326 



Anndides. 



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 ex- 

 aminations 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 di- 

 rectly asserts, either in the horse-leech, or in the medicinal species, 

 which has enlargements at every spot corresponding with each gan- 

 glion of the nervous cord. And although I have killed individuals 

 of both species, by means of alcohol, both before and after the inte- 

 gument 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 en- 

 veloped in a strong sheath, and in this sheath the blood-vessel ap- 

 pears 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 sys- 

 tem 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, but 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 T 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 



