The Hirudinea of Illinois. 481 



body is widest near the posterior end and very slender 

 anteriorly. Under such circumstances the head may be 

 much wider than the constricted region immediately follow- 

 ing it and has a somewhat cordate form. 



The dorsal surface is provided with numerous sensory 

 papilbe or cutaneous warts (PI. XLIL, Fig. 4). Except 

 toward the margins, where they are arranged somewhat in 

 two rows, they form a single series across each annulus. On 

 many specimens none of the papillae are at all conspicuously 

 elevated, and the surface may appear quite smooth ; in others 

 each annulus bears from ten to twenty quite large warts 

 which in general correspond in position with the larger warts 

 of P. rugosa, but have very different proportions. The median 

 series, so prominent in the latter, is in this species almost 

 obsolete. The largest and most constant are those which 

 overlook the dorso-lateral sensillpe. In all cases they are 

 low, smooth, and rounded, and seldom bear more than a 

 single sense organ ; never a rough rosette-like aggregation at 

 the summit. 



The shape of the head differs much according to conditions 

 of contraction and extension. Generally in the larger indi- 

 viduals it partakes of the even outline of the body and is 

 broadly rounded anteriorly. In the younger specimens, 

 especially in extension, it is somewhat expanded beyond a 

 neck-like constriction, and the preocular lobe is somewhat 

 sharply pointed. In such, also, the annuli are very clearly 

 differentiated, and in a few cases the preocular lobe, which 

 is generally simple, is distinctly subdivided into two rings 

 (PL XLIL, Fig. 1). 



A single pair of eyes — they are really compound— (Pl # 

 XLIL, Fig. 1), frequently united in a common pigment spot, 

 is situated on the posterior part of the second (occasionally 

 the third) distinct annulus. Then follow a short annulus, a 

 very large one, again a distinct short one, which forms the 

 posterior rim of the sucker below, and then two imperfectly 

 separated annuli V (a l-\-a 2) which completely unite ventrally 

 to constitute the postoral ring. On the ventral surface of 

 the sucker and lip most of these furrows may be readily 



