ON ANNELIDA. 129 



exterior angles proceed the extremely fine threads, to distri- 

 bute themselves to the different parts ; the last ganglion sup- 

 plies those which proceed to the posterior disk. 



The leeches feed on animal or vegetable matter, and on 

 the juices or substances of the former only, according to the 

 species. The medicinal leech, when feeding by suction, fas- 

 tens itself by the posterior disk, and having selected the spot 

 to bite, applies the anterior disk to it, which acts like a 

 cupping-glass ; the mouth is then advanced, its lips widened, 

 and the three dentiferous tubercles which carry the hooks are 

 erected and stiffened by a strong contraction of their muscu- 

 lar tissue. By the alternate contraction and slight expansion 

 of these tubercles, a combined action of pressure and rubbing 

 of the hooks is produced, and the smaller blood vessels thus 

 become ruptured, and the wound, slight as it is, some- 

 times produces a degree of inflammation, which would not 

 be likely to result from a simple cut produced by a smooth- 

 edged instrument. 



On opening the leech shortly after it has gorged itself with 

 the blood of its prey, it will be found that none of this blood 

 has passed into the intestines. 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 in- 

 testines in the ordinary manner, during all which period so 

 much of the blood as remains undigested in the stomach con- 

 tinues in a fluid state, and as if just taken in, notwithstanding 

 the vast difference in the heat of the body of a mammiferous 

 animal and that of a leech. 



The species which swallow animal matter in an entire state 

 take it only when alive or very recently dead. These do not 

 present the singularities in the structure of the oesophagus 



VOL. XIII. K 



