354 Univcrsitij of California Publications. [Zoology 



body, except that the beat may not be as strong. The niembra- 

 nellae are different in their action, reversal usually being but 

 momentary. An efit'ort was made to determine whether the beat 

 of the membranellae was independent' of the beat of the body 

 cilia. As a rule, reversal of the membranellae is accompanied by 

 a reversal of body cilia, especially during a change of direction 

 or when the motor reaction is given. The exceptions to this are 

 seen when the animals are traveling through a fairly clear field. 

 A reversal of the membranellae may occur, sufficient for the 

 ejection of particles without any stop or reversal of the move- 

 ment of the animal. The same is true of animals motionless on 

 the bottom of an aciuarium. when the membranellae may reverse 

 again and again without any movement on the part of the animal^ 

 Again during the motor reaction after the first reversal of both 

 the membranellae and cilia, the former resume their backward 

 beat while the cilia continue their reversed action for some time. 



That the membranellae nuist be largely responsil)le for the 

 gliding movement is indicated by the fact that v.hen on the 

 surface film often the only part close enough to the film to l)e 

 able to act on it is the buccal groove with its fringe of membra- 

 neDae. while the rest of the animal hangs down freely. Of course, 

 the body cilia must have some effect, but owing to the fact that 

 the buccal groove is held so close to the film that the body does 

 not rotate as in swimming, and that the direction of the move- 

 ment is to the right instead of to the left as in gliding on the 

 substratum and just opposite to the beat of the membranellae, it 

 cannot be doubted that these organs play a considerable part in 

 the animal "s movement. 



Fate of Gliding. — The rate of gliding is fairly constant under 

 normal conditions. Animals on the surface film have a faster 

 rate than those on the bottom of an aquarium. ^Measurements 

 of the time required for the animals to travel 1 mm. showed 

 that the distance is covered in about 2.9 seconds. The rate of 

 movement changes considerably when the animals are .stimulated, 

 apparently increasing to two or three times the ordinary rate. 

 The increase in rate on the surface film may be correlated with 

 the fact that the body of the animal is not in contact with the 

 film, and the cilia beat faster, as is the case when swimming. 



