308 



CCELHELMINTIIES. 



body may function as hearts (fig. 270, 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 couvo- 



c oe dg Ig a ph st on 



vd pt iig 



Fig. 276.— Anterior end of Po)i(of!ri7t(s moiioHis. (After Perrier.) a, vascular arches; 

 /i, ventral nerve chain: c, 'hearts'; co, oesophageal commissure; dg, dorsal blood- 

 vessel; d«, septa; gc, cerebrum; (, retractors of pharynx; Ig, lateral blood-vessel: 

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

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



Fig. :377.— Schematic cross-section of an annelid. (After Lang.") ar. aeicnlum: 'i. 

 cli<etu> ; hill, ventraJ nerve cord: i/c, dorsal cirrus: lip. notnpodium; k, gill: 'm, 

 loTigitudiiial muscles; 7/i(/, digestive tract ; /(/», ncpltridium; rjc, ovary: r/ii, circu- 

 lar muscles; tu\, trans\Trsc muscles; ii\ ncplirostnme; w, ventral cirrus: vd. vv, 

 dorsal and ventral blood-vessels; vp, ncuropodium. 



luted tube, and the external ojiening (fig. ()9). In many instances 

 (Oligoohajtes, some Polycha^tes) the nojihrostome is in one somite, 

 the external opening in the succeeding. The nephridia also usually 

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



