530 



ANNELIDA. 



The rectum resembles the skin in its lining epithelium, having 

 many unicellular glands. It receives the openings of the two anal 

 vesicles (Fig. 429, Ab) one on each side; these are elongated 

 tubular, sometimes branched, contractile structures which open into 

 the body-cavity by many ciliated openings. They are supposed to 

 be a pair of modified nephridia, the openings of which, as in some 

 Oligochaetes (p. 502), have got shifted into the rectum. 



The perivisceral cavity is spacious and entirely coelomic, and 

 contains a corpusculated fluid, the corpuscles of which are said, in 

 some forms, to contain haemoglobin. 



The vascular system consists of a dorsal vessel, lying on the 

 anterior part of the alimentary canal and continued along the 

 proboscis, and of a ventral supra-neural vessel. These two vessels 

 communicate in front and behind, and doubtless give off branches to 

 the organs, though such have not been described. 



The renal organs are constituted by the so- 

 called brown tubes or anterior nephridia, and 

 the anal vesicles or posterior nephridia. The 

 anterior nephridia closely resemble in form and 

 relations the brown tubes of Sipunculus ; they 

 are attached to the ventral body-wall at their 

 external openings, and project back into the 

 body-cavity as long blind tubes; the internal 

 opening, as in Sipunculus, is close to the ex- 

 ternal. They function also as generative ducts, 

 and the generative cells are collected in them 

 before extrusion, for which reason they are 

 sometimes called uteri. They vary in number 

 from one to four pairs. In Eclmirus there are 

 two pairs, but in TJialassema the number varies 

 in the different species from one to four pairs. 

 In Bonellia there is only one brown tube (Fig. 

 429), sometimes on the right side and some- 

 times on the left. The posterior nephridia or 

 anal vesicles have already been described. 

 The sexes are separate, and the generative 



FIG. 43o.-Pianarian-iike cells are derived from that portion of the 

 male of Bonellia (after coelomic epithelium which overlies the ventral 



Spengel). D intestine; . . . _,, 



TIT ciliated funnel of (supra-neural) vessel (Fig. 429 &). They are 

 the vas deferens (Vd), dehisced into the body-cavity, where they 



which is filled with . . 



mature and escape by the anterior nephridia. 



