The Hirudinea of Illinois. 521 



what outwards ; thus, together, they cover an arc of perhaps 

 160°. 



Annulus 6 — V (a 1+a 2) — unites with 7 (V a 3) ventrally to 

 form the broad postoral ring; but the immediate oral ring or 

 lower lip is a rim, more or less narrow and more or less dis- 

 tinct from 6, which is contributed by 5 (IV a 3). At the 

 sides the mouth is bounded by the 4th and 5th annuli, which 

 coalesce laterally (PI. XLVL, Fig. 33). 



In mature examples the clitellum (PI. XLV., Fig. 24) is 

 very distinct and equally well developed dorsally and ven- 

 trally. It is smooth, thick, and firm, and at its posterior 

 end as wide or wider than the succeeding annuli. Posteriorly 

 it is straight, anteriorly concave ; and it extends over fifteen 

 annuli (X£5-XIII « 2 inclusive). 



The Illinois specimens exhibit no variation in the position 

 of the genital pores, which are, invariably, the male in XI b 6 

 and the female in XII b6. The male orifice is situated close 

 to the anterior border of its annulus, which enlarges 

 mesially and encroaches slightly on the preceding annulus- 

 Occasionally the region immediately surrounding the orifice 

 is elevated as a low broad papilla. The size of the actual 

 opening, as well as its form, differs according to the state of 

 retraction or protrusion of the penis and related parts. The 

 female pore is rounded or slit-like, is smaller than the male, 

 and, like that, is usually close to the anterior border of its 

 annulus though more liable "to shift as far as half its width 

 caudad. The annulus' is enlarged and its anterior furrow 

 becomes obsolete in its middle part. 



The relatively small posterior sucker is circular, broadly 

 attached, thick posteriorly, and projects by about one third 

 of its diameter beyond the body, its anterior margin reaching 

 to XXV a 2. Just anterior to it is the large anus with its 

 much wrinkled margin cutting into XXVII. Prolapsus of 

 the rectum frequently occurs in individuals which contract 

 excessively as a result of irritation. 



There are 17 pairs of nephridiopores situated just anterior 

 to the posterior margins of the b'2 region of somites VIII to 

 XXIV inclusive (PI. XLVL, Fig. 34). Each is in a faint 



