213] TURBELLARIA FROM THE MISSISSIPPI BASIN— HIGLEY 19 



is the main inciting force which every season starts or retards the life processes, 

 and abnost never in nature is it found as a locaUzed stimulus. During 

 the winter the ice confines the cold to a layer at the surface and to some extent 

 limits and restricts the extent of its influence. It acts somewhat as a blanket, 

 preventing the cold of the air from penetrating too deeply, so that beneath it a 

 vast deal of Hfe continues thruout the year. It seems certain that rhabdocoels 

 remain in the layer of free water, which is, in fact, their regular habitat, rather 

 than hibernating in the more protected bottom mud. Thruout the winter of 

 1916 collections were made from a single pond every week. During January 

 and February the Cladophora was dragged up thru a hole in the ice. Two 

 or three species appeared in every haul, altho sometimes not in great numbers. 

 It is, of course, evident that myriads of individuals are killed by the cold every 

 winter, especially since by far the most five in the region of the surface and are 

 frozen. Those which are driven to deeper water carry on their existence, per- 

 haps with not so much vigor, but nevertheless very successfully. Another 

 factor which must be taken into consideration in drawing conclusions is that, 

 with the onset of cold, the manufacture of oxygen is largely cut off and a con- 

 siderable proportion of the destruction which occurs, results from the absence 

 of oxygen and the presence of an increasing amount of carbon dioxide. As an 

 interesting side light on this question I may note that in artificial ponds where 

 thru the bottom there is a constant stream of pure water all winter long, life 

 continues with unabated vigor. This is, of course, due partly to the supply of 

 oxygen, but the higher temperature also has much influence. 



With the melting of the ice in the spring relative conditions in the water are 

 entirely changed. The ponds lie open to the warming influence of the sun 

 and the response in many species is very ready. Thus it is that most planaria 

 and many rhabdocoels become sexually mature at this season. To a great 

 extent it is merely the change in temperature which incites greater cellular 

 activity. Those forms which do not have their reproductive seasons at this 

 time almost invariably go thru a period of very active budding or fragmenta- 

 tion which does not occur when the temperature is very low. Instances which 

 well illustrate the power of heat are to be found in cases where a warm spring 

 flows into a cold pond or stream. There, crowded into the water where the 

 temperature is high, are myriads of animal forms. Among these planaria of 

 all sizes are often very plentiful, while a few feet away, beyond the reach of 

 warmth, there are no specimens at all. 



The lowering of the temperature in the fall has two definite effects. First, 

 it retards the life processes of many species and, second, in a directly opposite 

 way, it induces an acceleration in the growth of the sexual elements. The first 

 instance is the more common and has been referred to above. The second is 

 rather difficult to understand. There are perhaps two theories to be offered 

 in explanation. One is that those rhabdocoels which, in their primitive habitat, 

 lived in cold regions become sexually mature when the temperature comes the 

 nearest to their ancestral condition. The other, is that eggs which are able to 



