CH^TOPODA. 343 



alternation of generations. The process is however in certain 

 types further differentiated. In Syllis (Quatrefages) fission 

 takes place, the parent form dividing into two, of which only the 

 posterior after its detachment develops sexual organs. The 

 anterior asexual zooid continues to produce fresh sexual zooids 

 by fission. In Myrianida also, where a chain of zooids is formed, 

 the sexual elements seem to be confined to the individuals 

 produced by budding. 



The cases of Syllis and Myrianida seem to be genuine 

 examples of alternations of generations, but a still better 

 instance is afforded by Autolytus (Krohn, No. 343, and Agassiz, 

 No. 333). 



In Autolytus cornutus the parent stock, produced directly 

 from the egg, acquires about 40 45 segments, and then gives 

 rise by fission, with the production of a zone of fission between 

 about the I3th and I4th rings, to a fresh zooid behind. This 

 after becoming fully developed into either a male or a female is 

 detached from the parent stock, from which it very markedly 

 differs. The males and females are moreover very different from 

 each other. In the female zooid the eggs are carried into a 

 kind of pouch where they undergo their development and give 

 rise to asexual parent stocks. After the young are hatched the 

 female dies. The asexual stock, after budding off one asexual 

 zooid, elongates again and buds off a second zooid. It never 

 develops generative organs. 



The life history of some species of the genus Nereis presents certain very 

 striking peculiarities which have not yet been completely elucidated. 



As was first shewn by Malmgren asexual examples of various species of 

 Nereis may acquire the characters of Heteronereis and become sexually 

 mature. 



The metamorphosis of Nereis Dumerilii has been investigated by 

 Claparede, who has arrived at certain very remarkable conclusions. He 

 finds that there are two distinct sexual generations of the Nereis form of 

 this species, and two distinct sexual generations of the Heteronereis form. 



One sexual Nereis, characterized by its small size, is dioecious, the other 

 discovered by Metschnikoff is hermaphrodite. 



Of the Heteronereis sexual forms, both are dioecious, one is small, and 

 swims on the surface, the other is larger and lives at the bottom. 



How these various generations are mutually related has not been made 

 out ; but Claparede traced the passage of large asexual examples of the 

 Nereis form into the large Heteronereis form. 



