PHYLUM ANNULATA 



487 



tion by proliferation of the segments at the posterior end ; then 



the appearance of a constriction separating off five or six of the 



most posterior segments followed by a fresh 



proliferation in front of the constriction ; and 



then a second constriction appears 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 become fully developed only after 



the zooids have separated from one another. 



In some of the Syllidse there is a distinct 

 alternation of generations. The asexual worm 

 developed from the ovum gives rise by a pro- 

 cess of posterior proliferation and constriction 

 (Fig. 381) to sexual zooids, a number of which 

 may remain for a time connected together 

 in a string before undergoing separation. 

 These sexual zooids become developed into 

 mature males or females, which may be re- 



mi 



markably unlike the parent form in the shape 

 of the parapodia, the character of the setse, 

 and other points ; and in some instances the 

 two sexes not only differ from the asexual 

 parent form but also from one another, so 

 that the three forms, before their relationship 

 was known, were set down as representing 

 three distinct genera. 



Si/His ramosa (Fig 382) which occurs in the interior of certain 

 deep-sea sponges, is exceptional among the Chaetopoda in giving 



FIG. 381. Budding in one 

 of the Syllidtt(Autotyttt* 

 cornutus) ; parent stock 

 with a male zooid nearly 

 ready to become de- 

 tached. (After Agassiz.) 



Fio. 382. Portion of Syllis ramosa. (From the Cambridge Natural History, after Melntosh. 



rise by lateral branching to a colony from which sexual zooids 

 afterwards become separated off. 



