1 68 Studies in Animal Behavior 



degree the distribution and arrangement of the plas- 

 tic tissues composing the parts. A very simple phys- 

 ical experiment serves to illustrate this point. A 

 cylindrical or square stick of sealing wax moved to 

 and fro 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 de- 

 creasing in thickness towards the edges and with 

 a rounded outline. The mechanical conditions re- 

 sulting from the movement 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 longi- 

 tudinal axis the effect of movement through the 

 water is lateral extension. In Leptoplana the un- 

 dulating 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." 



"There can be little doubt, in my opinion, that 

 these mechanical conditions constitute a factor in 

 the formation of the broad lateral regions in Lepto- 

 plana and more especially in other forms in which 

 the undulating movements of these parts occur. In 

 other words, the form is in some degree the result, 

 not the cause, of the characteristic method of ac- 

 tivity." 



