270 



ANNELIDA. 



Fig. 134. 



mixedly composed of blood returning from the tentacles and branchiae, 

 by both of which the function of respiration is performed. Here, again, 

 there exist but two principal directions in which the blood circulates, 

 viz. longitudinally and transversely, or circu- 

 larly, the former currents being connected with 

 the latter. The circular vessel (fig. 134, 6) acts 

 like an auricle ; it receives the blood from the 

 intestinal system and delivers it into the great 

 dorsal (a). The alimentary canal is embraced in 

 this genus, as in all Annelids, by a framework of 

 longitudinal and transverse vessels (/), in which 

 the blood moves backwards below, and forwards 

 above. 



(690.) In the dissection of Terebella nebu- 

 losa, figured by Milne-Edwards, a large vessel 

 (fig. 135, 1) is readily distinguishable towards the 

 anterior part of the animal, running along the 

 median line of the back, and situated imme- 

 diately beneath the integuments. This vessel 

 rests upon the alimentary canal, and exhibits 

 irregular contractile movements whereby the 

 blood contained in its interior is propelled from 

 behind forwards, and consequently performs the 

 functions of a heart ; and if we would compare it 

 with what exists in the higher animals, it might 

 be considered as physiologically representing a 

 pulmonic ventricle, seeing that the vessels that 

 convey the blood to the branchiae for the pur- 

 poses of respiration take their origin from its an- 

 terior extremity. 



(691.) By its posterior extremity, the great 

 dorsal trunk receives the blood which it is ap- 

 pointed to propel through the branchial organs, from several large veins 

 which are, for the most part, adherent to the walls of the intestine 

 (fig. 134, /), from which they receive a multitude of branches derived 

 from the rich vascular network distributed over the intestinal walls. 

 The principal veins, however, that communicate with this tubular heart 

 are two large transverse trunks which form a ring around the digestive 

 canal, beneath which they unite and become continuous with a large 

 median trunk (fig. 135, h) that runs along the under surface of the in- 

 testine, from which, in the same manner as the dorsal veins, it receives 

 numerous lateral branches derived from the vascular network already 

 mentioned. Lastly, there is a small median trunk, situated upon the 

 internal surface of the integuments of the back (fig. 135, m\ into which 

 open the veins derived from the different segments of the body, and 



Kespiratory and circula- 

 >aratus in Terebella. 



