The Hirudinea of Illinois. 491 



between the median and dorso-median rows, two between 

 the latter and the dorso-lateral, and two between this and 

 the marginal. A 2 sometimes bears one internal and one 

 external to the dorso-median sensilla. The marginal sensillaB 

 are small. 



At the posterior end somite XXIII, though triannulate, 

 shows some peculiarities. The median papilla on a 2 is 

 much reduced in size or absent, and on each side of the 

 middle line appears a very large rough wart not represented 

 on" any of the anterior somites. In surface views no trace of 

 the dorso-lateral sensillse can be found, but it is suspected 

 that they have been raised to the top of the large warts just 

 described, where a truncated clear area is sometimes visible. 

 Annulus a 3 is somewhat shortened, but in all other respects 

 this somite is typical. 



On XXIV the same features appear somewhat exaggerated ; 

 the papilla are as described for XXIII, and annulus a 3 is 

 distinctly smaller and mesially joined to a 2. 



XXV, XXVI, and XXVII show successive steps toward a 

 simpler condition. The first is generally divided into two 

 annuli nearly or quite to the middle ; the second, for a greater 

 or less distance from the margin ; and the third, just at the 

 margin or not at all. In all of these cases the anterior 

 annulus is the larger and bears a rather large median wart ; 

 the peculiar pair of warts of XXIII and XXIV is wanting ; 

 and the reduction in size of all other warts leads to the 

 relatively greater prominence of the dorso-lateral series. 



Reproductive Organs. — The genital organs differ in no im- 

 portant respect from those of P. parasitica. The only differ- 

 ence apparent in a number of dissections is that the prostate 

 cornua of this species are rather longer. 



Alimentary Canal. — Only two minor characters in which 

 the digestive tract of this species differs from P. parasitica 

 need be mentioned : the pharyngeal glands have no median 

 annectant lobe; and the cseca are less finely and numerously 

 divided. 



Color. — The colors of living examples of this species gener- 

 ally impress one as a pepper-and-salt mixture of various 



