The Ilirudiuea of Illinois. 507 



than b 6 and the secondary furrows are more nearly equal 

 to the primary. 



XI is a typically complete somite and presents characters 

 which are found with some slight individual variations as far 

 back as XXIV inclusive. In all of these the annuli are very 

 clearly indicated both on the dorsal and ventral surfaces. 

 ^ 1 is divided equally into h 1 and b 2, which are quite short 

 rings unmarked by any papillie ; a 2 is very distinctly divided 

 into a long 63, which bears a large median cutaneous papilla 

 and the three pairs of metameric sensillae, and a 6 4, of half 

 the size or less, which is the smallest annulus of the somite 

 and bears no papillae; a 3 is cut into h5 and 6 6, of which 

 the former is the largest annulus of the somite, both in length 

 and breadth, and which bears a large papilla similar to that 

 on 6 3, while 6 6 is a simple unadorned ring equal to 6 1 or 

 6 2. Thus each complete somite is divided into six annuli of 

 unequal size, and, leaving out of sight the limits of the 

 somites, this region of the body exhibits a regular alternation 

 of groups of three equal small rings with groups composed 

 of two large rings separated by a very small one. Each 

 of the large rings is marked by three cross ridges which 

 appear to be due to transverse muscle bands, while the 

 narrow rings present only two such ridges, 6 4 sometimes 

 only one ridge. 



The cutaneous papilla? (PI. XL 1 1 1., Fig. 8) are rather con- 

 spicuous conical elevations with the long axes of their ellip- 

 tical bases directed longitudinally. In the middle region they 

 are strictly confined to the annuli indicated, but posteriorly 

 somewhat overlap the succeeding secondary annuli. On 

 other specimens they will probably be found to begin further 

 forward than on the type, as opaque thickenings of the integu- 

 ments appear in the proper regions of somites X and IX, 

 though there are no elevations. 



Of the dorsal sensilUiB (PI. XLIIL, Fig. 8) the supra- 

 marginals cannot be distinguished, but the dorso-marginals 

 {dm), dorso-laterals (r//), and dorso-medians {md) are obvious, 

 and are very regularly arranged on all somites from V to 

 XXVI. Peripherad of these points they have not been traced. 



