CCELHEL-MIXTHES 



The blood-vessels usually arc represented by two main trunks which fre- 

 quently (as in earthworms) contain blood colored red by h;rmoglobin. One 

 trunk is dorsal, tlie other ventral, to the intestine, the two being comaected 

 bv vessels (ligs. 251, 255) in each segment. The lilood passes forwards 

 in tlie dorsal vessel, backwards in the ventral. It is propelled by con- 

 tractile portions of tlie vessels; usually tlie dorsal vessel pulsates, but as 

 in the earthworms, certain of the circular vessels in the anterior part of the 

 bodv may function as hearts (fig. 255, c ). Rarely, as in the Capitellida', 

 circulatory organs may be lacking. 



c oc dg Ig a pli St gc 



•Jfj 





ds I b 



9 vd pt Vg p 



Fig. 255. — .\nterior end of Pontodrilus marionis (after Perrier). a, vascular 

 arches; b, ventral nerve chain; c, 'hearts'; co, oesophageal comniissure; dg, dorsal 

 blood-vessel; ds, septa; gc, cerebruin; /, retractors of pharvnx; Ig, lateral blood-A'essel; 

 0, ovary; oc, cesophagus; p, receptacula seniniis; pJi, phan'nx; pt, ciliated funnels of \"as 

 deferens; s, nephridia; st, pharyngeal ganglion; vd, vas deferens. 



The e.^cretory organs (nephridia) were formerly known as 'segmental 

 organs,' since they occur in pairs in each segment. Strictly speaking, 

 each organ belongs to two segments. It usually begins in the anterior of 

 the two with a ciliated funnel (nephrostome), passes through the septum, 

 and, after convolutions, opens to the exterior in the second segment. 

 The nephridia! canal (usually lined with ciliated epithelium) often serves 

 to carry off the sexual products, wliich in aU chanopods, arise in the 

 ccelomic epithelium. 



The nephridia of Annelids seem to be derived from protonephridia (fig. 

 256, I, II), which finally opened into the co?lom (III, 1\"). In many species 

 they are simple or branched tubes, closed internally by soieiiocytcs, large cells 

 drawn out into a tube which empties into the blind end of the excretory tubule 

 and has a flagellum in the interior (fig. 69, p. 106). With the development of 

 the nephrostome the branched condition and the solenocytes are usuallv lost. 

 The relations of the nephridia to the sexual products a]ipear to be secondarv, 

 and are brought about by large ciliated grooves of the peritoneal epithelium. 

 There are three possibilities, (i) The sexual products are emptied by dehis- 

 cence of the body wall; the ciliated organs are phagocytic. (2) The ciliated 

 grooves at the time of sexual maturitv open directly to the exterior and carry 

 off the eggs and sperm (I and III). (3") They connect with the excretory 

 tubules, be these nephridial or protonephridial (II or R'), the segmental 



