OF ARTS AND SCIENCES. 81 



ferent points, so that the iiiimi>er corresponding to the dorsal half 

 of a ring is not the same as that of tlie ventral half. Gratiolet, 

 who was the first to emphasize the importance of a well-defined 

 starting-point in counting, recommended the unconventional and ex- 

 tremely awkward method of beginning with the ring in which the 

 last pair of nephridial pores is found, counting from this point for- _ 

 Avard. This is evidently an unnatural method, adopted under the 

 persuasion that no more convenient fixed point could be found. 



The simplest method, and the one least liable to confusion, seems 

 to be tliat which I have followed in my diagrams, the first pair of 

 eyes forming the starting-point. Each ring then has a definite num- 

 ber and precise relations, and homologous rings are easily recognized 

 in different species. For reasons that will be made clear elsewhere 

 by a comparison of different species, it is certain that the first three 

 pairs of eyes in Hirudo mark three successive rings. Beginning with 

 the first pair of eyes, we find the fourth and fifth pairs on the fifth and 

 eighth rings respectively. This simple arrangement of the eyes, which 

 is only slightly modified in the land leech (Htemadipsa), holds good 

 not only for Hirudo, but for ILTemopis, Aulostoma, Macrobdella, and all 

 the more closely allied genera. From the fifth pair of eyes onward, 

 the counting is rendered more easy by the size of the rings, and by 

 the metameric arrangement of the color-mai'kings and the papillae. 



It is certainly very desirable that species l)elonging to closely re- 

 lated genera should be described on a common plan ; and I know of 

 no better method than the one here proposed. It is quite certain that 

 no well-marked ring exists in front of tlie first pair of eyes. There 

 are here, to be sure, in some species, obscure traces of what, in the 

 opinion of some observers, might be regarded as one or two rings. 

 '\^ hile it is important to take note of all such evidences of rudimentary 

 rings, it is certainly advisable, for the sake of uniformity, to discard 

 them in countin<r. 



Figure 1 is designed to show all the important external characters 

 which are tvpical of the eenus Hirudo. The first rin<r of each somite 

 besrs the segmental papilla?, and their homologues, the eyes ; the 

 papillate rings, as we may call them, show us precisely how many 

 somites are represented between the first pair of eyes and the pos- 

 terior sucker (acetabidum). The papilla; on the sucker are not usu- 

 ally sufficiently distinct to form a reliable guide to the number of 

 somites represented in this part, and may therefore be left out of 

 account. The following definition of the genus Hirudo will serve as 

 a convenient standard of comparison. 



VOL. XX. (n. S. Ml.) 6 



