52 ILLINOIS BIOLOGICAL MONOGRAPHS [246 



pharynx in the heaviest part of the body and were visible even to the naked 

 eye. The specimens taken during April were all carrying eggs which generally 

 were extruded. As the animals Hved isolated in watch glasses, it was easy 

 to keep track of them. The first developmental stages were carried thru but 

 most of the eggs died before the second cleavage. 



This mud-loving form is, of all the Turbellaria, the most ethereal; it is 

 daintily tinged, a mere transparent shadow against the dark background. 



Strongylostoma rosaceum nov. spec. 

 Figures 1 to 20, 28, and 54 



Specimens were first seen during the early part of November in the ponds 

 at the United States Biological Station at Fairport, Iowa. They appeared 

 together with other rhabdocoel species, both swimming free in the water and 

 also coming out from masses of algae and water weed which were dragged up 

 from the sides and bottom of the ponds. They were present in larger or smaller 

 numbers in all the collections made during the next three months. Individuals 

 of this species were easily recognized by the rapidity of their movements thru 

 the water and by their habit of launching out openly rather than crawling over 

 the sides of the aquarium or on the plants. Altho frequent in most of the ponds 

 and the reservoir which supplied them, in certain places where conditions 

 seemed especially favorable they were exceedingly abundant. Such conditions 

 appeared, in general, to be a large amount of filamentous algae rather- than the 

 coarser water weeds, a depth of water of not more than eight feet, and as might 

 perhaps seem reasonable the presence of only a few fish. 



These forms are of especial interest for several reasons. Both color and 

 preciseness of structural plan are striking in intensity and definiteness. Among 

 the more common rhabdocoels the color is nearly lacking or at least dull and 

 varying with the surrounding conditions from a transparent white to an opaque 

 gray-green. In comparison with such fresh-water types, these specimens are 

 gorgeous, for even altho obscured by muddy water the clear delicate pink is 

 noteworthy, and makes them stand out sharply against the greenish back- 

 ground, easily distinguishable from all other animal species. While most 

 rhabdocoels and even many turbellaria are fairly simple structurally, in these 

 the several organic systems are clear-cut and more completely developed. 



Tho occurring in large numbers in these special ponds, they have not 

 been found in other ponds in this region, and members of this same genus have 

 been reported from only three locaUties in this country. Altho they are prob- 

 ably present in many places and will be found at some future time, they are 

 evidently not at all commonly found in the fresh water mud-holes of the Missis- 

 sippi valley. 



The facts, that in water under the ice the number of individuals is large, and 

 that the period of sexual maturity is mid-winter, may be correlated with the 

 northern habitat of related species. In a number of rather widely separated 

 regions in northern Europe, members of this genus have been described, and 

 taken altogether this seems to be a cold water loving form. 



