506 Illinois State Lahoratorij of Natural History. 



are about .4 to .5 mm. in length and perhaps .1 in diameter. 

 Each one (PI. XLIII., Fig. 11) contains a central gland duct 

 or perhaps a group of ducts surrounded by a sheath of 

 muscle fibers springing from muscular ridges which pass like 

 radii down the inner face of the sucker. The gland ducts 

 arise from a circle of glands which appear as a ring of whitish 

 spots arranged around the sucker about half-way between 

 the margin and the pedicle, and which raise the outer face 

 into a slightly marked encircling ridge. The anus is incon- 

 spicuous and appears to be situated in the usual position 

 posterior to somite XXVII. 



Annnli and Somites. — It is obviously unsafe to assign to a 

 species all of the details of annulation exhibited by a single 

 specimen. The arrangement of the annuli in certain regions 

 is so obscure and appears so differently under different con- 

 ditions of fixation and preservation, and, moreover, is so sub- 

 ject to individual variation, that the typical conditions in 

 many species can be determined only by the careful study of 

 many individuals. What follows, therefore, must be con- 

 sidered as applying in entirety only to the type specimen. 

 The characters of the complete somites and the composition 

 of most of the incomplete somites will in all probability be 

 confirmed by the study of further material. 



As the type specimen is interpreted the somites are consti- 

 tuted as follows (PI. XLIII., Fig. 8, 9) :— 



I to IV are uniannulate and the furrows which separate 

 the first three annuli are faint. No sensillte are recognizable. 



V, VI, and VII are biannulate, with three pairs of 

 distinct sensillfe on the first annulus of each. In each case 

 the second annulus is somewhat the larger. 



VIII is triannulate, and a 3 is somewhat larger than its 

 fellows and subdivided by a faint furrow. The sensillpe are 

 on al. 



On IX we find a 1 subdivided equally into b 1 and 6 2 by a 

 shallow furrow, a 2 more distinctly split into a larger anterior 

 and a smaller posterior ring, of which the former bears the 

 three pairs of sensillte, and a 3 subdivided equally. 



X is similar to IX except that 6 5 is considerably larger 



