330 FRESH-WATER BIOLOGY 



The regions occupied by different species of planarians are ap- 

 parently determined by temperature and food supply to a very 

 great extent. Those species which are adapted to low tempera- 

 tures become sluggish and inactive in higher temperatures, or 

 the reverse, and so will be less likely to find food than forms 

 especially adapted to that temperature. If the food supply is 

 limited this will necessarily lead to a crowding out of those less 

 perfectly adapted to the environment. The development of any 

 one species in a particular region is consequently limited by com- 

 petition with other species already established in the area. In 

 some cases two or more species may be found in almost equal 

 numbers in the same pond as Planaria maculata and Dendrocoelum 

 lacteum. In such cases a variety of food usually seems to be 

 abundant, thus reducing the competition which would otherwise 

 lead to the elimination of the weaker. Cannibalism sometimes 

 occurs among individuals of the same species when food is scarce 

 and different species are especially likely to prey upon each other. 

 Planaria agilis is a voracious feeder, and will exterminate a culture 

 of Planaria velata or Planaria maculata in a comparatively short 

 time even if other food is provided. This may account in part 

 for the fact that certain species are always found alone. 



Ordinarily a pond or stream shows no evidence of the presence 

 of Planaria even though large numbers of them may be hidden 

 away under stones or leaves. However, one sometimes finds 

 them moving restlessly about in great masses, either all in one 

 general direction or in disorder. Voigt has conducted some inter- 

 esting experiments with European forms under natural conditions 

 which would indicate that these apparently concerted movements 

 are the result of a response to some stimulus which may promise 

 food, and cannot be regarded as indicating the possession of any 

 inherited tendency toward periodical wanderings. The marine 

 Turbellaria, like the fresh-water forms, hide under stones and 

 among seaweeds. Some find shelter within the shells of molluscs 

 and a few are parasitic. 



The land planarians are in general characteristic of tropical 

 and sub-tropical regions where they attain a considerable length 

 and are usually brilliantly colored. In this country one may 



