Page:The Zoologist, 1st series, vol 1 (1843).djvu/354

326 vessel, and others have described it as merely a sheath for the nerve; and even at the present day anatomists are not agreed on this point. Knowing the discrepancies in their opinions, I have been induced to pay particular attention to this part of the anatomy of the horse-leech, and to that of a few others of the class Annelida as well, and my examinations lead me to maintain a different opinion from any of the authorities above alluded to. I cannot satisfy myself that there is a pulsating vessel surrounding the cord, as Dr. Rawlins Johnson directly asserts, either in the horse-leech, or in the medicinal species, which has enlargements at every spot corresponding with each ganglion of the nervous cord. And although I have killed individuals of both species, by means of alcohol, both before and after the integument has been removed, I have not been able as yet to make out this point to my satisfaction; but I find that the nervous cord is enveloped in a strong sheath, and in this sheath the blood-vessel appears to run, and it requires some little care not to confound one with the other. This agrees with observations which have been made by my friend Mr. Goadby, in his beautiful dissections of the nervous system of the king crab, (Limulus Polyphemus). In this animal it was extremely difficult to distinguish nerves from blood-vessels; but by filling the vessels with a mixture of size and vermilion, by means of a syringe, the difference between the two was rendered very manifest.

In the earth-worm I have carefully examined the nervous cord, and find an appearance as if there were two blood-vessels, one on each side of the cord, with a transverse connecting vessel every here and there, and in some places there appears to be a vessel in the middle, between the two component columns of the cord; and as the nerves are given off from the ganglia, a vessel runs between them for some distance. The examination of the nervous cord of a leech, in situ, with a pocket lens, shows two black lines, one on each side of the cord, which look like blood-vessels; but I have frequently placed a small instrument underneath the cord, and lifted it up, viewing it at the same time with a lens, so as to be able to see if the blood would dilate the sinus around any of the ganglia, bat such has not been the case. Portions of the cord cut off, dried, and mounted in Canada balsam, present an appearance of a blood-vessel on each side; but such appearance I consider due to the gravitation of the blood to the sides of the vessel in drying.

In addition to the general circulation in four trunks, as just alluded to, there has been described by some authors a lesser circulation, as