78 ILLINOIS BIOLOGICAL MONOGRAPHS [272 



there are several prominent tracts arranged in a narrow fan in the middle part 

 of the head. These lie dorsal to the brain, running back between the eyes, 

 but only far forward are they heavy enough to be noticeable. They are not 

 so conspicuous as those of Mesostoma angulare, where they are sharp against 

 the clear background. The parenchyma of this new Mesostoma simplex is 

 especially weU-developed, taking up most of the space in the body. The cells 

 are close together and there appears to be less fluid than in some forms. The 

 contained matter makes only the central portion darker and does not at 

 all obscure the pharynx which stands out dark and heavy. Its size is com- 

 paratively large and the rosette cells are rather long. The species has not 

 been taken when in the reproductive stage. The above outline summarizes 

 the notable characters. 



FAMILY GYRATRICIDAE 



The Gyratricidae, the smallest family among the Calyptorhynchia, or 

 proboscis-bearing rhabdocoels, was named by von Graff in 1905 to contain 

 one genus. This genus had been described by Ehrenberg as early as 1831 

 under the name Gyratrix. Because of one significant character, namely the 

 presence of two sexual pores instead of one, it was distinct from other nearly 

 related forms, standing entirely alone. Such a detail of structure made the 

 separation of this from other groups, the only logical division, and also elevated 

 the genus to the importance of a family. Von Graff's classification was a very 

 natural one. Thus far only three species have been named, one of which, 

 however, shows three varieties. 



In Europe the distribution is wide spread, the same forms being plentiful 

 in the freshwater lakes and along the salt shores of the sea, in the warm Medi- 

 terranean and along the icy coast of Helgoland. In America Gyratrix herma~ 

 phroditus and two of its varieties have been reported as occurring in fresh and 

 brackish water at Woods Hole, Massachusetts, and at two places in New York 

 state. 



Gyratrix hermaphroditiis Ehrenberg 1831 



My first specimens of this family were found at Havana, Illinois, in July. 

 They appeared rather frequently in nearly all of the collections made from 

 the many sloughs and overflow waters along the Illinois river. When buckets 

 of material were brought into the laboratory, almost as soon as the water was 

 quiet, these deUcate rhabdocoels could be seen swimming out from the felt- 

 work of algae. Under such conditions generally they were the first species 

 to venture from their hiding places. This was probably due to the fact that 

 the natural habitat is not the deep recesses of plant entanglements, but the 

 outskirts of the mass where the water is freer. This same free habit, when 

 circumstances were favorable, could be watched in native surroundings. If, 

 for example, a place was found where a large mass of Cladophora was floating 

 under the shadow of some tree, and if care was taken not to disturb or ripple 

 the surface, then, against the dark background of the algae, the tiny whitish 



