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 
extiemely 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 cesophagus 
VOL. XIII. K 
