308 S. J. HOLMES. 



stome expands, and another contraction soon follows. Hodge 

 and Acking found that the interval between successive contrac- 

 tions in Vorticella varied greatly. In one individual kept for a 

 long time under continuous observation contractions occurred at 

 one time about once in four seconds, at another once in eight 

 seconds, and at various other intervals in different times. Some- 

 times there was no rhythmic contraction at all. The stimulus to 

 the rhythmic contraction of the stalk apparently comes from the 

 body, for the stalks which I have isolated showed no independent 

 movements. 



Stentor. — Stentor ccemleus when attached and extended sways 

 about slowly in a circle. The swaying is a very regular move- 

 ment and is not due to any evident external stimulus. It is a 

 result of the contraction of the body instead of the action of 

 cilia, as the stalk is bent successively in different directions. 

 There is also a rhythmic movement executed by Stentor roeselii 

 during the construction of its tube. This species after attaching 

 itself alternately contracts and extends its body in a more or less 

 regular manner while it is secreting the gelatinous substance of 

 which its tube is formed. The swaying movements are not so 

 pronounced as in the preceding species. 



It is a noteworthy fact that rhythmical activities occur in those 



species which are either attached like Stentor and Vorticella, or 



which like Loxophyllum, Dileptus and Lachrymaria frequently 



remain for a long time near one spot. These forms do not have 



to wait for something to turn up, but are actively seeking for 



new stimulations, their rhythmical movements bringing them in 



a measure the advantages which in forms like Paramoeciimi are 



secured by almost continuous swimming. 



Biological Laboratory, 



University of Wisconsin. 



