284 ALTERNATIONS OF GENERATIONS. 



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 13th and 

 14th 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 sexual 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 Malmgreu 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 Metschnikofl' 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. 



BIBLIOGRAPHY. 



(332) Alex. Agassiz. "On the young stages of a few Annelids." Annals 

 Lyceum Nat. Hist, of New York, Vol. vin. 1866. 



(333) Alex. Agassiz. "On the embryology of Autolytus cornutus and alterna- 

 tions of generations, etc." Boston Journal of Nat. History, Vol. vn. 1859 63. 



(334) "W. Busch. Beobachtun/jeii ii. Anat. u. Entwick. einiger wirbelloser See- 

 thiere, 1851. 



(335) Ed. Claparede. Beobaclttungen it. Anat. u. Entwick. wirbelloser Thiere an 

 d. Kiiste von Normandie. Leipzig, 1863. 



(336) Ed. Claparede u. E. Metschnikoff. " Beitriige z. Keuntniss lib. Ent- 

 wicklungsgeschichte d. Chretopoden. " Zeit. f. wiss. Zool. Vol. xix. 1869. 



(337) E. Grube. Untersuchungen iib. Entwickhing d. Anneliden. Konigsberg, 

 1844. 



(338) B. Hatschek. "Beitrage z. Entwick. u. Morphol. d. Anneliden." Sitz. d. k. 

 Akad. Wiss. Wien, Vol. LXXIV. 1876. 



