HIRUDO. 23 
(ot 
throughout Europe, any notice of its peculiarities may here seem unne- 
cessary. But it is notorious, that many of the less skilful are frequently 
embarrassed in distinguishing the Horse Leech from the Medicinal Leech, 
and that nothing is more common than the substitution of the one, 
which is of no medical use in human distempers, for the other, sometimes 
proving of special benefit. Thus a few general observations on some of 
the characteristics of each, may assist those in case of necessity, who have 
no better means of ascertaining the difference—premising, however, that 
on this, as on all other occasions, my remarks are applicable to those 
animals alone that have been in my own possession. 
The Horse Leech presents a considerable diversity of aspect, accord- 
ing to the individual specimens. I do not know whether this may truly 
indicate the diversity of species ill-defined, whether variety, or to the 
want of uniformity among them, which may be identified. The whole 
tribe undergoes such extraordinary alterations and modifications of form, 
size, and colour, that the observer may be very readily deluded. 
Length above five inches ; figure round ; at rest, or while swimming, 
it is flattened. The body is divided into numerous segments, amounting 
to ninety-eight, according to M. Moquin Tandon, exclusive of the suckers. 
Ten ocular specks are marginal on the anterior extremity, set with 
tolerable regularity. But these must be sought for in younger specimens 
only, for, in adults, they are either quite inconspicuous, or to be disco- 
vered with the greatest difficulty,—easily to be credited in endeavouring 
to apply the microscope to so restless an animal. Amidst the different 
shades of colour, green is always predominant. This is either uniform 
throughout on the back, or speckled with darker lines: it is lighter on 
the belly, which is sometimes yellowish, when the speckling becomes 
more conspicuous. Occasionally the belly is dark-blue. Several slight 
variations frequently occur, so that such modifications are perplexing. 
—Plate III. figs. 1, 2. 
The observer may be farther embarrassed by the successive altera- 
tion of the aspect of some specimens. Leeches of full size are seen almost 
black ; but, if the light fall in a certain direction on the back, two 
variegated lines, still darker, may be discovered. A specimen extending 
