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, although it be not, in fact, the principal agent in carrying on the circulation, 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 circulation, 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 bronchiæ 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 bronchiæ, 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 sufficient, in proportion to the size of the animal, for the aeration of the blood, as well as for propelling it forwards.
In the Lumbricus terrestris the circulation of the blood, and the mode in 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 shown in the drawing intended to accompany this paper.
