THE LEECHES OF JAPAN. 367 
are easily sawn asunder, such as are found in the mouth and 
the nostrils. 
The genus Hemopis appears thus to rest on an insufficient 
basis ; and, as its rejection will be more consistent with our 
present nomenclature than its retention, I venture to propose 
its reabsorption in the genus Hirudo. 
Does then the number of denticles furnish any guide in 
the determination of genera? Is there any point in the reduc- 
tion of the number of denticles which can be taken as a limit 
to the genus Hirudo? All will agree that there is at least one 
such point; and I think a little reflection will show that there 
‘is only one. Solong as the denticles are sufficiently numerous 
and well formed to enable the Leech to live by sucking bload, 
it is plain that the reduction has not reached a point at which 
the formation of a new genus becomes imperative. When, 
however, the number and efficiency of the denticles have been 
reduced to such an extent that the Leech becomes incapable 
of drawing blood, and is thus compelled to accept a different 
kind of food and to adopt new methods-of obtaining it, it is 
obvious that the boundary line between two very distinct 
courses of life has been passed. The degeneration of the deuti- 
cles has been carried to a point that necessitates a complete 
revolution in habits, and a whole train of correlated morpho- 
logical changes sweep in. Such has been the history of Aulos- 
toma. The climacteric limit in the reduction of the number 
of denticles lies between Aulostoma and Hemopis, and this 
limit is the only one which, in this direction, can be found for 
the genus Hirudo. Between the maximum and minimum 
number of denticles compatible with the life of Hirudo, I can 
see no limit to variation that is entitled to generic rank. Any 
attempt to establish a limit to the genus inside of these ex- 
tremes, must be pronounced irrational, since it makes it impos- 
sible to draw any line between specific and generic distinctions. 
The futility as well as the absurdity of such an attempt has 
been shown in the use that has been made of the genus 
Hzmopis. Various Land Leeches have been referred to He- 
mopis, not on account of any real generic affinity, but simply 
