ECHIUROIDEA. 



529 



front this cord divides into two, which pass round the oesophagus 

 and extend quite to the anterior end of the proboscis, where they 

 join one another (Fig. 428). There are no ganglionic swellings, but 

 the whole system has a uniform coating of nerve-cells. The ventral 

 portion, moreover, contains a fine canal, which extends into the 

 commissures, but is absent from the hind end of the cord, and from 

 the supra-oesophageal portion. The central organ gives off nerves 

 to the adjacent parts of the body. 

 There are no special sense-organs. 



FIG. 429. Female of Bonellia viridis ; a, the whole animal; &, anatomy; c, integument and 

 generative organs after removal of the intestine (after L. Duthiers). Ab anal vesicles ; Ad 

 rectum; D alimentary canal ; Hd cutaneous glands; M mesentery; Ov ovary; R proboscis 

 (preoral lobe) ; Tr ciliated funnel of the single brown tube or anterior nephridium (uterus) ; 

 U uterus (anterior nephridium). 



The alimentary canal, in which various divisions can be made 

 out, consists of a long, thin-walled, much convoluted tube, along the 

 ventral wall of part of which is a ciliated groove. There is also a 

 siphon, or accessory intestine, like that found in Echinids and some 

 Chaetopods; this is a tube given off ventrally from the anterior 

 part of the long and coiled intestine; it lies ventral to the ciliated 

 groove, and opens posteriorly into the hind part of the intestine. 

 There is no special mesentery, but strands of tissue run from all 

 parts of the body-wall across the body-cavity, to be inserted into the 

 walls of the alimentary canal. 



2 M 



