ANNELIDA. 



193 



rendered quite tense, the sharp serrated edges of the teeth are 

 pressed firmly against it, and, a sawing movement being given to 

 each cartilaginous piece by the strong contractions of the muscular 

 fibres around the neck, these instruments soon pierce the cutis to 

 a considerable depth, and lay open the cutaneous vessels, from 

 which the creature sucks the fluid which its instinct prompts it to 

 seek after with so much voracity. The position of the teeth 

 around the opening of the mouth, as represented in the subjoined 

 figure, (fig- 79, A,) will at once explain the cause of the tri-racliate 

 form of the incision which a leech-bite invariably exhibits. 



On contemplating Fig. 79. 



this singular dental ap- 

 paratus found in the / \ 

 medicinal leech, and 

 considering the na- 

 ture of the food upon 

 which it usually lives, 

 it is difficult to avoid 

 arriving at the conclu- 

 sion that such a struc- 

 ture, which is indeed 

 only met with in one 

 or two species, is ra- 3 A 

 ther a provision in- 

 tended to render these creatures subservient to the alleviation of 

 human suffering than necessary to supply the wants of the animals 

 themselves. In the streams and ponds which they usually in- 

 habit, any opportunity of meeting with a supply of the blood of 

 warm-blooded vertebrata must be of rare occurrence, so that 

 comparatively few are ever enabled to indulge the instinct 

 which prompts them to gorge themselves so voraciously when 

 allowed to obtain it : neither does it appear that the blood 

 which they swallow with so much avidity is a material properly 

 suited to afford them nourishment ; for although it is certainly 

 true that it will remain for a considerable time in its stomach, 

 without becoming putrid, yet it is well known that most fre- 

 quently the death of the leech is caused by such inordinate reple- 

 tion, provided the greater portion of what is taken into the body 

 is not speedily regurgitated through the mouth. 



(236.) The internal digestive apparatus is evidently adapted in 

 the construction of all its parts to form a capaciovis reservoir for the 



