The Hirudinea of Illinois. 489 



median and the supra-marginal, and the more characteristic 

 papillae belonging to this ring. .4 3 is about half the 

 length of the larger double annulus, bears its characteristic 

 papillae, and forms the margin of the posterior rim of the 

 sucker. 



V is triannulate dorsally, a 2 is slightly wider than a 3, and 

 the latter than a 1, but all furrows are equally well developed, 

 until at the margin a l/# 2 suddenly disappears, leaving two 

 ventral annuli of approximately equal size. 



Eighteen somites (VI to XXIII) are completely triannulate. 

 A curious feature of these (PL XLII., Fig. 3) is that the 

 relative lengths of the annuli differ dorsally and ventrally ; 

 above, a 2 is the largest, a 3 slightly less, and a 1 obviously 

 the smallest; below, a 2 is the shortest, and a 3 just appreci- 

 ably shorter than a 1. This is readily explained by compari- 

 son of the relative positions of the furrows. Along the margins 

 of the body is an exceedingly thin expansion, crenulated in 

 correspondence to the annuli. Set in the emarginations be- 

 tween successive lobes are narrow wedge-shaped pieces which 

 in the contracted leech assume the position of more or less 

 vertical folds uniting the dorsal and ventral furrows. When 

 the leech is extended these pieces of course become horizon- 

 tal, and the dorsal and ventral furrows assume the following 

 relative positions ; a S/a 1 is ventrally in advance of its dorsal 

 part, a l/a2 is ventrally slightly behind, and # 2/« 3 again 

 ventrally in advance. This lack of alignment characterizes 

 all furrows from about VI/YII to XXIV/XXV. 



On the dorsal surface some very faint transverse wrinkles 

 pass between the principal papillae and are about equally 

 apparent on all of the annuli. Ventrally a 1 and a 2 may be 

 more or less divided, the portions toward the ends of the 

 somites being the smaller ; a 2 very rarely shows faint traces 

 of such a division. In this feature and in the rough cutaneous 

 papillae the species resembles Hcementeria officinalis. 



The disposition of the papillae on a typical complete somite 

 is shown in the figure (PI. XLII., Fig. 3), and is as follows: 

 On the dorsal surface of each annulus are two rows of small 

 smooth sense papillae separated by the transverse wrinkles 



