io6 C. M. Child. 



There can be little doubt, in my opinion, that these mechanical 

 conditions constitute a factor in the formation of the broad lateral 

 regions in Leptoplana 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 char- 

 acteristic method of activity. The experimental data to be de- 

 scribed support this view. 



In addition to its power of swimming, Leptoplana is able to 

 creep over surfaces rapidly and in a definite direction. Both 

 muscular and ciliary activity are concerned in the movements, 

 but one or the other may predominate according to conditions. 



When the animal is moving quietly, as for instance after a 

 slight stimulation, the cilia afford the chief motive power, although 

 the slight muscular movements of the margins of the body are 

 almost constant, portions being lifted from the substratum, 

 brought forward, and again attached. This muscular play of 

 the margins is especially marked in the anterior regions but extends 

 in some degree along the whole side of the body. 



After a strong stimulus the movements take on a different char- 

 acter, becoming chiefly muscular. The portions of the body In 

 which the undulating movements occur during swimming furnish 

 under these conditions the chief motive power. Parts of the 

 margin are lifted slightly, extended in the antero-lateral direction, 

 and attached to the substratum : contraction of the muscles fol- 

 lows and the body is drawn forward. These movements occur 

 in rapid alternation on the two sides of the body and the similar- 

 ity between this mode of progression and the use of legs can- 

 not escape the observer. The animals appear almost as if walk- 

 ing forward. 



At all times during creeping movements the body adheres 

 closely to the substratum as may be demonstrated by sudden at- 

 tempts to dislodge it. The chief regions of attachment are the 

 lateral margins and the posterior end. Frequently during creep- 

 ing small portions of the body margin which adhere more closely 

 than other parts are stretched posteriorly to a considerable degree 

 before they are torn away from the substratum. As In many 

 other Turbellaria the posterior end is an important organ of 



