CHAETOPODA. 607 



The process is more complicated in Chaetogastridae and Naidomorpha. A 

 ' zone of fission ' is formed between two somites as soon as the somites 

 increase beyond a certain (? fixed) number. The * zone ' divides into an 

 anterior and posterior portion, the latter forming a head for the posterior 

 set of somites, the former giving rise to a series of new somites. Chains of 

 zooids are thus produced, but the order of the zooids in a chain and the 

 number that constitute a chain, appear to be inconstant (Semper). Under 

 certain conditions, such as unfavourable climatic changes, the zooids be- 

 come free, lengthen and develope sexual organs. In the Serpulids Pro tula 

 and Filograna^ the posterior somites are set free from the anterior with 

 previous formation of a head. As to the Syllidae, the posterior somites in 

 the genera, Syllis, Trypanosyllis, and Opisthosyllis develope sexual organs 

 and bundles of long slender notopodial setae ( = Pubertats-borsten) ; a head 

 is formed, and then the sexual somites, which are not constant in number, 

 are set free as a separate worm 1 . The phenomena are more complicated 

 in the Syllidian tribe Autolyteae. Chains of individuals are produced of 

 which the last is the oldest. The derived zooids develope, one by one, 

 their sexual products and long notopodial setae, and are then detached. 

 An anal somite is always formed by the zooid in front of the one upon the 

 point of being detached. As the parent-form in these Syllidians remains 

 non-sexual, an alternation of generations is set up. Syllis ramosa, which 

 has been found inhabiting Hexactinellid sponges from the eastern seas is 

 a more remarkable form still. It occurs in branched colonies. Small 

 branches become sexual, develope a head with large eyes and long noto- 

 podial sexual setae, and are set free as males and females. 



The fissiparous individual among the Oligochaeta above mentioned 

 differs like an immature worm from the sexual individual by the absence 

 of clitellum, genital setae, &c. as well as of sexual organs, and by a 

 difference in the number of somites making up the body. The asexual 

 Syllidian differs from the sexual by the absence of the long notopodial 

 setae, and, where fission occurs, also in the shape of the head and size of 

 the eyes. The sexual HeUronereis-forrh. of certain species of Nereis 

 differs from the immature individual by the shape and size of the para- 

 podia of the genital somites which become adapted for swimming, by the 

 presence of long slender setae, the conformation of the dorsal cirri espe- 

 cially, and the anal somite, and an increase in the size of the eyes. These 

 changes are more marked in certain respects in the male, leading to a 

 slight degree of sexual dimorphism. Such dimorphism, however, is con- 

 system lies entirely in the hypodermis, and the sole pair of nephridia present appear to belong to the 

 peristomium. They are primitive forms, and are perhaps Oligochaeta. See the authors named ; and 

 for a discussion on the questions of affinity and generic names, Vejdovsky, System der Oligochaeten, 

 1885, pp. 164-66. 



1 The non-sexual parent in Trypanosyllis Krohnii developes previously to separation a new 

 peri-anal somite. Cf. Marion and Bobretzky, A. Sc. N. (6), ii. 1875, p. 36. 



