294 SEXUAL DIMORPHISM 



semen received previously from another individual. The 

 accessory generative organs are complicated, the semen being 

 emitted by vasa defercntia in one individual and received into 

 special receptacula in another. So far these are the very 

 conditions of structure and method which in other groups 

 of animals are so frequently accompanied with sexual di- 

 morphism, with the presence of secondary differences between 

 the males and females. But the Oligochseta are hermaphro- 

 dite, and each individual possesses not only the primary but 

 the accessory generative organs of both sexes. Hence copu- 

 lation is reciprocal, as in certain Gasteropoda, and there are 

 no individual differences related to reproduction. The girdle 

 or clitellum, a specialised glandular thickening of the skin in 

 a certain part of the body, is an accessory copulatory organ 

 analogous to some which, in the males of many animals, 

 serves for holding the female, and, if present in only one set 

 of individuals, would constitute a sexual dimorphism, but in 

 these hermaphrodite forms the clitellum is present in all 

 mature individuals, and therefore does not give rise to di- 

 morphism. At the same time there is no obvious difficulty 

 in attributing the evolution of the clitellum to the stimulation 

 of the epidermic glands in a particular region, such stimula- 

 tion resulting, as in other cases, in hereditary hypertrophy. 

 It is noteworthy in this connection that the clitellum, 

 although not differentiating one " form " of individual from 

 another within the species, resembles secondary sexual 

 characters in other cases, in the fact that it is absent in the 

 immature stage of life, and only reaches its full development 

 with the attainment of sexual maturity. 



In the marine Polychaeta the generative structures and 

 processes are very different. These animals are, with few 

 exceptions, of separate sexes, and fertilisation takes place 

 usually in the water without any special arrangements for its 

 accomplishment. The ova and spermatozoa are set free in 



