204 



ANNELIDA. 



worm has no foundation in truth. The whole intestinal tract of 

 one of these animals is represented in the figure (Jig. 82) : it con- 

 sists of a wide O3sophagus which terminates in a crop-like dilata- 

 tion ; to this succeeds a muscular gizzard (fc), and a long sacculated 

 intestine (/, /) which passes in a direct line to the anus. 



(247.) 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 earth-worm there are three 

 principal trunks connected with the vascular pj g ^ 33. 



system,* the arrangement of which is repre- 

 sented in the annexed diagram (Jig. 83). 

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

 length of the back in close contact with the 

 intestine (Jig. 82, o, o), upon which it lies ; 

 this vessel is tortuous, and exhibits constant 

 movements of contraction and dilatation, by 

 which 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 (Jig. 83, 2>), 

 which we shall call the ventral vessel, runs 

 immediately beneath the alimentary tube ; 

 while the other, which is situated close un- 

 der the skin, and consequently beneath the 

 ventral chain of ganglia composing the nervous 

 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 

 superficial, giving off innumerable vessels to 

 the integuments of the body, which, by rami- 

 fying through the skin, form an extensive vas- 

 cular surface adapted to respiration. 



The ventral vessel (6), like the dorsal (), 

 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 impor- 



* M. Duges, Annales des Sciences Nat. vol. xv. 



