96 C. M. Child. 



be less clearly marked than in those with relatively plastic 

 tissues. As is well known, the tissues of many of the poly- 

 clads are extremely tough and resistant. Stylochus, Thysanozoon, 

 and many other forms might be mentioned as examples of this 

 condition. As regards this feature also Leptoplana proved to be a 

 favorable form since its tissues are relatively soft and plastic, 

 though' much firmer than those of Planaria. With this form 

 as a basis it was possible to make some very interesting com- 

 parative observations upon various other species, which will be 

 discussed later. 



Nearly all the specimens used were collected about the Castel 

 dell' Ovo and were presumably Leptoplana tremellans. Since It 

 is Impossible according to Lang ('84, p. 482) to distinguish with 

 certainty the species of Leptoplana except by examination of the 

 copulatory organs In serial sections, my material may have In- 

 cluded other species — L. alcinoi and L. pallida. Nearly all the 

 specimens used, however, resembled closely the type of L. tremel- 

 laris represented In Lang's Figure i, Tafel IIL In no case did 

 individual specimens exhibit characteristic differences in the regu- 

 lative processes that could be regarded as specific, so the question 

 as to the species Is In any event of minor importance for the pres- 

 ent purpose. 



Specimens and pieces were kept Isolated or several together 

 according to the experiment. In Stender dishes of various sizes, 

 covered to exclude dust. The water was changed twice a week 

 or oftener, but the animals proved extremely hardy and capable 

 of living in small dishes even during summer for much longer 

 periods without change of water. 



An attempt was made early In the course of my work to de- 

 termine whether the activity of the animals was affected by light. 

 So far as I could determine, specimens kept In darkness were 

 slightly more active, but the difference was not sufficiently great 

 to exert any marked Influence on regulation. Later most of the 

 specimens were kept In darkness except when under examination. 

 All were kept without food. 



Extensive series of measurements were necessary In the study 

 of the form changes, and Leptoplana, like most of the turbellaria, 



