The Hirudinea of Illinois. 531 



Fortunately, in the posterior region of some aquatic indi- 

 viduals from Ohio the sensillre are comparatively distinct, so 

 that the determination of the values of the annuli and of the 

 limits of the somites is accomplished with greater ease and 

 confidence than would otherwise be possible. 



Reproductive Organs. — The reproductive organs also have 

 been described by Forbes, to whose account a few notes may 

 be added. The nerve cord may pass to the right of the 

 genital exits, as described by Forbes, or to the left, as here 

 figured. In two out of three dissections the latter condition 

 prevailed ; but the number is of course insufficient to 

 determine which is the more usual. 



The figure (PI. XLY., Fig. 27) will serve to show the 

 marked contrast in several respects between these organs in 

 H. lateralis and H. marmoratis, in most of which the former 

 approaches nearer to the H. sanguisuga of Europe. The 

 sperm-sacs (ss) and epididymes (ep) of H. lateralis are con- 

 fined to the distance between ganglia XI and XII, and the 

 latter are massive and compact and closely molded around 

 the sperm-sacs. The atrium (at) of this species is much 

 shorter, its posterior turn being at ganglion XIV ; the relative 

 lengths of the short to the long limb is as 1 to 1.7. The 

 penis is not protruded in any of the Illinois specimens, but 

 in the aquatic variety from Ohio is essentially similar to 

 that of H. marmoratis. 



Unlike the latter species the ovaries (ov) of this are situated 

 far in advance of the second pair of testes. Sometimes, at 

 least, they lie beneath the nerve cord and between the female 

 exit and ganglion XII. Similarly to the atrium, the vagina 

 is relatively short and never extends posterior to ganglion XIV • 



Alimentary Canal. — Counting the rudimentary denticles 

 which complete the series posteriorly the number on each 

 jaw is from 20 to 25 pairs (PI. XLY., Fig. 25), arranged, 

 as in H. marmoratis, in two contiguous series. They 

 are of more irregular shape than in that species and of 

 smaller size, but their greater number causes them to occupy 

 an approximately equal distance on the jaws. 



Color. — Forbes ('90) has described the colors from living 



