PHYLUM ANNULATA 



475 



number of segments, at first very small, becomes added to from 

 behind as the body gradually elongates. The establishment of 

 external segmentation is accompanied by the division of the meso- 

 derm bands into a series of segments, the history of which has been 

 sketched in describing the development of Nereis. The ectoderm of 

 the ventral plate develops a median thickening which gives rise to 

 the ventral nerve-cord. Anteriorly this becomes connected by a 

 pair of thickenings at the sides of the mouth the rudiments of the 

 oesophageal connectives with the developing cerebral ganglion. 



The completion of metamorphosis is brought about by the 

 increase in length of the body and concomitant increase in the 

 number of segments, by the full development of the various 

 systems of internal organs, and by the forma- 

 tion of the tentacles and other appendages. 

 The parapodia, when first formed, very usually 

 bear relatively long provisional setce, which are 

 subsequently thrown off to make way for those 

 of the adult. 



Asexual reproduction by simple fission 

 followed by regeneration of the lost segments, 

 or by proliferation followed by fission, occurs 

 in certain groups of Chsetopoda both among 

 the Oligochseta and the Polychaeta. Simple 

 fission occurs in Salmacina, one of the Ser- 

 pulids : a constriction becomes formed at a 

 certain point towards the posterior end, rudi- 

 ments of a new set of cephalic branchiae bud 

 out on one side at this point, and this pos- 

 terior 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 (hermaphrodite). In Nais 

 and Chcetogaster (Oligochseta) there is multipli- 

 cation 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 proli- 

 feration 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 process 

 of posterior proliferation and constriction (Fig. 388) to sexual 

 zooids, a number of which may remain for a time connected together 



FIG. 388. Building in one 

 of the Syllidae (Auto- 

 li/tus cornutus) ; parent, 

 stock with a male zooid 

 (bwl) nearly ready to 

 become detached. 

 (After Agassiz.) 



