Illinois Oligochcsta. 445 



of coelomic corpuscles Avith which the ccelomic fluid is crowded, 

 and which are readily visible through the delicate walls. In 

 nine specimens, apparently complete, the number of somites 

 varies from 215 to 365, the average number being 281. 



The setoe, except the genital ones, are all of the ordinary 

 uncinate, or cleft, type, and have the usual arrangement of 

 four bundles to each somite. Anterior to the clitellum the 

 number of setae in each bundle is usually five or six, but 

 posterior to it, throughout the greater part of the length, the 

 number is commonly four, while near the posterior end it is 

 more frequently one to three. The ordinary uncinate seta? 

 are slightly enlarged at a point about one third of their length 

 from the outer extremity. They are usually .09 to .1 mm. 

 in length and about .005 mm. in diameter at the middle. 

 The genital setae are of two kinds. On the ventral side of IX, 

 in place of each bundle of ordinary setae, there is a specially 

 modified seta (PI. XXXIX., Fig. 4, b, and PI. XL., Fig. 6— 

 viewed in different planes) .12 mm. in length and about .01 

 mm. in diameter at its largest part, which is near the proxi- 

 mal end. These seta? are in close relation to the openings of 

 a pair of glands to be described later. In one specimen ven- 

 tral setae of the same sort were present in X. The ventral 

 bundles of XI each consist of four to six peculiarly shaped 

 setae entirely different from the ventral setae of IX, the outer- 

 end of each being spatulate and rather abruptly hent (Fig. 

 4, c). These setae are from .14 to .16 mm. in length, and the 

 diameter is about .009 mm. at the inner end and .006 mm. 

 near the expanded outer end, while the outer end itself is 

 about .01 mm. in width. 



The clitellum is of the same general character as the clitel- 

 lum present in other Tubificida, and extends from about the 

 middle of X to the posterior part of XII, in some individuals 

 encroaching on XIII. It is incomplete ventrally. The pros- 

 tomium is relatively large and subcorneal. 



The brain has a. slightly convex anterior margin, without 

 any such median anterior extension as is present in some 

 Tubificidce. There are two posterior lobes extending back- 

 wards, one from each side of the middle of the ventral margin. 



