ASEXUAL CYCLE OF PLANARIA VELATA. 183 



mental modifications of the life cycle will be discussed in another 

 paper. 



II. THE PHYSIOLOGICAL RESISTANCE TO DEPRESSING AGENTS 

 OF YOUNG AND OLD WORMS. 



In these experiments the method of comparing resistances 

 which I have called the direct method was used. Here the 

 depressing agent is used in sufficiently high concentration to kill 

 the animals within a few hours and the occurrence of death is 

 determined by disintegration of the worms which begins within 

 a few moments after death. This method has been fully de- 

 scribed in another paper (Child, '130). In that paper it was 

 shown that with this method the animals with the higher rate 

 of metabolism or more strictly of cell respiration are less resistant 

 and therefore die and disintegrate earlier than those with the 

 lower rate. Thus the differences in resistance enable us to 

 compare the rates of respiration and so in a general way the rates 

 of metabolism. 



In Table I. the first vertical column gives the length of time 

 in the depressing agent in hours and minutes, the second the 

 serial numbers of the lots of worms compared, and the columns 

 I.-V. under "Stages of Disintegration" give the number of 

 worms of each lot in each stage of disintegration at each time. 

 As regards the five stages, I have found it convenient to distin- 

 guish more or less arbitrarily these stages in the process of dis- 

 integration, for disintegration usually appears first in certain 

 definite regions of the body while other regions are still alive and 

 show movement and it follows a more or less regular course 

 (Child, 'i3). The five stages are briefly characterized as fol- 

 lows : 



I. Intact, no disintegration. 



II. Disintegration beginning, usually in head region. 



III. Body beginning to disintegrate but form still retained. 



IV. Margins disintegrated, form disappearing in consequence 

 of swelling of tissues and separation of cells. 



V. All epithelium and pigment gone; swelling of tissues has 

 extended to all parts and original form has disappeared. 



The distinction of these stages makes it possible to compare 



