HIRUDO. 23 



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 



