NO 1160. LEECBES OF THE U. S. NATIONAL MUSEUM— MOORE. 545 



In the HirudinidflB generally this tendency is complete, and the five 

 annuli resulting are i)ractically equivalent so far as size, etc., is con- 

 cerned, al + 13 + Z/4 + 65 + hG or «1 + 63-6. TrachellohdeUa has all 

 three of the primary annuli subdivided, thus : 61 + 62 + 6.'J + 64 -|- 65 + 

 66 or more simply 61-6. But in some of the species the divisions are 

 incomplete, while in others those of the third order have set in, facts 

 which may be expressed by the use of brackets, as shown above for 

 Ilamentaria. In Dina the third actual (fourth secondary) annulus is 

 widened and distinctly bi-annulate, expressible thus: al-|-63+64 (c7 

 c8) + 65-|-66. The greatest complexity is found among the Ichthyob- 

 dellida', of which Cystobranchufi h-ds the six secondary annuli, the third 

 or sometimes the fourth being subdivided. 61 + 62 + e5 + rO + 64 + 

 65 + 66 or 61-2 + c5-6 + 64-6. Piscicola varies somewhat, but the most 

 frequent arrangement is that in which the full number of annuli of the 

 third order is developed, and two of these, namely, co and c8, are divided 

 into annuli of the fourth order, making in all fourteen annuli, expressed 

 by the formula cl-4 + d<) + dlO + r-G + c7 + ^15 + f716 + c9-12. In some 

 si)ecies the fourteen annuli become perfectly equivalent in size and the 

 plan of their formation obscure. No cases are known in which the 

 whole twenty-four of the possible annuli of the fourth order are devel- 

 oped, or in which annuli of the fifth order are more than very slightly 

 indicated. 



Partial or complete unions of adjacent annuli of neighboring somites 

 frequently occur, and possibly entire somites may be absorbed, or 

 simulati(jns of new ones formed in the prostomial region. All of 

 these conditions can be expressed in the formula, which could also 

 be adapted to indicate whether any given condition has arisen by sim- 

 plification or elaboration. The desirability of some more exact method 

 of defining the annulation of the Ichthyobdellidte must be obvious to 

 anyone who has noted the great confusion which reigns in this group 

 as to the number of annuli of each somite, and the scope of the genera. 

 Different authors have each usually attended to but one of the several 

 orders of division of the somite, and thus we have Piscicola {Ichthyob- 

 della) described with seven, twelve, or fourteen rings, each of which 

 expresses a part of the truth. 



By combining the somite formula with the Roman numerals by 

 •which the individual somites are indicated, we can describe any annu- 

 lus desired with the greatest precision. As to the order of the elab- 

 oration of the annuli in the Ichtliyobdellidie I have little light, except 

 that the process begins in the middle primary annulus, and there also 

 proceeds the farthest. There are good physiological and mechanical 

 reasons for this; but I hope soon to have sufficient data for a fuller dis- 

 cussion of the external morphology of this family. This preliminary 

 account is presented here in the hope that students of the Hirudinea 

 will find this sclieme of sufficient value to test and perfect it. The 

 systematic portion of the paper follows. 

 Proc. N. M. vol. xxi 35 



