CILETOPODA 297 



remarkable change in the structure of the parapodia and the 

 other changes described. It is obvious that the meta- 

 morphosis is essentially a somewhat sudden adaptation for 

 active swimming during the reproductive period. The theory 

 of selection among mere indefinite individual variations does 

 not satisfactorily explain the definite metamorphosis. That 

 theory must assume that the variations selected consisted 

 in changes in the individual occurring at a late period of 

 life. It gives no reason why these changes in the indvidual 

 occurred. The changes are actually associated with a 

 definite and important change in the habits and activities 

 of the individual, and the only reasonable conclusion is 

 that the change in the activities was the cause of the 

 transformation of structure which is now hereditary. 



But in addition to the difference between the immature 

 and the mature individual, there is also a difference between 

 the male and female in the mature state. The secondary 

 sexual characters give rise to a sexual dimorphism. Accord- 

 ing to Ehlers, the alterations taking place at sexual maturity 

 are usually more pronounced, of greater degree, in the male 

 than in the female. The number of unmodified segments 

 in the anterior part of the body is characteristic for the 

 species, but there is often in this respect a sexual difference, 

 the number of unaltered segments being smaller in the male. 

 The modification also is usually complete in the first modified 

 segment in the male, while in the female there are several 

 transitional segments. These differences would naturally 

 result if the males were more active than the females. I 

 know of no direct evidence on this point, but it is reasonable 

 to suppose that the males, as in other cases, actively seek 

 the females. 



The history of one species of Nereis, N. dumerilii, accord- 

 ing to the investigations hitherto made, presents remarkable 

 peculiarities, which are very difficult to explain on any 



