446 



ZOOLOGY 



SECT. 



number of segments, by the full development of the various 

 systems of internal organs, and by the formation of the tentacles 

 and other appendages. The parapodia, when first formed, very 

 usually bear relatively long provisional seta 1 , which are subsequently 

 thrown off to make way for those of the adult. 



Asexual reproduction by simple fission followed by regenera- 

 tion of the lost segments, or by proliferation followed by fission, 

 occurs in certain groups of Chaetopoda both among the Oligochaeta 

 and the Polychseta. Simple fission occurs in Salmacina, one of 

 the Serpulids : a constriction becomes formed at a certain point 

 towards the posterior end ; rudiments of a new set of cephalic 

 branchiae bud out on one side at this point, said this posterior 

 part becomes a distinct zooid, which is eventually separated off 

 and develops the full number of segments characteristic of the 

 adult. This is not in any way a case of alternation of generations, 

 as both parent and offspring are similar and sexual (hermaphro- 

 dite). In Nais and Chcetogaster (Oligochaeta) there is multiplica- 

 tion by proliferation of the segments at the 

 posterior end ; then the appearance of a con- 

 striction separating off five or six of the most 

 posterior segments, followed by a fresh pro- 

 liferation in front of the constriction, and 

 this in turn by the appearance of a second 

 constriction five or six segments further 

 forwards the result being the development 

 of a chain of zooids which remain for a time 

 connected together. The sexual cells be- 

 come fully developed only after the zooids 

 have become separated from one another, 

 In some of the Syllidae (Errantia) there is 

 a distinct alternation of generations. Thfc 

 asexual worm developed from the ovum gives 

 rise by a process of posterior proliferation and 

 constriction (Fig. 348) to sexual zooids, a 

 number of which may remain for a time con- 

 nected together in a string before becoming 

 separated. These sexual zooids become de- 

 veloped into mature males or females, which 

 may be remarkably unlike the parent form 

 in the shape of the parapodia, the character 

 of the setae, and similar points ; and in some 

 instances the two sexes not only differ from 

 the asexual parent form but from one another, 



so that the three forms, before their relationship was known, have 

 been set down as representing three distinct genera. 



Syllis ramosa, which occurs in the interior of certain .deep-sea 

 sponges, is exceptional among the Chaetopoda in giving rise by 



FIG. 348. Budding in one 



cornutus), parent stock 

 with a male zooid nearly 

 ready to become de- 

 tached. (After Agassiz. ) 





