SANGUISUGA. 



looking appendage, represented until recently as being in- 

 tended to furnish some important secretion, bat which late 

 discoveries have shown to be connected with the propulsion 

 of the blood over the walls of the breathing-vesicle, in a man- 

 ner to be explained immediately. It would seem, however, 

 that the respiratory function is not exclusively carried on by 

 the agency of the lateral sacculi ; the entire surface of the 

 body is permeated by innumerable delicate vascular ramifica- 

 tions, and from the thinness of the integument it is evident 

 that the blood which traverses the cutaneous net-work thus 

 extensively distributed rmist be more or less completely ex- 

 posed to the influence of oxygen contained in the surrounding 

 medium ; nay, it would even appear from careful examination 

 of the movements of the blood, as seen in the transparent 

 bodies of some of the Hirudinidce^ that a kind of vicarious 

 action occurs between the capillary vessels of the skin and 

 those of the respiratory sacs, so that when the circulation pro- 

 ceeds languidly through one set of vessels, it is carried on 

 with greater activity in the other. 



The vessels appropriated to the distribution of the circu- 

 lating fluid in the leech come now under consideration. 

 There is no heart, but the movements of the blood are entirely 

 due to the contractions of the canals in which it flows. The 

 principal vascular trunks are four in number, which, although 

 they all communicate extensively with each other, perform 

 distinct offices in effecting the circulation ; two of them being 

 specially connected with the supply of the general system, 

 while the other two seem subservient to the distribution of 

 the blood over the respiratory sacculi. 



The two systemic trunks run along the mesian line of the 

 body, one upon the dorsal and the other upon the ventral as- 

 pect. The dorsal vessel seems to be arterial in its character, 

 and no doubt corresponds in function with the heart of more 

 perfect forms of the Articulata, receiving the blood from all 

 parts of the system, as well from the respiratory vessels as 

 from the venous capillaries, and by successive undulatory con- 

 tractions, which may be observed to proceed from the tail 

 towards the anterior extremity, propelling it through all the 

 arterial branches derived from it. The ventral vessel, on the 

 contrary, seems to be venous, collecting the blood after its pas- 

 sage through the systemic capillaries, and returning it partly 

 into the dorsal artery from which it set out, and partly to the 



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