The Hirudinea of Illinois. 523 



sensillfe is present on the posterior portion of the dorsal 

 surface, but the ventral ones have escaped notice, if present. 

 The second, smaller annulus (aS) has one series of goblet- 

 shaped organs. 



On VI a partial furrow indicates the line of division between 

 the constituents a 1 and a 2 of the much enlarged an- 

 terior annulus. The extent of this furrow is variable. It 

 extends from the dorsal mid-line, where it is deepest, laterad 

 sometimes as far as the dorso-marginal or even to the supra- 

 marginal sensillse, but may reach as far as the eyes 

 only. All of the sensillre, both ventral and dorsal, are 

 well developed and, including the 5th pair of eyes, are on the 

 a 2 constituent. Annulus a 3 is the last on which a complete 

 row of goblet-organs is distinguishable. 



VII is triannulate, a 1 being slightly shorter than a 2 and 

 a 3 much longer, the latter constituting about two fifths of the 

 total length of the somite. Occasionally in large specimens 

 a very evident furrow divides this annulus into two equal 

 halves on the dorsal surface, and at least a shallow furrow is 

 always present. The sensilhe are normal and on a 2. 



VIII is quadriannulate. Al is slightly wider than VII a 3, 

 and like the latter shows a partial division into the secondary 

 annuli. A% bears the sensilhe and a 3 is completely divided 

 into b 5 and b 6, each of which equals a 2 in size. 



The series of complete quinqueannulate somites begins 

 with IX and ends with XXIII, making 15 in all. The five 

 annuli of each of these are of equal length. The sensilla; are 

 small but very conspicuous on properly prepared material ; 

 but the exact size of the sensory areas is difficult to figure, as 

 they appear as circular white spots in small unpigmented 

 areas the limits of which are rather vaguely defined. All of 

 the sensillse are much subject to variation, even the dorso- 

 medians and dorso-laterals being frequently subdivided, 

 changed in position, or entirely wanting. But the two 

 marginal series are especially prone to subdivision ; and they 

 are very commonly represented by a chain of contiguous 

 smaller sensory areas, not infrequently made up of four or 

 five members. 



