tal apparatus of the medicinal leech "was rather a provision intended to render these creatures subservient to the alleviation of human suffering, than necessary to supply the wants of the animal itself?"
Blood.—The blood of the horse-leech is of a red colour, and when examined microscopically is found to consist of a reddish liquid, or liquor sanguinis, containing disks, which are very scantily diffused throughout the liquor sanguinis, and present a strange contrast, both in point of number and figure, to those of the human subject or any vertebrate animal. A person unskilled in microscopic observation would readily overlook them. They are for the most part of a circular figure and of a grey colour, and vary considerably in size; the average diameter of the most common disk is about the 16000 of an inch; some few are to be seen of an oval figure, these are generally about the 14000 of an inch in their long by 15000 in their short diameter. Now and then larger disks are to be observed, which present a granular appearance; these are not at all constant in size, some being as large as the 11000 of an inch, whilst others, which are no doubt of the same character, are not larger than the 14000. The disks are figured, of their relative dimensions, at g, p. 92.
These disks, soon after their removal from the body, become very indistinct; and when the blood has been suffered to remain at rest, it will undergo a species of slow coagulation, and if examined in this state, small patches of the red colouring matter will be found aggregated together in small masses. The coagulation, however, is exceedingly slow, which is to be accounted for by the small quantity of fibrine contained in the liquor sanguinis. The colour of the blood appears darker in the vessels belonging to the venous than in those of the arterial system, and M. Derheims states[1] that if equal quantities of the two kinds of blood be placed on glass, side by side, they are readily to be distinguished by their shade of colour.
The blood, then, of this animal, as well as of many of the Annelida, differs from that of any of the Invertebrata lower in the scale of creation in being of a red colour, and from that of any of the Vertebrata in not having the colouring matter contained in the envelope of the disks, but diffused generally throughout the liquor sanguinis.
Vessels.—The blood which we have just described circulates throughout the body in a peculiar series of vessels, which can be to-
- ↑ Hist. Nat. des Sangsues; 8vo. Paris, 1825.
