CILIARY AND M CSTULAR LOCOMOTION". 139 



until they entirely disappear. Ciliary progression was observed 

 more frequently than muscular in the case of snails confined in. 

 aquaria. The latter type now and then appeared when the animals 

 were moving in a lead-lined tank, over a glass surface and often 

 when they were climbing up the sides of glass aquaria. Ciliary 

 locomotion was successfully accomplished over pebbly bottoms as 

 well as sandy ones. In the former situation the foot, especially the 

 propodium, molded its surface to the irregularities of the substrate 

 and appeared as if it were flowing over them. The remarkable 

 effectiveness of this form of progression is readily understood, 

 however, if it is remembered that irrespective of the character of 

 the substrate the cilia are always beating against a thick layer of 

 mucus which adheres to the underlying materials, and forms a 

 smooth bed over which the foot can readily glide. 



If, when moving over a pebbly bottom, the propodium is thrust 

 beneath the stones, the foot is worked forward by muscular con- 

 tractions, the anterior end in contact with the glass bottom of the 

 aquarium. Thus when considerable resistance is to be overcome 

 the muscular form of progression is employed. This fact is well 

 illustrated by the behavior of a snail burrowing into the sand. 

 Rhythmic contractions go on as in ordinary locomotion. The pro- 

 podium is directed downward, and when the pedal wave reaches 

 the organ it is extended and worked farther forward and down- 

 ward into the sand. By holding back a burrowing animal, sand 

 grains may be seen travelling along the sole of the foot and also 

 over the dorsal surface of the propodium, showing that the cilia 

 are beating. By this procedure a snail may completely disappear 

 beneath the sand in less than a minute and a half to a little over 

 two minutes. When the anterior end of the foot was cut off, 

 Poliniccs could not burrow successfully. By continued pedal con- 

 tractions it was able to work into the sand somewhat, but at the 

 end of about an hour it rested with a part of the shell still exposed. 

 The propodium, therefore, although not necessary in locomotion, 

 is essential for burrowing. 



CILIARY CONTROL. 



It is now well established that the transmission which excites 

 activity over ciliated epithelia is not ordinarily directed through 



