WOODWOETH: ILLINOIS TURBELLARIA. 11 



powerful clearing reagents. The length of the specimen was 2.8 mm., the 

 oreatest breadth 1.8 mm. ^Yhen alive the worm probably must have attained 

 a length of from 8 to 10 mm. 



Whether the occuiTeuce 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 BdeUoura 

 (Leidy, 1852*) and Syuccelidium (Wheeler, 1894), or whether its occurrence 

 was purely accidental, can only be determined by careful and extended search. 



Mesostoma ehrenbergii 0. Schmidt. 



Figure 6. 



Mesostoma ehrenbergii O. Schmidt, 1848, p. 47. 

 Mesostoma wardii Woodworth, 1805, p. 95; 1896% p. 241. 



No. 13,521, Station L ; No. 13,626, Illinois Eiver. 



In a report on the Turbellaria collected by the Michigan Fish Commission I 

 (1896, 1896") described what I believed to be anew species of Mesostoma under 

 the name of M. v:ardii, basing the species chiefly upon the uniformly small 

 size and the absence of cephalic tracts of rhabditi or " Stahchenstrassen." A 

 comparison, however, with a hirger number of Illinois ISIesostoma has convinced 

 me of the identity of both the Michigan and Illinois forms with M. ehrenbergii 

 of Europe, and I must hereby cancel the species estalilished by me in 1896. 

 The Illinois specimens range from 1^ 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-sheUed winter 

 eggs and the smaller translucent summer eggs, and pointed out the fact that 

 in the vi\'iparou3 condition the young ones arise from the thin-shelled summer 

 eggs. The 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-sheUed ■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 indi\aduals 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 weU knoAvn species a cosmopolitan 

 distribution. 



