NOTES ON THE FREE-LIVING NEMATODES. 479 



of the Neniatoda it is found that now the female, now the 

 male, carries the characters of the other sex in a latent state, 

 and when these are wakened to activity secondary herma- 

 phroditism is developed. In Mendelian terminology either 

 sex may be heterozygous. Moreover if the cytological phe- 

 nomenon described by Maupas (p. 491) for Rhabditis 

 elegans really shows that the male in that species is 

 heterozygouSj we are then forced to the hypothesis that both 

 sexes are heterozygous in one and the same species, and at 

 the same time. The phenomena of cytology and heredity as 

 at present known in other groups, e.g. the Iiisecta, are 

 capable of such diverse interpretations that it is impossible to 

 say whether such a case as this suggested above is anomalous 

 or no. 



(5) Self-f ertilisation in Animals. 



Among hermaphrodite animals authentic cases of self- 

 fertilisation are by no means common. In the Trematoda 

 the rule of cross-fertilisation may occasionally be departed 

 from, but only possibly in cases where the spermatozoa dis- 

 charged into the body-cavity of the host find their way back 

 into the female aperture of the same individual. Veiy little 

 is known about the methods of fertilisation in the Cestoda. 

 The evidence for self-fertilisation rests upon two observations, 

 one by Leuckart of a penis inserted in the vagina of the same 

 proglottis, and the other by Pagenstecher of similar i-elations 

 between penis and vagina of adjacent proglottides.^ 



In the MoUusca it is easier to prove by the isolation of 

 individuals the possibility of reproduction without cross- 



1 In the Rhabdoccel Turbellaria self-fertilisation is a very widely 

 spread phenomenon and often the nsnal method of reproduction. Its 

 existence has been put beyond douljt Ijy the observations of individuals 

 raised from the egg, but sncli experiments have not apparently l)een 

 continued over several consecutive generations. In some forms the 

 penis effects self-impregnation, in others there is no copulatory organ 

 or female aperture and the spermatozoa migrate through the Ijody tissue 

 to the ovaiy (see Bresslau, ' Verb, deutseh. zool. GeseU.,' 1903, p. 126, 

 and especially ' Sekera Zool. Anz.,' Bd. xxx, 1906. pp. 112-153). It must 

 be noticed that in the three chief cases, the Turbellaria, the Neniatoda, 



