53 



An Account of the Circulation of the Blood in the Class Vermes ofLin- 

 nteus, and the principle explained in which it differs from that in 

 the higher Classes. By Sir Everard Home, Bart. V.P.R.S. Read 

 November?, 1816. [Phil. Trans. 1817, p. 1.] 



The circulation of the blood in these animals is different from that of 

 any other animal known to the author. There is one point to which 

 all the blood is brought, and from which it is again emitted to all 

 parts of the body. To this the author gives the name of heart, al- 

 though it be not, in fact, the principal agent in carrying on the cir- 

 culation, and although so small as hardly to deserve the name of 

 ventricle. It is situated directly in the middle line of the belly, and 

 is that point in a vessel which comes from the head, where the rest 

 of the blood is received from two auricles that lie above it at the back 

 of the animal, one on each side. 



Sir Everard, after describing the unsuccessful attempts which he 

 had made to discover the arrangement of the vessels by transparency 

 of the body of the animal in full sunshine, acknowledges that he is 

 indebted to Mr. Clift for the means of detecting the course of its cir- 

 culation, by steeping it in vinegar, which presently coagulates the 

 blood in the vessels, and gives it a deep black colour. 



From the central point before mentioned, the artery proceeds down 

 towards the tail, giving off branches by pairs to the bronchiae as it 

 passes, which may be termed pulmonary arteries. From these the 

 blood is returned to a single vein, which passes up the middle of the 

 back to the head. From the external situation of the bronchiae, the 

 circulation of the blood in them is readily seen, and is described not 

 to be simultaneous in adjacent pairs of them, but successive from the 

 head backwards. The blood propelled forward by these toward the 

 head, is returned from thence by the longitudinal vein along the 

 belly to the original centre of motion. 



In the mean time, another portion of the blood which has supplied 

 the viscera and skin, is returned by two large veins that pass up on 

 each side of the animal, and is by them delivered to the two auricles 

 mentioned as placed in the middle of the back. 



The great peculiarity in the circulation of this animal, appears to 

 consist in the muscular structure of .the branchial vessels. Of these 

 there are as many as thirteen pair, which appear abundantly suffi- 

 cient, in proportion to the size of the animal, for the aeration of the 

 blood, as well as for propelling it forwards. 



In the Lumbricus terrcstris the circulation of the blood, and the 

 mode ha which it is aerated, the author says, are very different. A 

 large artery passes along the belly, giving off lateral branches, and a 

 corresponding vein runs up the back, with its branches on each side. 

 But the two great trunks also communicate laterally through five 

 pair of reservoirs, filled from the vein, and emptying themselves into 

 the artery. On each side of the great vein on the back, are also a 

 row of vesicles with external openings, for the purpose of aerating the 

 blood, as showu in the drawing intended to accompany this paper. 



