306 SEXUAL DIMORPHISM 



formation of the head in the separated hinder portion, and 

 the growth of new hinder segments in the original front 

 portion, are simply processes of recrescence such as occur in 

 almost any segmented worm if it is artificially divided. The 

 processes being repeated in every generation have become 

 constant and hereditary, and all different degrees in the 

 evolution of the process are seen in different species of the 

 family Syllidae. 



Again, with regard to the dimorphism of the sexual 

 zooids, this corresponds in all probability to some difference 

 in the habitual use made of the sensory cephalic tentacles by 

 the male and female. As in many other animals, the male 

 seeks the female in order to fertilise the ova. We find the 

 sexual dimorphism developed to a greater degree in the 

 family Syllidse than in any other marine Polychaeta, and 

 this family is peculiar in the fact that the females carry the 

 embryos, so that a kind of copulation takes place. I do not 

 mean to assert that fertilisation is internal, but since the ova 

 never leave the mother, the male must approach the female 

 to effect fertilisation. It is possible that the palps of the 

 male possess the power of smell as well as touch. Whatever 

 may be the exact mode in which the cephalic appendages of 

 the male are used, there can be little doubt that they are 

 used in seeking the female. It may be that they are merely 

 moved about more actively than the similar parts in the 

 female or asexual stock. The sexual relations imply some 

 stimulation to the sensory organs of the head in the male 

 zooid. The tendency originally was merely to develop by 

 recrescence in both male and female zooid parts of the head 

 similar to those of the asexual stock. It is the conditions 

 which have modified the developments. These conditions 

 being associated with sexual maturity, the resulting modifica- 

 tion is inherited only in association with sexual maturity, and 

 thus we have a complete explanation of the evolution of this 



