MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 565 



Loeb as the oue most frequently taken by plauarians in a 

 dish containing- only water. This behavioui* towards light 

 is not, however, an absolutely precise reaction. Many times 

 during experiments I have seen specimens come to rest in 

 the very lightest parts of the dish and remain there ; but in 

 general this reaction will cause most of the animals to gather 

 in shaded areas. It is probably the principal factor in 

 causing the animals to take positions beneath stones in their 

 natural habitat. 



The second immediate cause in determining where the 

 animals shall come to rest is the goniotaxis mentioned above. 

 If an animal in the proper physiological condition of reduced 

 tonus comes to an unevenness in the surface on which it is 

 moving, it will in most cases come to rest there. This, again, 

 is not a very precise reaction; not sufficiently so as to make 

 it possible to predict beforehand where any given individual 

 will stop. In this case, just as in the case of light, much 

 depends on the animal's physiological condition, and when in 

 the proper condition they may come to rest on a perfectly 

 smooth surface. Thus in a dish individuals will always be 

 found at rest on the smooth sides and bottom, yet there is a 

 distinctly marked tendency, when the animals are put under 

 experimental conditions and closely observed, for them to 

 come to rest in the angle of the dish. This reaction probably 

 also plays a considerable part in the habit of coming to rest 

 among the branches and leaves of the plant material. In 

 the natural habitat it is undoubtedly the factor which causes 

 them to take positions on the uneven parts of stones. It 

 may be that the immediate cause of the stopping in this case 

 is the increased resistance to movement afforded by the 

 unevenness of the surface. This, acting on an animal in a 

 fatigued condition, might give the necessary stimulus fur 

 the stopping. 



The third factor in determining where the animals shall 

 come to rest is one about which I am doubtful. There seems 

 to bo some evidence, from the behaviour of tlie animals 

 themselves, that in the foi-nuition of the groups or collections 



