THE LEECHES OF JAPAN. 359 
which assist in the determination of species. I have satisfied 
myself that not only the number and position of the rings, but 
the relative size and general appearance of each ring even to 
very minute details,’ are accurately reproduced in every normal 
individual of a species. The obscurity that is supposed to exist 
in regard to the precise number of rings which enter into the 
composition of the cephalic lobe or the hind end of the body, 
affords no excuse for the meagre descriptions usually given of 
these regions, but furnishes rather an argument for describing 
them with the utmost care and detail. As to the difficulties in 
the way of counting, these are scarcely worth mentioning in 
the various species of Hirudo, or of the allied genera, Aulo- 
stoma, Hemopis, Macrobdella, &c. It is only necessary 
to adopt some method of counting that can be safely followed 
in all these genera. What my own method is, I have made 
clear in the foregoing descriptions ; and it now remains only to 
show its advantages over those proposed by other writers. As 
before pointed out when comparing the Land Leech with the 
Medicinal Leech of Japan, it will not do to follow Moquin- 
Tandon, Diesing, and others in counting from the ventral side, 
for some of the more important rings are not seen from this 
side; and the dorsal aspect of some rings, particularly the 
buccals and post-buccals, differ very much from the ventral. 
Besides, the abbreviated somites can only be clearly described 
by an accurate study of the dorsal side; and it is here that the 
sense-organs attain their highest development, and the colour- 
markings their more important diagnostic distinctions. The 
total number of annuli, the position of the sexual orifices, the 
nephridial pores, and the segmental papille, must therefore all 
be determined by reference to the dorsal side, the differences 
between this side and the ventral being noted wherever 
necessary. 
A still more objectionable method is that of counting the 
annuli from the anterior end, but from two different points, 
one on the dorsal the other on the ventral side. Thus the 
orgaus of the two sides, being located with reference to two 
* Colour alone excepted, 
