74 ILLINOIS BIOLOGICAL MONOGRAPHS [268 



Mesostoma ehrenhergii, mentioned above, is especially noteworthy. For, al- 

 though recorded only twice before now, and then each time as only a few indivi- 

 duals, it was the first of the whole group to be discovered in this central region. 

 Twenty years ago Woodworth found it in the Illinois river at Havana. Now, 

 in one of my collections from the same place a single specimen was obtained. 

 This crawled out from a tangle of waterweed which was brought in from over- 

 flow water. Persistent efforts to find other individuals failed. 



The general characters were very distinct so that the identification was 

 easy. Some variations, however, are worthy of note. The size was 7mm. 

 The shape of the body was a regular long rectangle. Both ends instead of 

 being more or less pointed, were truncate. This was less noticeable when the 

 animal was in motion but even then the square projecting corners were evi- 

 dent. These portions of the head seemed sensitive and constantly investigated 

 the immediate surroundings. They call to mind the auricular appendages of 

 Planaria. Together with the smooth contour the very flat leaflike condition 

 was conspicuous. Even allowing for the lack of thickness due to the small 

 undeveloped eggs, my specimen was very much flatter than the European 

 forms. And compared with them the whole body shape was especiaUy char- 

 acteristic, more regular, smoother and depressed to the thinness of paper. 

 The extreme transparency added to the weird effect. Against this clear back- 

 groimd the small eyes showed black and distinct. They were very near to- 

 gether, regular in shape, lying on the dorsal surface of the brain ganglia. In 

 front of the brain the rows of rhabdites were large and well defined so that 

 the anterior part of the head appeared gray. The reproductive organs showed 

 very clearly their outlines and the row of summer eggs lying regularly arranged 

 in the uterus. Almost in the middle of the body, the pharynx lay with its 

 wall heavier and more noticeable than in foreign individuals. 



It is impossible to draw any general conclusions from a single worm but 

 these points of variation may lead to a better understanding of the adaptability 

 which has been so evident in old world specimens. 



Mesostoma angulare nov. spec. 

 Figures 29, 41, 45, and 55 



Twenty specimens of this unique rhabdocoel came in the hauls from a pond 

 near Urbana. Later the same form was found in the Illinois river at Havana. 

 It is interesting to note that here is a form resembhng in some respects the 

 European types and yet in many ways distinctly different. These worms 

 are essentiaUy bottom lovers, tho they very quickly creep into and thru any 

 heavy mass of filamentous algae. The length was always between three and 

 five millimeters, the width not more than one-fifth the length, while the greatest 

 depth was only a little less than the width. Both these measurements, how- 

 ever, differed greatly in the different specimens and in the same specimen at 

 different times. The whole shape was slender, cylindrical in a general way. 

 The head end showed the characteristic conical, bluntly pointed portion with 



