Studies on Regulation. IV. 105 



movements do not cease abruptly as they pass posteriorly but 

 gradually decrease in amplitude until no longer visible. During 

 extreme activity they may extend much further posteriorly than 

 under ordinary conditions and frequently slight undulations of 

 the margins appear along the sides and pass posteriorly even 

 when the animal is creeping. During swimming the anterior 

 region of the body is considerably broader than in Figure i. 



It can scarcely be doubted that these movements play a part 

 in shaping the regions in which they occur. A comparison be- 

 tween frequency, amplitude, and force of the undulating move- 

 ments and the degree of lateral development in the regions in 

 which they occur is most striking. According to the usual point 

 of view this correlation between structure and function is merely 

 one of the many remarkable cases of adaptation, but in my opin- 

 ion it is, at least in part, the direct result of function in the indi- 

 vidual. Some experimental evidence bearing on this point will 

 be offered elsewhere. 



As regards the manner in which the movement may affect the 

 tissues it is not difficult to see that the movement of these parts 

 to and fro through the water must subject them to tension in the 

 in the lateral direction. This must affect in greater or less degree 

 the distribution and arrangement of the plastic tissues composing 

 the parts. A very simple physical experiment serves to illustrate 

 this point. A cylindrical or square stick of sealing-wax moved 

 to and fro in one plane in water sufficiently warm to soften it 

 will undergo flattening in a plane at right angles to the direction 

 of movement. The change in form is more strikingly shown if 

 a rigid axis is present; a mass of wax molded in cylindrical form 

 about a stiff wire will become in a few minutes a thin, flat plate 

 decreasing in thickness towards the edges and with a rounded 

 outline. The mechanical conditions resulting from the move- 

 ment of the wax through the water are not widely different from 

 those which the undulating margins of Leptoplana produce. If 

 the wire axis of the wax be considered as the longitudinal axis the 

 effect of movement through the water is lateral extension. In 

 Leptoplana the undulating movement is confined chiefly to the 

 lateral regions in the anterior third of the body and it follows that 

 the conditions described are limited chiefly to these parts. 



