278 POLYCHAETA 



assumption of a Heteronereid condition in some cases and not in 

 others, are unknown. 



A somewhat similar phenomenon is exhibited by members of 

 the family Syllidae. 1 In this family sexual reproduction is 

 frequently accompanied by the asexual modes of fission and 

 gemmation. In some genera, such as Eusyllis, Odontosyllis, and 

 Exogone, there occur changes quite similar to those characteris- 

 ing " Heteronereis " that is, the posterior segments in which the 

 genital organs exist become altered, so that the worm consists 

 of two distinct regions, and is termed a " Heterosyllis." The 

 most marked change is the appearance of a dorsal bundle of long 

 capilliform chaetae in each of the genital segments (Fig. 148, I). 



But in other genera the hinder genital region of the body 

 becomes separated, on maturity, from the anterior non- sexual 

 region. Various stages of this " schizogamy," or fission into a 

 sexual and a non-sexual zooid, have been observed in different 

 genera. In the genus Syllis the first segment of the sexual 

 zooid, after its separation from the asexual zooid, proceeds to bud 

 forth a head. The character of the head is alike in both sexes, 

 though different species present heads of different shapes ; and 

 as the worms were originally described as distinct genera, the 

 names then given are retained as descriptive terms. Thus the 

 " Chaetosyllis " form has only two tentacles ; the " Ioda " form 

 has three tentacles and a pair of palps. One and the same species 

 (e.g. S. hyalina) may successively pass through these stages. 



With regard to the asexual portion, there is a regeneration of 

 the tail segments after the sexual zooid has separated; and the 

 number of segments so regenerated is usually equal to those that 

 have become sexual. After a time these newly formed segments 

 will produce generative organs, and take on the characteristic 

 natatory chaetae, and this region will in its turn separate. 



But in other genera, such as Autolytus, the regeneration of 

 segments may commence before the separation of the sexual 

 zooid ; and the head of the sexual zooid becomes budded out 

 before separation from the asexual portion. So that the animal 

 now consists of two worms, each with its own head, separated by 



1 Malaquin gives a detailed account of the asexual reproduction in Syllidae in 

 Jiecherches sur les Syllidiens, Lille, 1893, and in Revue Biol. d. Nord de la France, 

 iii. 1891. See also St. Joseph, "Les annelides polychetes des cotes de Dinard," 

 Ann. Sci. Nat. Zool. (7th ser.) i. 1886, p. 134. 



