. J . whitman: [Vol. IV. 



as I have before pointed out, seems to have begun at both 

 extremities, and to have advanced from these points towards 

 the middle region of the body. Its advance has been centrip- 

 etal, and the extent of its advance shows how far a form has 

 departed from the ancestral condition of uniform somites. It 

 is here that we discover a very important guide to the systematic 

 rank and relationship of different forms. This is most clearly 

 illustrated in Hirudo and Hsemadipsa. 



It may be well here to call attention to a fact hitherto over- 

 looked ; namely, that metamerism among the leeches has under- 

 gone modification in two opposite directions. Variation by 

 centripetal reduction of the number of rings is universal ; varia- 

 tion by multiplication of rings characterizes, as a rule, only the 

 higher forms, Hirudo, Nephelis, etc. Clepsine rarely exhibits 

 the second mode of variation, and never to the extent that 

 Hirudo does. The difference between the Clepsinidae and the 

 Hirudinidae in this respect has a physiological explanation. 

 Hirudo swims, and for this purpose a long flexible body is 

 required ; Clepsine, with few exceptions, habitually creeps, and 

 for this mode of locomotion, supplementary rings have not been 

 essential. In variation by multiplication we have another means 

 of determining close systematic relations.^ I would not be 



1 I am reminded of an error into which I fell in my paper on Japanese Leeches. 

 The error was the assumption that all somites having less than five rings were abbre- 

 viated. The assumption should have been, as I now feel convinced, that all somites 

 with less than three rings are abbreviated, and all with more than three have been 

 increased by the division of one or two of the three primary rings. I have collected 

 considerable evidence, which cannot be given here, going to show that in the evolu- 

 tion of Hirudo, it was the second and third rings that underwent division, while the 

 first remained undivided. In the Hirudinidae, then, we have supplemented somites 

 (five rings, rarely four), type-somites (three rings), and abbreviated sotnites (0-2 

 rings). The type-somites I formerly regarded as abbreviated. The view here taken 

 helps to understand what before seemed unaccountable, that Hirudo and most of its 

 congeners present three successive somites (4-6) with only three rings each. Allows 

 ing these to be type-somites, we recognize in them a sort of neutral zone, standing 

 between the abbreviated and the supplemented somites. Usually one of these type- 

 somites only is preserved in the posterior region of the body; and sometimes we find 

 this somite already enlarged \.o four rings, by the division of its third nng, as is well 

 shown in the Japanese Leptosoma acratiulatnm. 



Mr. Apathy — who, as I observe, seems to look upon "Some New Facts about 

 Leeches," which I recently published, as worthy of being claimed as his own dis- 

 coveries — advances a different view, according to which the type-somite is supposed 

 to have had twelve rings. A review of his position must be postponed until I can 

 bring forward the evidences which seem to me fatal to such a view. 



