CILIARY AND MUSCULAR LOCOMOTION. 135 



hind part of the foot shows no muscular movement during the 

 entire reaction. 



The preceding observations lead to the conclusion that pedal cilia' 

 are beating actively at the time of locomotion, when the foot is 

 functioning as a righting organ, and when it is being moved about 

 as the snail is held out of the water, and that cilia are quiescent, or 

 nearly so, when the animal is at rest either in the water or out of 

 it. The results of certain experiments may now be recorded which 

 in my opinion leave little doubt that these cilia function as organs 

 of progression. 



A comparison of the rate of locomotion with the speed at which 

 objects were driven over the pedal surface by beating cilia, or what 

 may be termed the ciliary rate, was obtained in the following way. 

 A snail which was moving around the periphery of a shallow glass 

 dish filled with sea water was stopped by holding back the shell, 

 but grains of sand adhering to the mucus were carried along the 

 under surface of the foot by the active cilia as the animal attempted 

 to continue its course. By the use of a millimeter rule and mirror 

 placed beneath the dish it was possible with a stop-watch to obtain 

 records of the rate of their movement. One of the lateral margins 

 of the foot was slightly curled upward and some of the records 

 were taken as the sand passed over this area ; other grains were 

 timed as they moved with the mucus nearer the central and poste- 

 rior portions of the expanded foot. The average rate of move- 

 ment over a distance of 5 mm. in ten trials was found to be 2.7 

 seconds. Immediately after the ciliary rate was obtained the snail 

 was allowed to proceed over the bottom of the dish and ten records 

 of its speed were secured, which showed that it was moving at the 

 average rate of 5 mm. in 2.6 seconds. The remarkably close cor- 

 respondence between the locomotor and ciliary rates strongly sup- 

 ports the view that progression is accomplished by ciliary action. 



Watching Polinices glide over the bottom of an aquarium one 

 gains the impression that the animal is too heavy to be propelled 

 merely by the lashing of cilia in a film of mucus spread over the 

 substrate. That such a conclusion is in reality a false one was 

 proved by a few simple tests. As has been pointed out, the pedal 

 cilia are active when the snail is attempting to right itself, and at 

 that time it is possible to determine their driving power by placing 



