LOCOMOTION IN GASTROPODS. 133 



ments were sometimes seen at rest, again beating slowly and 

 frequently at a high rate of speed, all accounting perfectly for 

 the erratic movements of the fragments. There is no evidence 

 that the locomotor mechanism of Alectrion triviUata contains 

 any important factor not represented in that of Alectrion obsoleta, 

 and I believe that progression in both species is accomplished 

 by cilia. 



If, as it appears, the rapidity of ciliary movement in Alectrion 

 trivittata tends to be reduced when the animal is held with the 

 ventral side of its foot at the surface of the water, some explana- 

 tion of this peculiarity should be sought. I believe it is to be 

 found in its habit of surface floating and moving, one not shared 

 by adult mud snails. Observation of the snails' movements at 

 the surface of the water gives one the immediate impression that 

 this form of activity is a decidedly leisurely one associated with 

 feeding. A snail may remain floating with extended proboscis 

 for a long time, and when it moves, it is often for short distances 

 in an apparently aimless way. At such times the cilia probably 

 beat more slowly than they do when the animal is moving in a 

 definite course over a substrate, as it was when the locomotor 

 rates were taken. 1 When the snail is fixed to a support with 

 its expanded foot at the surface of the water, its normal floating 

 position is closely simulated. It accepts the inverted position 

 often without showing any extended disturbance, and always 

 exhibits the feeding reaction when stimulated with fish meat. 

 It would not be surprising, therefore, if the behavior of the cilia 

 were similar under these two conditions, and I believe that a 

 somewhat reduced ciliary activity is characteristic of both. 



CILIARY CONTROL. 



In connection with the investigations described in the preceding 

 pages there developed the exceedingly interesting problem of 

 ciliary control. Doubtless it has occurred to the reader that the 



1 That snails move more slowly at the surface of the water than they do over a 

 glass surface is perfectly clear, the average speed of locomotion in the former 

 situation, based on thirty-four trials, being at the rate of 5 mm. in 2.7 seconds. 

 Drawing conclusions, however, on the rapidity of ciliary movement by direct 

 comparison of these two locomotor rates may not be justified, for a substrate is 

 not present in both cases. 



