526 RAYMOND PEARL. 



numerous observations have been made on tlieir general 

 natural llistor3^ It is thought desirable to present a general 

 account of this here for two reasons : first, because there is no 

 adequate discussion of the natural history of the fresh-Avater 

 triclads in the literature; and furthermore, because it will 

 bring out prominently the phenomena for which we are 

 seeking an explanation. In other words, it will present the 

 problems with which this study has had to do. 



I. Occurrence and Distribution. 



The species of Planaria (maculata, dorotocephala, and 

 gonocephala) used in this study have been collected mainly 

 from the Huron River near Ann Arbor. They are found, for 

 the most part, on the under surfaces of stones in places where 

 the current is of moderate swiftness, and the substrate on 

 which the stones rest is rather soft. They are also found 

 among the fronds of such water plants as Ceratophyl lum 

 and El odea, although less abundantly than on stones. I 

 have obtained these species only very I'arely in collections 

 fi'om ponds and small pools of stagnant water. They appear 

 to be, in general, much more abundant in shallow water than 

 in deep. 



Rhabdocoeles I have found in great abundance in small 

 ponds and pools of stagnant water, and, with the exception 

 of Stenostoma leucops, almost never in running water. 

 Dendrocoelum, sp., is also much more abundant in stagnant 

 Avater than in streams. 



There is no marked seasonal distribution of the species of 

 Planaria studied. They appear to be slightly more abun- 

 dant in the fall than in the spring. I have found no evidence 

 of any migration into deep water during the winter in the 

 case of these forms, as has been described by Child (: 01, pp. 

 978 — 981) as occurring in Stichostemma. The seasonal 

 distribution of Dendrocoel um,sp., appears to be well marked, 



iiat.ui-al conditions, williout necessarily implying Uie same idea as that embraced 

 in tlie term " habit " as used by the psychologists. 



