PHYLUM ANNFLATA 



499 



neph.fun 



ant.neph 



ventyess 



simple character, consisting of mere spots of pigment, are present 

 in some of the Inermia. 



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

 appendages of the 

 rectum are, as al- 

 ready mentioned, 

 to be regarded as 

 posterior nephiidia. 

 In addition there 

 are present an- 

 terior nephridia. In 

 Bonellia (Fig. 393, 

 ant. neph.), and 

 in some Inermia, 

 there is only a 

 single one : in the 

 majority of cases 

 there is one pair, 

 while in various 

 species of Thalas- 

 acma there are from 

 one to four pairs. 

 They are tubes 

 which open extern- 

 ally on the ventral 

 surface, and intern- 

 ally communicate 

 with the coelome 

 by means of ciliated 

 apertures, the form 

 and position of which vary in different cases. They act as 

 efferent ducts for the reproductive elements (gonoduds) ; 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, developed 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 detached, and undergo the later 

 stages of their development while float- 

 ing in the ccelomic fluid. 

 A remarkable instance of extreme sexual dimorphism occurs 



K K 2 



Fifi. 3113. — Bonellia^ general view of the internal organs. 

 aii. anus; iiiiL neji/i. anterior nephridium ; int. intestine; 

 m ph. fun. ncplirostome ; vs. oesophagus; ov. ovary; ph. 

 phcirynx ; j^ost. neph. posterior nephridium ; jjro&. proboscis ; 

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



Fig. 304.— One of the ciliated 

 funnels of the posterior 

 nephridia i if Echiurus. 

 (After Greef.) 



