Studies on Regulation. IV. loy 



attachment although in Leptoplana it is not so exclusively em- 

 ployed for this function as in many other forms. 



Leptoplana differs from many other species of polyclads in the 

 definite direction of its movements. In some forms, e. g. Stylo- 

 chus, the direction of movement is very indefinite, movements in 

 other directions being almost as frequent as anteriorly directed 

 movements. In Leptoplana, however, the deviation from the 

 longitudinal direction is slight. 



As a general rule the more posterior portions of the margin 

 and the posterior end itself are used more frequently as organs 

 of attachment than the more anterior regions. 



In consequent of the adhesion to the substratum by the margins 

 and posterior end, the body of Leptoplana is subjected to mechan- 

 ical tension in the longitudinal direction, often visibly in a con- 

 siderable degree, during creeping. As in the case of Stenostoma 

 (Child, '02, '03), this longitudinal tension constitutes a factor 

 in determining the general form and outline of the body. The 

 fact that the margins as well as the posterior end are employed 

 as organs of attachment accounts for certain characteristic fea- 

 tures in connection with the form. 



From the facts above cited regarding movement we must con- 

 clude that the posterior portions of the body are subjected more 

 frequently than the anterior parts to longitudinal tension In con- 

 sequence of their poslton and more frequent use for attachment, 

 and moreover, that the tension is greater than that In the anterior 

 regions since all ciliary impulses and muscular contractions aiding 

 in forward movement anterior to the point of attachment com- 

 bine to produce it. This statement is correct in a simple case 

 but frequently various points along the margin may become at- 

 tached simultaneously and the tension Is distributed among them. 

 The continual muscular play of the margins, the rapid transitions 

 which a given region undergoes from attachment to reattachment 

 are of course accompanied by great variation in mechanical con- 

 ditions. The Important point is that the tissues are subjected to 

 longitudinal tension and the posterior regions more than the an- 

 terior. 



