4IO WHITMAN. [Vol. IV. 



ject of special remark merely because they happen to touch the 

 case in hand. 



Let us now consider briefly in what particulars these descrip- 

 tions fail to meet our needs. First of all they are not accom- 

 panied with any figures, and without such aids it is often 

 extremely difficult, if not impossible, to get at the author's exact 

 meaning, however skilful he may be at depicting. The oral 

 sucker in these species presents a number of important diag- 

 nostic characters, not one of which is mentioned. The position 

 of the mouth varies for different species, but it is not alluded to. 

 Important specific distinctions are to be found in the number 

 and condition of the rings represented in the head, and espe- 

 cially in the buccal and post-buccal rings, all of which are 

 ignored. Nothing whatever is said about the number of rings 

 and segments in the animal, and the peculiarities of the abbre- 

 viated posterior somites pass unnoticed. The metameric sense- 

 organs were not then known, and although conspicuous enough, 

 escaped notice. The nephridiopores are neither numbered nor 

 otherwise defined, and the genital pores are not so much as 

 mentioned. All this for the external features. Not one of the 

 internal features received any notice, not even the diverticula 

 of the stomach. 



Some time ago I attempted to show that a satisfactory basis 

 for the classification of the ten-eyed leeches (Hirudinidse) was 

 to be found in the external metamerism. I shall now show that 

 the same method may be extended to the Clepsinidae. The 

 determination of the number and homology of the rings is often 

 more difficult here than in the Hirudinidae ; but still it is always 

 possible, I believe, and it affords the only safe basis for distin- 

 guishing genera and species. It is the remarkable constancy of 

 these metameric characters that gives them such high value for 

 diagnostic purposes. With metamerism as a basis, all the external 

 features are readily defined topographically, and with a precision 

 and definiteness that cannot otherwise be attained. The first 

 thing to be done in describing a new form is, therefore, to 

 determine with all possible precision the number of the rings 

 and somites. This task is comparatively easy except at the 

 two extremities of the body, where, it must be confessed, it is 

 often quite difficult to decide upon the number of rings. This 

 is more especially true of the head region. Here it is often 



