Japanese Freshwater Triclads. Q [ 



shows that PI. vlvlda is apt to thrive in the upper parts of the 

 streams, Pohj. aurlculatd in the middle courses, and Fl. gonocfphah. 

 in the lower parts. This is a state similar to what we find in 

 Central Europe, where the most familiar species are Dendrocoelum 

 lacfeu/ii, Fl. (jonocep/iala, Fl. alpina and Foli/. cornuta. According 

 to some investigators (Voigt (81-90), Volz (91), Zschokke (103, 

 104) WiLHELMi (93) and others), Fl. gonocephala flourishes in the 

 lower reaches of the stream, Foly. cornuta in the middle courses, 

 and PI. aljnna in tlie upper parts. Between each two there is a 

 section, in which the two species are found side by side. 



As regards the causes underlying this condition of the dis- 

 tributi(ni we can enumerate six infiuences to account for this: 



(1) food-supply, (4) nature of the substratum. 



(2) struggle for existence, (5) property of water, and 



(3) temperature, (6) light. 



Generally the food of planarians consists of small animals and 

 plants, which will, doubtless, vary in kind and quantity in different 

 localities. Migrations of planarians may be greatly dependent upon 

 the nature of the food-supply, as is the case with several animals. 

 With regard to this sui)ject, however, I have no observation to 

 put forward. 



According to ol)servalions in a small aquai'ium, PI. rirlchi and 

 Poljj. auriculata are apt to shun PI. gonocephala which is rather 

 stoutly built. Sometimes it may be seen that some forms, when 

 they are bruised so that some of the tissues underlying the epi- 

 dermis is exposed, are seized and eaten by anothei- individual of 

 the same or different species. But to my mind, the struggle for 

 existence does not appear to exercise a paramount influence. 



The geological structure of the locality seems to a certain 

 extent to be an important factor in affecting the distribution. So 

 far as my observation goes, a sandy soil seems to be the most 

 favourable substratum for PI. vlvida, whereas a muddy soil is more 

 favoQi'able to Folg auriculata. The latter species is mostly found 

 cHnging to th.e under side of deca3ing leaves and logs rather than 

 beneath stones, though PI. gonocephala abounds under stones, or 

 under decaying vegetable matter, on both sandy and muddy 



