272 



CCELHELMINTHES 



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

 quently (as in earthworms) contain blood colored red by haemoglobin. One 

 trunk is dorsal, the other ventral, to the intestine, the two being connected 

 by vessels (figs. 251, 255) in each segment. The blood passes forwards 

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

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

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

 body may function as hearts (fig. 255, c ). Rarely, as in the Capitellida?, 

 circulatory organs may be lacking. 



dg Ig a 



oe 



ph st gc 



I b 



CO 



pt i-g p b < 



FIG. 255. Anterior end of Pontodrilus marionis (after Perrier). a, vascular 

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

 blood-vessel; ds, septa; gc, cerebrum; I, retractors of pharynx; Ig, lateral blood-vessel; 

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

 deferens; s, nephridia; 5/, pharyngeal ganglion; vd, vas deferens. 



The excretory 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 nephridial canal (usually lined with ciliated epithelium) often serves 

 to carry off the sexual products, which in all chtetopods, arise in the 

 ccelomic epithelium. 



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

 256, I, II), which finally opened into the ccelom (III, IV). In many species 

 they are simple or branched tubes, closed internally by solenocytes, 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 usually lost. 

 The relations of the nephridia to the sexual products appear to be secondary, 

 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 maturity 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 IV), the segmental 



