. A manual of zoology. Zoology. 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 hear
. A manual of zoology. Zoology. 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
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectzoology, bookyear1912
