PHYLUM ANNULATA 



459 



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. 363). This is not unlike a Planarian in appearance, com- 

 pressed and covered with cilia, with a pair of ventral hook-like 

 seta3. In the interior of the body bundles of dorso-ventral 

 muscular fibres placed at regular intervals give an appearance 

 of rudimentary segmentation. The alimentary canal is rudi- 



ant.nep 



post.neph 



FIG. 361. Echiurus, internal organisation, an, 

 anus ; ant. neph, anterior nephridia ; int, in- 

 testine ; int. vess, intestinal vessel ; ces, oeso- 

 phagus ; post . ncph, posterior nephridia ; rent. 

 r.;.s.<, ventral vessel. (After Greef.) 



FIG. 302. Echiurus, general out- 

 line of the animal with the 

 nervous system (diagrammatic) 

 ne. co, nerve cord ; ne. ri, nerve 

 ring. (After Greef.) 



mentary and completely closed, 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 ccelome 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. 364) has a well- 

 developed prse-oral or prostomial lobe with a prae-oral and post- 

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

 trochosphere embryo of a ChaBtopod. The posterior part of the 



