X 



PHYLUM ANNULATA 



445 



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 prse-oral circlet, there is a peri-anal circlet round the anal 

 end (telotrocJial larvae). In some cases, instead of a pra3-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 (polytrockal). 



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

 segmentation become visible ; sometimes a series of constrictions 



--'-=> ink 



J73 



.;. 347. A, B, C, three stages in the development of the Trochosphere of Exipomatus, from 

 the side. an. anus ; fit, blastocoele ; m, polar cells of the mesoderm ; ///, mid-gut ; n, larval 

 head-nephridium ; of. otolith; sp, neural plate ; si. stomodfBum ; wk, preoral ciliated ring; wki 

 post oral ciliated ring. (From Lang's Comparative Anatomy.) 



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 becomes 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 the metamorphosis is brought about by the 

 increase in length of the body, and concomitant increase in the 



