The Distribution of Senescence 



appears to be definable in terms of ins tars, D. longispina living 

 for 19-22 instars, the duration of which depend upon the con- 

 ditions of culture (Ingle, Wood and Banta, 1937) and D. magna 

 for 17 instars (Anderson and Jenkins, 1942). Detailed studies 

 upon factors which retard or accelerate the rate of development 

 and life-span in Daphnia have been carried out. (Mc Arthur and 

 Baillie, 1926 seq.; Ingle, Wood and Banta, 1937; Anderson and 

 Jenkins 1942, etc., see pp. 143 seq.). In view of the availability 

 of life-tables for Daphnia, the pattern of its normal growth is of 



Observed growth 

 Calculated growth 

 Growth rate 1st cycle 

 Growth rate 2nd cycle 



r -. i_ 



60 Days 70 



Fig. 25 (a).— Growth of Daphnia magna— first type (Edlen, 1938). 



particular interest. Edlen (1938) showed that the growth of 

 normal daphnids takes place in two cycles, the first levelling off 

 after three or four instars, and the second coinciding with the 

 development of the gonad. He found three types of pattern in 



fed upon living cultures of protozoans or algae, the food organism itself may 

 metabolize and remove pantothenic acid. It is highly questionable in view 

 of Fritsch's findings how far the life -tables obtained by workers using 

 different culture techniques, or even by one worker at different times, are 

 comparable. This is unfortunate, as Cladocerans are most useful organisms 

 to gerontologists — further standardization of culture techniques seems 

 essential if they are to be used in this way, however. 



93 



