The Hirudinea of Illinois. 525 



nerve cord and the genital apparatus usually obtains, it is 

 not constant and diagnostic of the species. Cases have been 

 found in which the nerve cord passes to the left of the genital 

 exits or to the right of one and the left of the other, or in 

 which the atrium or vagina crosses the nerve cord dorsally 

 and lies partly on each side of it. 



Ten pairs of testes {tl, ^3), belonging to somites XIV to 

 XXIII inclusive, appear to be constant. Each occupies 

 the first two annuli of its somite and the posterior 

 annulus, or even two annuli, of the preceding somite. They 

 are largest in the middle of the series, and become smaller 

 toward both ends. The twenty short vasa efferentia are simi- 

 lar in structure and appearance to the paired vasa deferentia 

 into which they empty. Each of the latter (vd) is a rather 

 conspicuous glandular tube of yellowish color which takes a 

 more or less sinuous course just entad of the line of nephridial 

 vesicles. When about opposite to the male pore it turns 

 sharply caudad, having become narrower and of firmer, less 

 glandular, texture, and soon passes into a much convoluted 

 region, the epididymis (e/>). The latter is neither compact 

 nor massive, and in the posterior part of XIII opens into the 

 sperm sac (s-s), a fusiform enlargement with which the ductus 

 ejaculatorius {dc) begins. Throughout its greater part the 

 latter is a delicate tube which extends forward to a point 

 opposite to the male pore and then again bends on itself 

 sharply caudad to open into the fundus of the atrium or 

 penial sheath {at) at the anterior limit of somite XV. 

 Throughout its entire length the ductus ejaculatorius has 

 firm glistening muscular walls. 'Generally the right ductus 

 passes beneath the nerve cord at its anterior turning point 

 behind ganglion XI, but occasionally it is the left which 

 makes this crossing. 



The atrium or penis sheath (at) is very long and slender. 

 Beginning at the anterior end of somite XV it reaches caudad 

 to ganglion XVII, bends sharply on itself, and passes directly 

 cephalad to the male orifice. In the specimen figured, which 

 measures 92 mm. in total length, the atrium has a length of 

 40 mm., the ratio between the short and long limljs l)eing as 



