304 SEXUAL DIMORPHISM 



short. In the female sexual zooid the tentacles of the head 

 are similar, and there are two eyes, each with two lenses, as 

 in the asexual stock. Behind the head is a segment 

 hearing tentacular cirri. Behind this come six segments, 

 which have no dorsal bristles, and resemble those of the 

 asexual stock. The segments behind these bear, in addition, 

 the characteristic bundle of long dorsal bristles. 



In the male zooid the chief differences are in the head. 

 There are three tentacles as in the female, the median being 

 very long and the two lateral very small. In front of these 

 are two long bifid palps, to which there is nothing corre- 

 sponding in the female. The segment behind the head, 

 which is without bristles, bears two pairs of cirri. 



There is also a constant difference between the sexes in 

 the position of the generative organs. In the male the 

 testes are confined to the anterior unmodified segments, 

 which do not possess the long natatory bristles, while in the 

 female there are ova, and probably ovaries, in every segment. 



I have given illustrations of the three forms in a worm 

 called Myrianida, in which the dorsal cirri are foliaceous. 

 In this genus the hinder segments are not modified into the 

 sexual form after they are fully formed, but a large number 

 of new segments of small size are formed in front of the last 

 segment of the asexual stock.; heads are formed at intervals 

 in this series of segments, and the zooids thus indicated pro- 

 ceed to develop into the sexual forms. This means merely 

 that the course of development which occurs in Autolytus 

 is hastened, and the future destiny of the posterior segments 

 is indicated at a very early stage. 



There is good reason to believe that all the differences above 

 described correspond to differences in the conditions under 

 which the three forms are developed. ' This fact is evident 

 enough in the original paper of Agassiz, for he states that the 

 asexual stock forms a case attached to the stems of the fixed 



