PHYLUM ANNULATA 



481 



The anal vesicles or posterior nephridia (Fig. 394, post, neph) are 

 narrow and elongated, and their funnels are sessile. There is a 

 pair of anterior nephridia (ant. neph) with nephrostomes and 

 nephridiopores situated ventrally behind the seta?. The sexes 

 are alike and the ovary or testis is a ventrally situated median mass 

 of cells in the posterior half of the ccelome. The anterior nephridia 

 act as genital ducts. 



Thalassema is allied to Echiurus, but has a shorter proboscis and 

 no peri-anal setae. There are from one to three pairs of anterior 

 nephridia. 



Bonellia (Fig. 396) is ovoid in shape and green in colour, and is 

 covered with large papilte which are not arranged in rings. The 



ant.nep 



yr-ne. 



ant .neph 



-post.neph 



FIG. 394. Echiurus, internal organisation. 

 an. anus ; ant. neph. anterior nephridia ; int. 

 intestine ; int. vess. intestinal vessel ; ces. oeso- 

 phagus ; post. neph. posterior nephridia ; set. 

 setigerous sacs ; vent. vess. ventral vessel. 

 (After Greef.) 



-ne.ro 





FIG. 395. Echiurus, general outline 

 of the animal, with the nervous system 

 (diagrammatic), ne. co. nerve-cord ; 

 ne. ri. nerve-ring. (After Greef.) 



proboscis is very long and very extensible, and bifurcated terminally. 

 There are two ventral setaa, sometimes more. There is only one 

 anterior nephridium (Fig. 397, ant. neph} which is greatly enlarged 

 and acts as a uterus. The anal vesicles (post, neph) are ovoid, and 

 the funnels terminate the branches of narrow branching tubes 

 given off from the main vesicles. It is the female which has these 

 characters : the male (Fig. 399) is minute, Turbellarian-like, 

 ciliated, with in most species a pair of seta3, no proboscis, and a 

 reduced alimentary canal without mouth or anus. In the young 



VOL. I. II 



