HIRUDO. 39 
tem, allowing that to be the interpretation ; nor are my doubts altogether 
dispelled. 
Animals may be identified with others from general resemblance, 
or the peculiarities of their parts ; but it is the constant subject of regret 
that in too many descriptions and representations, the obscurities and the 
fallacies utterly preclude recognition of the truth. 
It is quite obvious that the vast majority of the leeches offered in 
illustration have been delineated from dead, preserved, or rather decay- 
ed specimens, or from those half-starved by injudicious treatment. 
The generic character may be said to consist in the presence of the 
posterior sucker ; the specific character, in the absence, the presence, the 
number and position of the eyes ; but of any one of all these the skilful 
naturalist will avail himself. 
The anterior extremity is either a flattened disc, or merely a ten- 
dency to an obtuse triangle. Such a condition appears in the octo-oculata, 
the medicinalis, and sanguisuga, the complanata and stagnalis, but often 
very indefinite. Or the disc is lanceolate, as m those of which we have 
still to speak, the ¢essellata, vitrina, and flava. 
I could not discover eyes in any of the four marine species, wdonella, 
grossa, muricata, vittata. 
In the fresh-water leeches, especially the subject of examination, 
the eyes have been as follows :— 
Two on the front of the head—Hirudo bin-oculata—Hirudo flava. 
Six in three pairs on the front of the head—Hirudo complanata. 
Eight set around the margin of the head—Hirudo octo-oculata. 
Hight set in four pair in the middle of the anterior surface—Hirudo 
tessellata—Hirudo vitrina. 
Ten set around the anterior margin—Hirudo medicinalis—sangui- 
Suga. 
These are the only arrangements I have seen in twelve different 
leeches occurring to me in Scotland. 
Although both extremities possess the faculty of fixture, it is chiefly 
predominant in the posterior organ,where the adhesion is generally very 
powerful if the subject be vigorous. 
