221] TURBELLARIA FROM THE MISSISSIPPI BASIN— HIGLEY 27 



size, and partly to the abundant supply of individuals. The distribution is 

 very much wider than was earlier supposed. Silliman (1885) was the first 

 to find it in the United States. He discovered it in large numbers in certain 

 ponds in the east. The knowledge of its occurrence in other places came 

 slowly. Every few years it was reported from some new locality, but not until 

 within the last fifteen years have the collections been adequate enough to prove 

 its presence common thruout the country. Now it is known to be living in 

 three fourths of all the ponds wherever there is other animal or plant life. 

 During the winter under the ice it seems to carry on a successful existence and 

 can generally be found at any time. Of the whole group of rhabdocoels this 

 species is the most common, in fact almost cosmopolitan, and it is likely that 

 future collections will prove it entirely so. 



In the way of biologic relationships, some few new observations may be 

 of value. One character which makes possible the very general habitat is the 

 ability to exist under varying conditions. Stenostoma leucops is able to live 

 in situations where the oxygen content is extremely low and where often the 

 amount of carbon dioxide present is so large as to kill other members of this 

 group. This explains the fact that it may be present in small puddles and 

 ponds where there is almost no plant life, or in places where few animal types 

 exist. Often, too, in cases where the water is thick with bacterial growth and 

 disintegrating material of all sorts, this Stenostoma is able to live long after 

 the conditions seem entirely imfit. That it is sensitive to the presence or 

 absence of oxygen and carbon dioxide is shown when specimens are placed in 

 water where part of it is clear and fresher than the rest. They invariably 

 find the freshest parts, even tho the difference be very slight, and are always 

 to be found in the clearer portions of an aquarium. 



The reaction to light is negative, tho not very prompt or definite. That 

 is, the specimens are always on the side of the aquarium away from the light 

 but it takes some time for them to find that position and many seem not to 

 prefer the very darkest places. It may perhaps be said that both the brightest 

 and darkest portions are unfavorable, and that subdued light is preferred. 



The reactions are not at aU precise or prompt and the animals seem to be 

 sensitive to general influences rather than to direct stimuli. Heat and cold in 

 a general way seem to have almost no appreciable eft'ect. Small weak indivi- 

 duals are found as well in summer as in winter; large plump specimens in cold 

 as well as in warm water, and, too, th^ period of sexual maturity is not so exactly 

 dependent upon change in season and consequent change in temperature as 

 in most rhabdocoels. It is true that in winter, in ponds where a small stream 

 of water raises the temperature, the numbers which find the warmer parts are 

 very large, but other conditions very probably hold here, since in such places 

 there will be more food and often since the water is fresher, more oxygen. As 

 a whole, the response to any ordinary stimulus is never strong. If the stimulus 

 is so intense as to cause a decided reaction, it is then of such a nature as to be 

 injurious or perhaps fatal. 



