18 ILLINOIS BIOLOGICAL MONOGRAPHS [212 



however, at any point a little current passes thru, thus raising the oxygen con- 

 tent, at least a few individuals are there. Out in the middle of the river, on 

 the contrary, where the flow is exceedingly swift, making a boiUng, seething tor- 

 rent as it tumbles over stones, the oxygen content is greater. Here the 

 number of individuals in a given area is limited only by 'standing room,' even 

 clam, shells and bits of wood being entirely covered. Thus it seem.s that in 

 respect to their use of oxygen the turbellarians are not essentially different 

 from other higher phyla. 



All the turbellarians are, to some extent at least, sensitive to hght, but 

 they react toward it in very different ways, so that no general statement can 

 be given. The presence or absence of eyes cannot be taken as the main factor 

 in this response, for altho the eyes are primarily light detecting organs, species 

 in which they do not occur may possess the ability to distinguish between light 

 and darkness. The planarians and rhabdocoels live under such different con- 

 ditions that a difference in reaction is to be expected. 



The planaria are definitely and conspicuously negative in their response 

 both to diffuse and localized light. In three species, comm.on to this region, 

 the eyes are large and well-developed. The animals ordinarily live under stones 

 or in the depths of algal masses where a good deal of precision in their reaction 

 is necessary. If placed in a large open dish, they investigate very thoroly 

 every part and invariably come to rest at the point of greatest shadow. Their 

 ability to estimate the size and density of the shadow is evidently keen, since 

 they seem able to distinguish even small gradations. Another factor which is 

 perhaps to be correlated with the perfection of eyes is the relative amount of 

 color. The pigment which here varies in amount with light and dark locations, 

 is not present, at least to any extent, in forms which do not possess eyes, and 

 it is not an unreasonable conclusion that color and eyes have evolved together. 



The rhabdocoels have a more variable habitat than the planaria and are not 

 capable of so exact a response to light, even tho some species possess as highly 

 developed ej^es. The color has no direct connection, as for instance, Strongy- 

 lostoma, a form which has relatively very large eyes possesses no pigment. 

 This is also the condition in several Dalyellias. The members of these two genera 

 are positively heliotropic, except when the hght is intensely localized. The 

 genus Stenostoma, on the other hand, is without eyes or pigment and gives a 

 negative response to light, be it strong or weak. These forms do not habitually 

 frequent deep, shaded places but rather a dimly lighted situation. This fact 

 perhaps explains the rather general reaction, and their habit of being content 

 with merely the less Hghted regions. This habit, however, is constant, tho in 

 just what way the light is sensed is not known. As a whole, the genus Stenos- 

 toma is very simple in structure, all of the reactions being general rather than 

 precise, and this response to light is the most definite of all. 



The effect of temperature is very hard to ascertain, in any detailed or definite 

 way, since it is of general influence, having to do with large quantities of water 

 rather than with any very limited portion. In the broad sense temperature 



