FISSION 281 



is marked out. The zone of proliferation immediately in front of 

 the new head now proceeds to form new segments, and a second 

 jsooid results. This process goes on till a considerable number of 

 new worms have been formed at the tail of the original one, 

 the oldest of these new ones being the most posterior, the 

 youngest next the original " stock." In each zooid there is 

 a zone of activity which adds to its number of segments, so that 

 as we pass backwards the zooids increase in size. As many as 

 twenty -nine such zooids may be formed in this way entirely 

 by gemmation ; and as each zooid becomes completed, genital 

 organs make their appearance, and when these are ripe the zooid 

 separates from the " colony " and leads an independent life. Here, 

 as in Autolytus, the sexes are dimorphic, the male and female 

 resembling those of that genus. 



The process of gemmation, as seen in Autolytus, closely 

 resembles that exhibited by certain Oligochaeta (J\ T aididae), 

 where there exists a definite alternation of generations ; the 

 production of new individuals by gemmation occurring through- 

 out the greater part of the year, and sexual reproduction recur- 

 ring only at certain intervals. In the Polychaeta such alternation 

 exists in Myrianida ; but it is only the terminal link of a series, 

 which takes its starting-point in the process exhibited by the 

 majority of Annelids, where no sexual character marks maturity. 

 The next stage is presented by " epigamous " forms like Hetero- 

 nereis and Heterosyllis ; then " schizogamy " makes its appear- 

 ance in certain Syllidae, resulting in the formation of two 

 morphologically and physiologically distinct individuals which 

 lead independent lives. The appearance of a head and of a zone 

 of proliferation leading to the formation of a chain of sexual 

 zooids is accompanied by a delay in the appearance of the 

 genital organs, for in Autolytus these arise during the formation 

 of the new individuals, as part of the general process of new 

 formation ; whilst in Myrianida the delay is prolonged, and the 

 generative elements do not make their appearance till after the 

 new individuals have reached some size. 



More simple cases of the separation of the body into two 

 parts, sexual and asexual, occur also in some of the Serpulidae, 

 Thus in Filigrana and Salmacina the generative elements make 

 their appearance in the hinder segments, as they do throughout 

 the Sabelliformia : and this hinder part of the body separates 



