136 SMITHSONIAN MISCELLANEOUS COLLECTIONS [vOL. 45 



the longitudinal axis of the body, but the direction of its growth 

 depended entirely on the direction of movement of the piece. In this 

 manner specimens were obtained in which the tip of the regenerated 

 tail was overlapped by the head. The body was not simply tem- 

 porarily bent, but it was impossible for the animals to straighten 

 themselves. The new tissue had grown out at an angle to the 

 longitudinal axis, the size of the angle varying inversely as the size 

 of the circle in which the piece moved. In other words, the re- 

 generating tail being used constantly for attachment was subjected 

 to tension, but, owing to the circular movement of the piece, this 

 tension was not in the direction of the longitudinal axis, but formed 

 an angle with it. The tension was effective either directly or in- 

 directly in determining the arrangement of the tissue. 



All of these facts and many others observed show the great in- 

 fluence of mechanical factors in determining the form of these 

 animals. 



It may be said that in Leptoplana, as in Stcnostoma, the charac- 

 teristic form or outline of the body is determined by mechanical con- 

 ditions of tension. In the normal environment, however, these con- 

 ditions depend on the characteristic activity of the animal and the 

 characteristic properties of its tissues. From this point of view the 

 forms may be regarded as predetermined, but even so, only indirectly, 

 since it is the result rather than the cause of function. These prob- 

 lems will be discussed more fully at another time : here it is possible 

 only to call attention to them briefly. 



II. Cestoplana 



The species of Cestoplana employed was a small white form aver- 

 aging about 10 mm. in length and less than i mm. in transverse 

 diameter. A slightly differentiated head with a number of eyes was 

 present. Posterior to the cephalic ganglia the much-branched in- 

 testine filled the whole body : the pharynx was situated in the middle 

 region of the body with some slight variation in position. 



In general regenerative power this form differed widely from 

 Leptoplana. In no case did extended growth in the posterior direc- 

 tion from a cut surface occur : the wound healed and a very small 

 bud of new tissue appeared, but this was all. The result was the 

 same whether only a small portion had been removed from the pos- 

 terior end of the body or all except two or three millimeters at the 

 anterior end. 



From an anterior cut surface, anterior to the brain, new tissue 



