Annelicles, 91 



tal apparatus of the medicinal leech " was rather a provision intended 

 to render these creatures subservient to the alleviation of human suf- 

 fering, 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 circu- 

 lar figure and of a grey colour, and vary considerably in size ; the 

 average diameter of the most common disk is about the -^^V^r of an 

 inch ; some few are to be seen of an oval figure, these are generally 

 about the ^^y^ of an inch in their long by ■^'■-^ 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 t^W of an inch, whilst others, which are no doubt of 

 the same character, are not larger than the -^^^. 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 aggre- 

 gated together in small masses. The coagulation, however, is ex- 

 ceedingly 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* 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 Anneli- 

 da, 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 Verte- 

 brata 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. 



