The phenomena so far can hardly be described as cases of 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 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 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 Claparède, 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 Claparède 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. VIII. 1866. (333) Alex. Agassiz. "On the embryology of Autolytus cornutus and alternations of generations, etc." _Boston Journal of Nat. History_, Vol. VII. 1859-63. (334) W. Busch. _Beobachtungen ü. Anat. u. Entwick. einiger wirbelloser Seethiere_, 1851.