The Hirudinea of Illinois. 501 



the last triannulate somite, lacks this subdivision, but it exists 

 on all other complete somites. 



XXIII, in addition to the differences of papulation already 

 mentioned, shows peculiarities in annulus ad, which is the 

 smallest of all and is differentiated from a 2 at the margins 

 only. 



On XXIV the complete individuality of all three annuli is 

 suppressed, their boundaries being evident only at the 

 margins. On XXV only the marginal notches a 2/a 3 are 

 indicated. 



XXVI and XXVII are uniannulate and contracted to form 

 the supporting pedicle of the sucker. 



The arrangement of the papilla: is as follows : The median 

 series (mp) begins in small isolated papillae of about equal 

 size, usually on VI a 2 and a 3. By about IX they are well 

 established on all three annuli, a 3 bearing the largest, a 2 

 the next in size, and a 1 a very small one, and this relative 

 size is maintained throughout the series. A few somites 

 caudad they are higher, more prominent, crowded, and 

 elevated on a ridge. Nearing the posterior end the ridge 

 increases in height and the papilla on a 2 and a 3 increase in 

 size, but the crowding becomes relieved and the papillae on a 1 

 smaller or more frequently absent. Both the ridge and its 

 papilla: cease completely and abruptly on XXII a 3. 



The paired carina? (dip) are slightly nearer to the margins 

 than to the middle line. In essentials they are precisely 

 similar to the median one, but present the following special 

 features : the carina? as a whole are slightly less prominent ; 

 they require a greater distance in which to become fully 

 established, i. e., the papillae are not supported on a con- 

 tinuous ridge, nor present on every annulus until about 

 XII or XIII ; scattered papilla? of small size are found on a 1 

 and a 2 as far forward as VI, but they are very small ; in the 

 region of full development the papilla on a 2 is by far the 

 largest of this series and generally larger than any of the 

 middle series, and the papilla on a 1 is very small and 

 frequently absent, especially in the posterior region. The 



