Australasian Alpheidae by Coutière(14), 

 no mention is made of the presence of a testis: it is therefore probable that this forni must 

 be associated with Sylon in its manner of reproduction. 



An investigation of the manner of propagation of these forms would give highly inter- 

 esting results, and material for the study of Sylon should not be very difficult to obtain for 

 a Norwegian or Danish naturalist. 



6. General discussion. 



To recapitulate the detailed evidence which has now been given — the Rhizocephala, 

 with the exception of Sylon and perhaps Thylacoplethus and Duplorbis, are hermaphrodites in 

 which reproduction is carried on by a continuous round of self-fertilization. Degenerate com- 

 plemental males may be present, but these no longer play any part in fertilization. Sylon 

 (and perhaps we may add Thylacoplethus and Duplorbis) is structurally female, and reproduction 

 is carried on either by separate males or parthenogenetically. 



It is perhaps of interest to establish the unbroken occurrence of self-fertilization in a 

 large group of related genera, and to study the interesting adaptations connected with the 

 process, especially the remarkable economy in the production of sperm efFected by the hyper- 

 trophied degeneration of the greater number of the male germinai cells. The occurrence of 

 perpetuai self-fertilization, in any extensive group of animals or plants, has rarely been proved 

 to occur, and in view of the striking contrivances for efiecting cross-fertilization in the great 

 majority of animals and plants, it is always mysterious how any group can withstand the evil 

 consequences which are inferred to result from continuous in-breeding. The existence of such 

 a group as the Uhizocephala, which are among the most dominant and prolific Crustacea, is 

 certainly paradoxical especially Avhen we consider that self-fertilization in other Cirripedes is 



Zool. Station zu Neapel, Fatina nnd Flora, Golf von Neapel. Bhizocephala. 5 



