Annelides. 21 



wards, as in the act of biting with great force and rapidity ; one mus- 

 cle appears to be continuous with that of the oesophagus, and is 

 attached to one extremity of the jaw, whilst another is attached to 

 the opposite end, so that by their alternate contraction and dilatation 

 the jaws have a semicircular movement. 



But the dental apparatus of the medicinal leech is much more 

 marked than in this species, and this fact, combined with the striking 

 differences in their alimentary canals, affords a most satisfactory ex- 

 planation of the reason why the horse leech is unfit for medical pur- 

 poses, for instead of the fifteen blunt molar-like teeth with which the 

 latter is provided, the jaws of the medicinal species are much larger, 

 more convex, and of very much firmer consistence ; each jaw is pro- 

 vided with a row of teeth, varying from sixty to eighty in number ; 

 their points or apices are much sharper than those of the ^^lest needles 

 (fig.^); they are likewise strongly imbedded in the cartilaginous matter 

 of the jaw: in the horse leech, on the contrary, the teeth appear to 

 be loosely connected to the upper part of the jaw, for when a leech 

 has undergone slight decomposition the little calcareous teeth readily 

 separate from it, and, on examining the mouth and squeezing it, the 

 three sets of teeth will be found quite detached, being only entangled 

 in the mucous matter, which is always very abundant at this part ; 

 they can then be washed in water and placed on glass, and mounted 

 as microscopic objects. 



I cannot forbear mentioning, in this place, an error which is likely 

 to be committed in describing the shape of the teeth: if the jaws of 

 the medicinal leech be examined, when laid on their sides, the teeth 

 appear like so many sharp-pointed canines, imbedded, like the teeth 

 of Mammalia, to half their depth in the cartilage of the jaw, all that 

 part projecting above the margin of the jaw being of a darker colour 

 than the remaining half which appears imbedded in it : that this is 

 deceptive may be at once proved by turning the jaw in such a man- 

 ner that the cutting edge may be seen (fig. h) ; the teeth then will be 

 found to be of precisely the same figure as those of the horse leech, 

 (fig- /)> the only differences being that they are more numerous and 

 much sharper than in the latter animal. Having fallen into this error 

 myself, and only by mere accident discovered the mistake, I have 

 thought proper to caution others : Moquin Tandon has represented 

 two rows of teeth in each jaw of the medicinal leech, and makes 

 each tooth pointed like a canine ; in this he is certainly wrong. 



John Quekett, 



(To be continued). 



