474 



ZOOLOGY 



SECT 



cmbolic gastrula. In the great majority of forms, however, an 

 epibolic gastrula is formed after the manner already described 

 in the case of Nereis ; but forms of the process of gastrulation 

 intermediate between these two extremes have been observed. 

 The blastopore of the gastrula, however formed, does not usually 

 give rise directly either to the mouth or to the anus. It becomes 

 elongated into a slit which becomes closed up, and the anus and 

 proctodaeum are formed by a fresh invagination in the original 

 position of its posterior end, while another invagination of the 

 ectoderm further forwards gives rise to the mouth and stomodseum. 

 The embryo then passes into the trochophore stage. 



The arrangement of the cilia on the surface of the trochophore 

 varies in different Polychseta. Sometimes, though rarely, the 



JZ* 



FIG. 387. A, B, C, three stages in the development of the trochophore of Eupomatus, from 

 the side. an. anus \fh. blastocoele ; m. polar cells of the mesoclerm ; md. mid-gut ; n. larval 

 head-nephridium ; ot. otplith ; sp. neural plate ; st. stomodceum ; wk, pre-oral ciliated ring; 

 tfti, post-oral ciliated ring. (From Lang's Comparative Anatomy, after Hatschek.) 



pre-oral circlet is absent and the surface is covered uniformly with 

 cilia in addition to an apical tuft : such larvse are said to be atrochal. 

 Typically there are two circlets close together, the one pre-oral, 

 immediately in front of the mouth, and the other post-oral, immedi- 

 ately behind it. Sometimes, in addition to the pre-oral circlet, 

 there is a peri-anal circlet round the anal end (telotrochal larvse). 

 In some cases the pre-oral circlet is absent and the post-oral is 

 situated about the middle of the body (mesotrochal), 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 

 rudiments of the latter with their setae are developed first. The 



