540 RAYMOND PEAKfu 



a solid (viz. tlio surfaco film). The only possible exception 

 to tliis is tlie ]iassa<>-e of the animals from the surface film 

 to the l)ottom on a string of mucus, as described above. 

 Furthermore, so far as I can find in the literature, no one 

 else has ever seen a fresh- water triclad swim freely through 

 the water. 



The movement takes place with the body in contact with 

 a surface either of a solid or of the surface film. There is, 

 of course, between the ventral surface of the body and the 

 surface on which it is moving, the thin layer of mucus which 

 is constantly being secreted. It is in this mucus layer 

 rather than the free water that the cilia beat. 



This gliding movement is, so far as I have been able to 

 ascertain, brought about by the action of the cilia on the 

 ventral surface. There may also be some very slight 

 muscular movement of the ventral body-wall comparable to 

 that in the foot of a mollusc like Physa, which assists in the 

 locomotion; but in the case of the flat-worm this factor, if it 

 exists at all, is very insignificant. Only in a few instances 

 have I been able to satisfy m3^self that any such movement 

 was taking place, and then it did not have the characteristic 

 rhythm seen in a mollusc. ■ If this factor has any effect at all 

 on the gliding movement it must be an extremely slight one. 



The cilia beat strongly backward, i. e. towards the poste- 

 rior end of the body. I have not been able to induce any 

 reversal of the ciliai*y beats in these ventral cilia. Bardeen 

 (loc. cit., p. 15) states that when the head is suddenly drawn 

 back from some object the movement of the cilia on the 

 antero-lateral margin of the head is reversed, and further 

 suggests that " this reversed action may possibly be set up 

 by the mechanical friction of the water." It would appear 

 that the suggestion is the correct explanation, and that this 

 is not a true reversal of effective beat. 



The cilia which are mainly effective in producing the 

 gliding luovcment are distributed on the ventral surface of 

 the body, as shown in Fig. 2. There is a band down the 

 centre of the body, which wid(Mis out at the anterior end so 



