500 



ZOOLOGY 



in Bonellia. The ordinary large individuals (Fig. 390), to various 

 points in the structure of which reference has been already made, 

 are females. The single, greatly enlarged nephridium contains a 

 spacious cavity, which has been termed the uterus. In the 

 interior of this is found the very small male (Fig. 397), which is 

 not unlike a Planarian in appearance: it is compressed and covered 

 with cilia, with a pair of ventral hook-like setse. In the interior 

 of the body bundles of dorso- ventral muscular fibres placed at 

 regular intervals give an appearance of rudimentary segmenta- 

 tion. The alimentary canal is vestigial and completely closed, 



ant.nep 



r 



post.neph 



Fig. 395. — Echiurus, Internal organisation, an. 

 anus ; ant. neph. anterior ncphridia ; int. in- 

 testine ; int. vess. intestinal vessel ; at. oeso- 

 phagus ; post. neph. posterior ncphridia ; vent, 

 vess. ventral vessel. (After Greet.) 



Fig. 3'Jti.— Echiurus, general out- 

 line of the animal, with the 

 nervous system (diagrammatic). 

 ne. co. nerve-cord ; ne. ri. ncrvc- 

 ring. (After Greef.) 



both mouth and anus being absent. There is a pair of 

 nephridia placed posteriorly. The sperms, developed from 

 modified ccelomic cells, reach the exterior through a duct, dilated 

 externally into a vesicula seminalis, and opening internally into 

 the coalome by a funnel-shaped aperture. In Hamingia, also, there 

 are imperfectly developed males which are lodged in the nephridia 

 of the female. 



Development. — The larva of Echiurus (Fig. 398) has a well- 

 developed pre-oral or prostomial lobe with pre-oral and post- 

 oral circlets of cilia, and in other respects closely resembles the 

 trochophore embryo of a Chsetopod. The posterior part of the 



