200 



MANUAL OF ZOOLOGY 



SECT. 



enclosed in it. All the Polychaeta, with one or two excep- 

 tions, have the sexes separate, and there is a free-swimming 

 pelagic larva, the Trochosphere (Fig. 108) with one or several 

 circlets of cilia. 



When a common Earthworm is compared with Nereis, 

 certain resemblances are at once discernible. The Earth- 



FIG. 108. A, B, C, three stages in the development of the Trochosphere of 

 EupomatUS, from the side. an. anus ; ///. blastoccele ; vi. polar cells of the 

 mesoderm ; md. mid-gut ; . larval head-nephridium ; ot. otolith ; sp. neural 

 plate ; st. stomodaeum ; ivk. preoral ciliated ring ; <wk\. post-oral ciliated ring. 

 (From Lang's Comparative Anatomy?) (After Hatschek.) 



worm (Fig. 109) has a similar elongated cylindrical body, 

 divided by ring-like grooves into a large number of seg- 

 ments or metameres. But the well-developed head-region 

 is absent, as are also the eyes, palpi and tentacles ; and 

 there are no parapodia and no cirri. Setae, however, are 

 present (Fig. no), though so short as to be distinguish- 

 able with difficulty : they are arranged in two double rows 



