486 ZOOLOGY sect. 



in the original position of its posterior end, while another in- 

 vagination of the ectoderm further forwards gives rise to the 

 mouth and stomodrcium. The embr3-o then passes into the 

 trochophore stage. 



The arrangement of the cilia on the surface of the trochophore 

 varies in different PolychsRta. Sometimes, though rarely, the pre- 

 oral circlet is absent and the surface is covered uniformly with 

 cilia : such larvae are said to be atrochal. Sometimes there are 

 two circlets close together, the one immediately in front of, and 

 the other immediately behind, the mouth. Sometimes in addition 

 to the pre-oral circlet, there is a peri-anal circlet round the anal 

 end (telotrochal larvae). In some cases, instead of a pre-oral circlet 

 there is one further back round the middle of the body (meso- 

 trochal), or there may be several between the mouth and the anal 

 end (polytrochal). 



The post-oral portion of the larva elongates, and traces of 

 segmentation become visible ; sometimes a series of constrictions 

 are developed before there is any trace of parapodia, sometimes rudi- 

 ments of the latter with their setae are developed first. The number 

 of segments, at first very small, becomes added to from behind as the 

 body gradually elongates. The establishment of external segmenta- 

 tion is accompanied by the division of the mesoderm 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 becames connected by a pair of thick- 

 enings 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 formation of the tentacles 

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

 usually bear relatively long provisional se(a\ which are subsequentlj' 

 thrown off to make way for the 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 Chsetopoda both among the Oligochaeta 

 and the Polychaeta. Simple fission occurs in Sahnacina, 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, and 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 Chmtogaster (Oligochaeta) there is multiplica- 



