No. 3] DESCRIPTION OF CLEPSINE PLANA. 411 



necessary to resort to various expedients. It is well to have 

 several individuals killed in weak chromic acid {\ to \%)y in an 

 extended condition. If the leeches do not die extended, they 

 may easily be straightened by stretching a little. After lying 

 in the acid a short time, the rings usually become sufficiently 

 well defined to admit of study and comparison. It is advisable 

 always to supplement this study with that of the living leech. 

 Sometimes I have found it necessary to cut off the head and 

 study it in all positions while it is contracting and expand- 

 ing. For such study, a good dissecting microscope {e.g. that 

 of Zeiss) is indispensable. The metameric sense-organs enable 

 one to fix the limits of the somites, and are thus a most impor- 

 tant guide in the analysis of the head region. Having deter- 

 mined the composition of the first two or three somites, the 

 next important point to settle is the number of the ring which 

 forms the posterior boundary of the oral sucker. This buccal 

 ring is sometimes united either above or below with the preced- 

 ing or following ring, and hence both the buccal and the post- 

 buccals require careful examination from all sides and in all 

 states of contraction and extension. The ocular ring is fre- 

 quently a double ring, i.e. two rings more or less consolidated. 

 In order to describe accurately such rings, it is necessary to 

 note the relative width of the successive rings, particularly in 

 the head and the posterior extremity of the body. The method 

 of numbering rings and somites is shown in Figs, i and 3, 

 PI. XV. 



My chief reason for offering the following description at this 

 time is the fact that I have had to refer to this leech by name 

 in my paper on spermatophores, and it seems desirable to 

 show that the name stands for a reality. The description may, 

 however, be regarded as a preliminary one, inasmuch as I hope 

 to be able to describe this in connection with the more closely 

 allied species, and to furnish with the descriptions the much- 

 needed colored figures. The preparation of such figures, with 

 due attention to all the details which require to be accurately 

 reproduced, is already in progress. 



Clepsine plana, ;/. sp. 

 The largest of the five specimens obtained measured as 

 follows : — 



