458 



ZOOLOGY 



SECT. 



oes 



neph.fun 



ant.neph 



ventyess 



Priapulus is devoid of nephridia. In the Armata a pair of 



appendages of the rectum are, as already mentioned, to be regarded 



as posterior nephri- 

 dia. In addition 

 there are present 

 anterior nephridia. 

 In Bonellia (Fig. 

 359, ant.neph.), and 

 in some Inermia, 

 there is only one. 

 In the majority of 

 cases there is one 

 pair, while in vari- 

 ous species of Tha- 

 lassema there are 

 from one pair to 

 four. They are 

 tubes which open 

 externally on the 

 ventral surface, and 

 internally commu- 

 nicate with the 

 coelome by means 

 of ciliated aper- 

 tures, the form and 

 position of which 

 varies in different 

 cases. They act as 

 efferent ducts for 

 the reproductive 



elements (gonoducts) ; but their function as excretory organs 



has not been definitely established. 



, The sexes are usually distinct, and the reproductive organs 



are of very simple character, consisting 



merely of ridges or clumps of cells (gonads), 



sometimes enclosed in a membrane, de- 



veloped at various points on the body-wall, 



or on the wall of one of the main blood- 



vessels. The cells of these ovaries or 



testes may develop in situ into perfect ova 



or sperms ; more usually they become deT 



tached, and undergo the later stages of 



their development while floating in the 



ccelomic fluid. 



A remarkable instance of extreme 



sexual dimorphism occurs in Bonellia. 



The ordinary large individuals (Fig. 356), 



post. neph. 



FIG. 350. Bonellia, general view of the internal organs. 

 an. anus ; ant. neph. anterior nephridium ; int. intestine ; 

 neph. fun. nephrostome ; ms.i oesophagus ; or. ovary ; ph. 

 pharynx ; post. neph. posterior nephridium ; prol>. proboscis ; 

 vent. vess. ventral vessel. (After Greef.) 



Flo . 360 ._ O ne of the ciliated 

 funnels of the posterior 



' 



