80 rnOCEEDINGS of the AMERICAN ACADEMY 



such as are supposed by the writer, in common with many others, to 

 exist between annelid worms and vertebrates. At all events, the exist- 

 ence of such organs in tlie leech furnishes a broader basis for the dis- 

 cussion of the question, whether the vertebrates and annelids have been 

 derived from a common form possessing metameric sense-organs, as 

 was first argued bj' Dr. Eisig of the Naples Station. Assuming that 

 the sense-organs of the lateral line of the vertebrate and the segmental 

 papilla) of the leech may be traced to a common origin in some remote 

 ancestral form, it does not follow that they should now present dose 

 structural resemblances. It is far more important to show that they 

 possess certain general features in common. The most important of 

 their common features is undoubtedly their metameric origin. The 

 nerve-supply forms another feature of fundamental importance, in 

 which, according to the interesting observations of Mr. Beard, on " the 

 segmental sense-organs of the lateral line" (Zool. Anz., VIL, Nos. 161 

 and 1 62) of the vertebrate, there is essential agreement. The develop- 

 mental history of these lateral organs in the fish, where they make 

 their first appearance as segmental papilla; in the strictest sense of these 

 words, cannot at present be explained on a more satisfactory hypothesis. 



The fact that the eyes of the leech are metameric sense-organs 

 representing merely structurally improved forms of the segmental 

 papilliB, will be placed in still clearer light by the following study of 

 the abbreviated somites. 



Hirudo and Allied Genera. — In order to arrive at satisfactory con- 

 clusions respecting the characters which distinguish the genus Hirudo 

 from allied genera, we must make a thorough study of the rings and 

 somites composing the body. The importance of this first step in a 

 compaiative study of genera will be seen as we proceed. 



The obscurity that is supposed to exist in regard to the precise 

 number of rings in the ceiihalic lobe and in the hind end of the body, 

 affords no excuse for the meagre descriptions usually given of these 

 reirions ; but furnishes rather an argument for describinij them with 

 the utmost care and detail. The diUiculties in the way of counting 

 the rings has been greatly overestimated. It is only necessary to 

 adopt some method of counting that can be followed in the different 

 genera. Some authors count the rings from the ventral side, begin- 

 ning with the buccal ring (5th and 6th in my figure), and take no 

 account of the rudimentary anal (or post-anal) ring: thus counted 

 there would be only 9o rings, which is the number usually given for 

 Hirudo medicinalis of Europe. According to another mode of 

 counting, the rings are counted from both sides, but from two dif- 



