The Hirudinca of Illinois. 501 



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

 on all other complete somites. 



XXIII, in addition to the differences of papillation already- 

 mentioned, shows peculiarities in annulus <i 3, 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 XXY only the marginal notches (f 2/a 3 are 

 indicated. 



XXYI and XXYII are uniannulate and contracted to form 

 the supporting pedicle of the sucker. 



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

 series {:m])) begins in small isolated papillcB of about equal 

 size, usually on VI « 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. Hearing the posterior end the ridge 

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

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

 smaller or more frequently al)sent. Both the ridge and its 

 papilla; cease completely and abruptly on XXII a 3. 



The paired carina; {(Up) 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 



