HEAD-FREQUENCY ■ IN PLANARIA 405 



all tabulated data are in percentages. It may be pointed out 

 once more that in these head-frequency data only differences 

 greater than 10 per cent are to be regarded as significant. The 

 sources of error are such that it is not safe to attribute definite 

 significance to differences less than this. 



HEAD-FREQUENCY IN RELATION TO SIZE OR PHYSIOLOGICAL AGE 



Tt has been shown that susceptibility to lack of oxygen (Child, 

 '19 c), rate of CO2 production (Child, '19 a), and oxygen consump- 

 tion (Hyman, '19 a, b, c) all decrease with increasing size in P. 

 dorotocephala, and Allen ('19 a) has recently recorded a similar 

 decrease in oxygen consumption with increasing size in two other 

 species. As regards the question whether size may be regarded 

 as a criterion of physiological age, attention may be called to 

 several points: First, animals sexually produced are approxi- 

 mately equal in size at the time of hatching, therefore the size of 

 such an animal at any given time represents the amount of growth 

 it has undergone, and if physiological senescence is associated 

 with growth and progressive development, the animal must grow 

 old as it becomes larger. In cases where comparison has been 

 possible, animals asexually produced have been found to show 

 approximately the same physiological condition in all respects as 

 sexually produced animals of the same size. Second, the physio- 

 logical changes which normally accompany growth and progress- 

 ive development have been prevented, at least so far as could be 

 determined, in Planaria velata by giving only sufficient food to 

 maintain a given size (Child, '14 c). Third, animals of the same 

 size and living under similar conditions are much more alike 

 physiologically than animals of different size. Fourth, sexual 

 maturity occurs only at a relatively advanced physiological stage 

 when a certain size and metabolic condition are attained, and 

 can be prevented from occurring by insufficient feeding or by 

 frequent regeneration. Fifth, the physiological changes with in- 

 creasing size in Planaria are similar to those usually regarded as 

 characteristic of physiological senescence in the higher animals 

 and man. In short, certain progressive physiological changes 



