Annelides. 93 



ones. Durondeau* has mentioned the presence of a heart, or central 

 organ of circulation, in the medicinal leech. He describes it as a 

 fleshy pouch, of a conical figure, attached to the back by large vessels 

 but having its apex free. This statement has not been confirmed by 

 any other author ; and in all my examinations of the horse-leech, I 

 have never been able to detect such an organ. 



The lateral vessels (fig. a a) run from one extremity of the body to 

 the other, in a wavy course, forming a series of festoons. Some au- 

 thors describe them as quite straight when the animal is in motion or 

 stretched out, but wavy when the animal is at rest in its constricted 

 state : I have always found them, both in the horse-leech and the me- 

 dicinal species, to form a series of festoons in every position of the 

 animal, whether the body was dilated by the distention of the alimen- 

 tary canal with injection, or when in a state of contraction produced 

 either by alcohol or other corrugating fluids. These vessels are lar- 

 gest in the middle of the animal, and diminish gradually in size as 

 they approach the extremities, at which points they become continu- 

 ous with one another, and here they give off* numerous branches to 

 supply the locomotive sucking disk, the mouth, and the organs of 

 sight. As these lateral trunks proceed in their wavy course, they send 

 off" branches (dorso-lateral and abdomino-lateral) at nearly equal in- 

 tervals ; by means of these numerous anastomoses, when injection 

 is forced into one lateral vessel, the vessel of the opposite side, and 

 the ventral and dorsal, become filled as well. On opening a leech 

 a well-marked systole and diastole can be seen in the vessels above 

 described ; and as no heart can be detected, we must conclude that 

 these vessels, which are so much larger in diameter and thicker in 

 their parietes than either the dorsal or ventral trunks, must perform 

 the function of propelling the blood through the system, in which re- 

 spect they are analogous to the dorsal vessel of insects. Dr. Raw- 

 lings Johnson counted the pulsations, and found that in the first 

 minute there were ten, in the second nine, and in the third eight ; 

 they then became irregular and indistinct, and the leech died. 



On a careful examination of the lateral vessels by the microscope, 

 the truth of this assertion is fully borne out by the structure then 

 displayed. If a small portion of one of these trunks be made the 

 Subject of investigation, it will be found that its parietes are to all ap- 

 pearance highly muscular, it is composed in fact of two sets of bands, 

 arranged like the rings of a trachea, which take an oblique direction 



* Journal de Physique, 1782, p. 287. 



