312 ORDER SUCTORIA; LEECH. 



cartilaginous bodies, usually called teeth, but more properly jaws, 

 are seen to be disposed around it, in such a manner that the 



FIG. 532. HIRUDO OFFICINALIS. 



three edges form three radii of a circle. Each of these has two 

 rows of minute teeth at its edge, so that it resembles a small 

 semi-circular saw. It is imbedded at its base in a bed of 

 muscle, by the action of which it is worked, in such a manner as 

 to cut into the skin, a sawing movement being given to each 

 piece separately. It is in this manner that the tri-radiate form 

 of the leech-bite is occasioned ; each ray being a separate little 

 saw. The lacerated character of the wound is very favourable 

 to the flow of blood ; which is further promoted by the vacuum 

 created by the action of the sucker. The alimentary canal is 

 straight ; but is furnished, at its posterior portion, with a large 

 number of little sacs, or caeca, opening from it. The operation 

 of digestion is extremely slow, notwithstanding the rapid and 

 excessive manner in which the Leech fills its stomach ; a single 

 meal of blood will suffice for many months ; nay, more than a 

 year will sometimes elapse, before the blood has passed through 

 the alimentary canal in the ordinary manner, during all which 

 period so much of the blood as remains undigested in the sto- 

 mach continues in a fluid state. Leeches are furnished with 

 eight or ten simple eyes, which may be detected by the aid of 

 a magnifying-glass, as a semicircular row of black points, situated 

 above the mouth upon the sucking surface of the oral disc ; this 

 position is evidently calculated to render these organs of use in 

 the discovery of food. Each of these visual specks would seem 

 to be merely an expansion of the extremity of a nerve, derived 

 from the cephalic ganglia, spread out beneath a kind of cornea 

 formed by the delicate and transparent cuticle, and having 

 behind it a layer of black pigment ; nothing like a crystalline 

 lens can be discovered ; so that the vision of these animals must 



