PHYLUM ANNULATA 



199 



possess branchice, organs which are absent as such in Nereis, 

 in the form of simple or branched vascular processes ar- 

 ranged in pairs on the dorsal side of the parapodia, through- 

 out the whole or a part of the length of the body, or (Fig. 

 113) confined to the head-end. All such marine worms 

 belong to the sub-class Polychseta of the Chaetopoda. 

 Though many of them move about freely like Nereis, others 

 live permanently in tubes of a membranous or shelly mate- 



FlG. 114. — A, B, C, three stages in the development of the Trochosphere of 

 Eupomatus, from the side, an, anus; fh, blastoccele; 711, polar cells of the 

 mesoderm; met, mid-gut; n, larval head-nephridium; ot, otolith; sp, neural 

 plate; st, stomodseum; wk, preoral ciliated ring; ivk x , post-oral ciliated ring. 

 (From Lang's Comparative Anatomy.) 



rial. The tube-inhabiting Polychseta (Fig. 113) usually 

 present marked modifications of form in accordance with 

 their mode of life. The branchiae when present are usually 

 confined to the head-end, so that they can easily be thrust 

 out through the opening of the tube, and the body is fre- 



