The Hlrudinea of Illinois. 483 



discernible ; the others are obscure and must be sought for 

 in sections. 



III is biannulate. The first annulus is large and generally 

 divided by an incipient cross-furrow, posterior to which are 

 the eyes and a full set of metameric sensillae. The second 

 annulus (a 3) is smaller, and loses its individuality at the 

 margins. 



IV is also biannulate, the annulus (a 1 + a 2) being very 

 large, but sometimes scarcely more visibly divided than its 

 preceding homologue. All of the four pairs of sensillre except 

 the dorso-median are very distinct. This somite forms most 

 of the lateral margins of the sucker, and its posterior ring 

 contracts suddenly and sweeps caudad and mesiad as the 

 narrow posterior ventral rim of the latter. 



V. When the head is expanded as described above, this 

 somite forms its posterior limit. The furrow a \\a 2 is more 

 evident than in somite IV, but a 1 and a 2 together may still 

 be regarded as a single large and incompletely divided 

 annulus. In no case does this furrow extend beyond the 

 margins of the body, so that the ventral surface of the 

 annulus is entirely undivided. The dorsal metameric sensillae 

 occupy the posterior constituent (a 2). The small annulus 

 a 3 is a trifle more than half the length of the large one, and 

 the furrow a 2/a 3 is very much deeper and more distinct 

 than a l/« 2, though somewhat less so than V/VI ; it is also 

 fully developed ventrally. 



VI may be provisionally and arbitrarily regarded as com- 

 plete, but even in this somite the furrow a 1/a 2 is dorsally 

 much less obvious than a 2/a 3 and becomes on the ventral 

 side very faint, and in small specimens especially obsolete. 

 These two potential annuli are also smaller and more inti- 

 mately associated on the ventral than the dorsal surface. 

 Viewed from either above or below the marginal curvatures 

 of a 1 and a 2 are more homogeneous and the emarginations 

 much less deep and abrupt than between al and a 3, thus 

 further indicating the closer growth relations of the former 

 two. 



Much the same conditions prevail in succeeding somites, 



