HIRUDO. 39 



tern, 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 in those of which we have 

 still to speak, the tessellata, vitrina, andyfom. 



I could not discover eyes in any of the four marine species, udonella, 

 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 flam. 



Six in three pairs on the front of the head Hirudo complanata. 



Eight set around the margin of the head Hirudo octo-oculata. 



Eight 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. 



