The Distribution of Senescence 



sudden change, especially of diet and of physiochemical com- 

 position of the water, induces the appearance of mictic females' 

 (Hyman, 1951). The reason for the difference in longevity 

 between Lansing's old and young orthoclones is not, it should 

 be noted, entirely comparable to that between mictic and 

 amictic females. It does not seem to represent a difference in 

 specific age due to a shortening of that part of the survival- 

 curves which, owing to the low early mortality in rotifers, is 

 usually horizontal, but rather a 'breaking away' of this plateau 

 by the introduction of a higher and higher early mortality, the 

 curve becoming less and less rectangular and more and more 

 oblique. If conclusions are to be drawn upon the effect of 

 maternal age upon senescence, this difference is important. 



It is possible that the uniform specific age of rotifer popula- 

 tions is due to depletion. Reproductive exhaustion has already 

 been discussed. It is also known that the limited regeneration 

 observed in Stephanoceros takes place at the expense of reproduc- 

 tion and of somatic growth. Little is known of the metabolic 

 capacity of rotifers — they apparently store glycogen, but may 

 be incapable of assimilating carbohydrate (Hyman, 1951). Sud- 

 den senescence might well represent the exhaustion of a meta- 

 bolic substrate, or of a non-renewable system. Accumulation has 

 also been suggested: pigment certainly does accumulate, prob- 

 ably secondarily to the ageing process. Lansing (1942) demon- 

 strated the accumulation of calcium in old rotifers, and suc- 

 ceeded in prolonging their life by immersion in dilute citrate 

 solution — it is not clear how often this process can be repeated. 

 A more curious factor influencing the life-span was observed by 

 Edmondson (1945) in Floscularia conifera, where individuals 

 growing in aggregation reach twice the length, twice the age, 

 and a higher level of fertility than solitary specimens. 



The peculiarities of rotifer organization are so numerous that 

 some, if not all, of the mechanisms controlling their longevity 

 are likely to be peculiar to the group. On the other hand, their 

 short life-span makes them a suitable object for study, and they 

 provide an unequivocal example of senescence coupled with 

 cellular non-renewal which calls for further investigation. 



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