80 ILLINOIS BIOLOGICAL MONOGRAPHS [274 



Against this light background the proboscis is conspicuous both because 

 of its structure and because of its constant motion. The proboscis itself is a 

 transparent narrow barrel-shaped organ criss-crossed by exceedingly fine 

 shimmering lines. The anterior cone-shaped piece is different, being clearer 

 and lacking the lines. The whole lies in a sac-hke sheath which opens by a 

 narrow but distensible canal toward the front. The proboscis is at frequent 

 intervals violently jerked forward and back, or sideways, in its sheath. Its 

 function is a little obscure, for though evidently sensory, it is deeply buried and 

 not protrusible. 



Of the whole body the reproductive organs are the most conspicuous parts. 

 The difference between right and left sides is apparent even to the naked eye. 

 The large vitelline gland on the right gives that side a thicker whiteness. The 

 small ovary together with the egg capsule are both distinct enough to be easily 

 made out lying in the center. It was almost impossible to see the testis in any 

 specimen since the color was nearly that of the underlying intestine. The 

 complicated copulatory apparatus in the extreme posterior region is the most 

 noticeable detail of aU. The stiletto is chitinous, very large and heavy. Together 

 with its sheath it makes a heavy rod reaching up thru fully one-fourth the body 

 length. It lies a little diagonal to the middle line, extending from a ventral 

 posterior point up thru to the dorsal side. The relation of this one strong 

 stiff part to the neighboring soft tissue is very easily seen as the animal turns 

 over or contracts. 



In the same surroundings and often in the same collection both the type 

 species Gyratrix hermaphroditus , and the variety, Gyratrix hermaphroditus 

 hermaphroditus, were found. The difference between these two forms was 

 quite evident since in the specimens of the variety, the point of the stiletto- 

 sheath was very sharply recurved. The numbers were about one of the variety 

 to ten of the typical individuals. It is interesting to note that the form.s were 

 living side by side. 



About the first of August a number of specimens of Gyratrix hermaphrodi- 

 tus were found in Cedar Lake. This is one of the smallest lakes of the group 

 in northern Illinois which forms the source of the Illinois river. It is con- 

 nected with other lakes only by a very small stream which is during part of 

 the summer entirely dry. The general conditions here, however, are practical- 

 ly the same as in the Havana locality, except that in many places the bottom 

 is sandy and the water clear. Since most of the aquatic fauna is the same in 

 both places, this similarity among the rhabdocoel species is to be expected. 



