308 



CCELHELMINTIIES. 



body may function as hearts (fig. 276, c). Rarely, as in the 

 Capitellidse, circulatory organs may be lacking. 



The excretory organs (nephridia) were formerly known as 

 * segmental organs/ since they occur in pairs in each segment. 

 These supplant the embryonic protonephridia; each consists of an 

 internal ciliated funnel, the nephrostome, a more or less convo- 



dg lg a 



oe 



St QC 



ds 



CO 



s o vd pt vg p 



FIG. 276. Anterior end of Pontodrilus marionis. (After Perrier.) a, vascular arches; 

 b, ventral nerve chain; c, 'hearts'; co, oasophageal commissure; dg, dorsal blood- 

 vessel; ds, septa; gc, cerebrum; Z, retractors of pharynx; lg, lateral blood-vessel: 

 o, ovary; oe, oesophagus; p, receptacula seminis; ph, pharynx; pt, ciliated 

 funnels of vas deferens; 8, nephridia; vd, vas deferens. 



FIG. 277. Schematic cross-section of an annelid. (After Lang.) etc, aciculum; b, 

 chsetae; hrn, ventral nerve cord: dc, dorsal cirrus: dp, notopodium; /c, gill: In?, 

 longitudinal muscles; rnd, digestive tract; ?ip, nephridium; oy, ovary; rm, circu- 

 lar muscles; tm, transverse muscles; fr, nephrostome; uc, ventral cirrus; vd, uv, 

 dorsal and ventral blood-vessels; vp< neuropodium. 



luted tube, and the external opening (fig. 69). In many instances 

 (Oligochaetes, some Polychaetes) the nephrostome is in one somite, 

 the external opening in the succeeding. The nephridia also usually 

 serve as genital ducts, carrying away the reproductive cells, which 



