WOODWORTH: ILLINOIS TURBELLARIA. EL 
powerful clearing reagents. The length of the specimen was 2.8 mm., the 
greatest breadth 1.8 mm. When alive the worm probably must have attained 
a length of from 8 to 10 mm. 
Whether the occurrence of the animal on the mantle of Unio is an indica- 
tion of a parasitic mode of life, like that of the Triclads of the genera Bdelloura 
(Leidy, 1852") and Syncolidium (Wheeler, 1894), or whether its occurrence 
was purely accidental, can only be determined by careful and extended search, 
Mesostoma ehrenbergii O. Scumipr. 
Figure 6. 
Mesostoma ehrenbergii O. Schmidt, 1848, p. 47. 
Mesostoma wardii Woopworru, 1896, p. 95; 1896°, p. 241. 
No. 13,521, Station L; No. 13,626, Illinois River. 
In a report on the Turbellaria collected by the Michigan Fish Commission I 
(1896, 1896") described what I believed to be a new species of Mesostoma under 
the name of M. wardii, basing the species chiefly upon the uniformly small 
size and the absence of cephalic tracts of rhabditi or “ Stäbchenstrassen.” A. 
comparison, however, with a larger number of Illinois Mesostoma has convinced 
me of the identity af both the Michigan and Illinois forms with M. ehrenbergii 
of Europe, and I must hereby cancel the species established by me in 1896. 
The Illinois specimens range from 14 mm. to 6 mm. in length, while the 
Michigan specimens measured only from 2 to 3 mm. In all of the small 
Illinois specimens the cephalic tracts of rhabditi are lacking, and are prominent 
only in the largest individuals. The largest of the specimens, 6 mm. in length, 
contained eight young worms in the left uterus, the right uterus being empty. 
The viviparity of M. ehrenbergii was known to Focke (1836), and the same 
author has described the differences between the brown hard-shelled winter 
eggs and the smaller translucent summer eggs, and pointed out the fact that 
in the viviparous condition the young ones arise from the thin-shelled summer 
eggs. he simultaneous occurrence of both summer and winter eggs in the 
same individual was observed by Leuckart (1852). Four of the Illinois speci- 
mens contained the characteristic large opaque brown hard-shelled winter 
eggs, and therefore agree with the European form in producing both kinds of 
eggs at the same season, though in no case were there both kinds of eggs actually 
present in the same individual. About 40% of the individuals contained the 
translucent summer eggs, the smaller specimens showing no signs of sexual 
organs. ‘The winter eggs have the same shape as those of the European spe- 
cies, such as would result if one half of a hollow sphere were infolded into 
the other half. As in the European species, too, the diameter of the winter 
eggs considerably exceeds that of the summer eggs, the former measuring 
0.525 and the latter 0.350 mm. in diameter, The occurrence of M. ehrenbergii 
in the United States also gives to this well known species a cosmopolitan 
distribution. 
