C. M. CHILD. 



most anterior piece and much larger in the two posterior pieces 

 c and d which together equal b of Series 19. And finally, in 

 Series 27 which consists of ^ pieces all the pieces encyst except 

 30 per cent, of a and 10 per cent, of b. When the pieces are 

 cut still smaller all encyst. 



The frequency of encystment then shows in pieces of equal 

 size a gradation from the anterior to the posterior end of the 

 body and indicates the existence of some sort of a physiological 

 gradient in the animal. Encystment may, however, occur in 

 pieces from any region if they are sufficiently small, but in general 

 anterior pieces must be smaller than posterior pieces to give the 

 same frequency of encystment. This fact indicates that the 

 physiological state of the piece differs in some way with its size. 

 As a matter of fact this species possesses essentially the same sort 

 of gradient in rate of metabolism as Planaria dorotocephala 

 (Child, '130, 'i3c) and the relation between frequency of encyst- 

 ment, region of the body and size of the piece depends upon the 

 existence of this gradient and the changes in rate of metabolism 

 which occur in pieces from different regions of the body and of 

 different size after isolation. Further consideration of these 

 points is postponed to another time. 



3. The Frequency of Encystment of Pieces in Relation to 



Temperature. 



Series 5J, October 5, IQII. Animals 9-10 mm. in length were 

 selected from a stock which had been kept at a temperature of 

 20 C., the heads removed and the bodies cut into four equal 

 pieces a-d. Lots of ten each of each of the four pieces were 

 placed in three different temperatures, 10, 20 and 28-30 C. 

 Table VI. gives the results in percentages. 



It is evident at once from Table VI. that the frequency of 

 encystment is greater with higher than with lower temperature, 

 i. e., the higher the rate of metabolism in the pieces the greater 

 the frequency of encystment. Numerous other series give the 

 same results without exception, not only for pieces, but for whole 

 worms. Worms which have been kept at a temperature of 20, 

 when placed in a temperature of 30 will often encyst entire 

 while at 20 they remain active until they fragment and the 

 pieces encyst, and at 10 many of them do not encyst at all. 



