544 RAYMOND PEARL. 



in part tlic expression of local changes in the tonic contrac- 

 tion of the muscles. 



In the gliding movement the body is in close contact with 

 the surface along which the animal is moving. When an 

 animal passes from the resting condition into movement one 

 can see the body lengthen and flatten so as to hug the 

 surface. By observing with a compound microscope an 

 animal gliding along the vertical side of a dish so that the 

 edge is brought sharply into view, the closeness of the con- 

 tact of the margin of the body with the surface can be well 

 seen. Furthermoi'e, in specimens in which the posterior part 

 of the body has been split longitudinally in the middle line 

 to a point just behind the head, it is found that the half of 

 the body which is determining the direction of the move- 

 ment is always in close contact with the surface, while the 

 other half only lightly touches it. 



It would appear from all the observations which have been 

 given that the gliding movement is brought about in the 

 following Avay : — The ventral surface of the body constantly 

 secretes mucus in greater or less quantity. This mucus can 

 be shown experimentally to be very sticky immediately after 

 it is secreted into the water. As it is secreted under normal 

 conditions it immediately sticks to the surface on which the 

 animal is reposing. Thus there will be constantly between 

 the animal and the surface on which it is moving a layer of 

 mucus which is adherent to the substrate. We can think of 

 the lowest part of this mucus layer where it is stuck to the 

 surface as of denser consistency than its upper layers which 

 are in contact with the animal. In this upper layer of the 

 mucus the cilia are beating and constantly pushing the 

 animal forward. Of course, what really takes place is that 

 the cilia are pushing the secreted mucus backward, but as 

 this layer of mucus becomes fixed to tiie substrate as soon as 

 it is secreted, the practical result is that the animal is pushed 

 ahead. 'IMiis relation is shown in Fig. 4. A represents a 

 side view of a gliding worm ; D is the substrate ; C the cilia 

 on the ventral side of the organism ; and 13 the mucus secre- 



