CIRCULATION IN THE EAETHWOEM. 



231 



Fig. 112. 



taneous surface is the scene of a dense plexus of blood-proper vessels. 

 It is always enveloped in a stratum of viscid fluid, which is remarkable 

 for the property of absorbing and dissolving atmospheric air. This air, 

 brought thus into immediate and intimate contact with the surface of 

 the body, operates directly upon the blood-proper circulating in the 

 cutaneous plexus. In the Abranchiate Annelids, as in many of the 

 Tubicola, the alimentary canal is always profusely supplied with a vas- 

 cular tissue which shares in the respiratory process, and may be regarded 

 as a species of intestinal respiration. 



(600.) The circulation of the blood in the 

 terricolous Annelidans has been the subject of 

 much discussion, and. until recently was but very 

 imperfectly understood. In the Earthworm there 

 are three principal trunks connected with the 

 vascular system*, the arrangement of which is 

 represented in the annexed diagram (fig. 112). 

 First, a dorsal vessel (a) runs along the whole 

 length of the back, in close contact with the in- 

 testine (fig. Ill, o o), upon which it lies; this 

 vessel is tortuous, and exhibits constant move- 

 ments of contraction and dilatation, whereby the 

 blood is propelled in continuous undulations from 

 the tail towards the head. Two other large 

 vessels occupy the ventral region of the body : of 

 these, one (fig. 112, 6), which we shall call the 

 ventral vessel, runs immediately beneath the ali- 

 mentary tube, while the other, that is situated 

 close under the skin, and consequently beneath 

 the ventral chain of ganglia composing the ner- 

 vous system, by which it is separated from the 

 last, may be distinguished as the sub-ganglionic 

 vessel. These three great trunks are united by 

 important branches, and form two distinct 

 systems, one of which is deeply seated, being 

 distributed to internal viscera ; the other is su- 

 perficial, giving oif innumerable vessels to the 

 integuments of the body, and these, by ramifying 

 through the skin, form an extensive vascular 

 surface adapted to respiration. 



(601.) The ventral vessel (6), like the dorsal (a), 

 may be traced quite to the anterior extremity of 

 the worm, where numerous small anastomosing branches unite the two 

 trunks ; but these inosculations are of little consequence in describing 

 the circular movement of the blood, a more important communication 

 * M. Duges, Ann. des Sci. Nat. vol. xv. 



Circulation in the Earth- 

 worm. 



