532 KAYiMOND PEARL. 



digitate processes." I incliue to the view that in Plan aria 

 ib is the mucus which attaches the organism to the support, 

 although it must be said that the jippearance is at times 

 strikingly as if the anterior and posterior ends acted as 

 suckers. 



c. Periods of Activity and Pest. — There is in the 

 case of freshly collected planarians a certain periodicity in 

 the activities. First, there is the rather marked difference 

 in the amount of activit}^ in the night aud day. It has been 

 stated by a number of investigators that planarians were 

 probably nocturnal in their habits, i. e. more active at night 

 than during the day. This can easily be seen to be the case 

 in the following way : — In a dish containing a large number 

 of planarians, together with some plant material like Cera- 

 tophyllum, usually comparatively few specimens will be 

 seen during the day. Nearly all will be in among the fronds 

 of the plant material in a quiet condition. If, however, one 

 comes into the laboratory at 8 p.m. or later at night, so that 

 (in case of winter days) there has been two and a half or 

 more hours of darkness, a large number of the specimens 

 will be found on the sides and bottom of the dish in active 

 movement. Again, one will frequently find in the morning 

 that the specimens are scattered about all over tlie sides 

 and bottom of the aquarium dish at rest. By noon numy 

 of these will have disappeared, or, in other words, gone 

 in among the plants, where they are protected from the 

 light. 



Besides this day and night periodicity there is another 

 fact that may be mentioned; this is that during the day, 

 at any rate, they seem to be incapable of continuing 

 movement more than a certain, not very great, length of 

 time. Then a period of rest must intervene. Thus one may 

 see a specimen which has been moving about come to rest, 

 and after a length of time, varying from a comparatively few 

 minutes to several hours, it Avill start into spontaneous move- 

 ment again, and repeat the Avhole cycle over and over. It 

 seems that the periods of ({uiet are really for the })urpose of 



