524 Illinois State Lahoratorn of Naticnd History. 



Somite XXIV is quadrianiiulate, but it is the posterior 

 end ((/3) whicli is least developed, instead of the anterior 

 end {al) as in the quadriannnlate somite YIII. In some 

 examples the large posterior annnlus (a S) is marked by a 

 slight farrow, which is more frequent on the ventral surface. 



XXV. Because of its variations this normally triannulate 

 somite is one of the most interesting. A 1 is always longer 

 than XXIV a 3. In four specimens it exhibits no trace of a 

 subdivision dorsally or ventrally ; in six there is more or less 

 evidence of a ventral furrow ; four examples, while lacking 

 any trace of a furrow, show two distinct integumental ridges 

 at the margins of this annulus ; two others have both the 

 marginal ridges and the furrow ; and in one individual of 

 large size the furrow extends even half way around the dorsal 

 side. The remaining annuli a 2 and a 3 are of equal size 

 and present no noteworthy features. 



XXVI is uniannulate above, and the sensilhe, with the 

 exception of the dorso-median, which are at about the middle 

 of its length, are situated close to the posterior border. On 

 the ventral side a partial annulus is developed in many cases 

 posterior to the line of sensilbe. 



XXVII. For the reason just stated the short preanal 

 annulus is regarded as XXVII al, while al and (f 3 are 

 united in a single sensilliferous annulus which is cut into 

 by the anus. The furrow al/a^ is sometimes incomplete 

 mesially. 



The dorsal surface of the sucker is marked by a variable 

 number of concentric furrows crossed and connected by 

 irregular wrinkles. The sensilhx^ are difficult to distinguish, 

 but generally about three belonging to each of the dorsal 

 series are present. 



Reproductive Onjaiis. — The dissection represented in the 

 figure (PI. XL v.. Fig. 26) was made upon a well-extended 

 mature specimen of medium size. In this the nerve cord 

 passes to the right side of both the genital orifices, and the 

 unpaired portions of the genital ducts lie to its left. The 

 dissection of a number of specimens of this species from 

 several localities shows that while this relation between the 



