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 ncph riclia . 

 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 

 ccelome 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 - 

 \ i ^sels. The cells of these ovaries or 

 testes may develop in situ into perfect ova 

 or sperms: more usually they become de- 

 tached, and undergo the later stages of 

 their development while floating in the 

 ccelomic fluid. 



A remarkable instance of extreme 



1 EG. 300. One of the ciliated 



sexual dimorphism occurs in Bonellia. funnels of the posterior 



rni i i i i /TTi- c\<-n nephridia of Echiurus. 



Ihe ordinary large individuals (Fig. 356), (After Greet.) 



]-'IG. ?..'.'.!. Bonellia ; general view of the internal organs. 

 an. anus; n.,>t. ;n./i/i. anterior nephriflium ; int. intestine ; 

 nep7i. fun. nephrostome ; cfs.t oesophagus ; oi\ ovary ; t >l. 

 pharynx ; poxt. ,/./>/(. posterior nephridium ; i>rob. proboscis ; 

 vent. >'. ventral vessel. (After Greet.) 



