The Biology of Senescence 



under study — an extraordinary deficiency, which is a measure 

 of the equally extraordinary lack of interest in age processes. 

 It is fairly evident, however, that the distribution both of sen- 

 escence in general and of any one process of senescence, such 

 as depletion or mechanical deterioration, is quite discontinuous 

 in phylogeny. This evolutionary discontinuity is what we should 

 expect if 'exhaustion of programme' is the common basis of 

 adverse age changes. 



Senescence in some shape or form probably occurs in every 

 group where the power of regeneration or fissile reproduction 

 is less than total, or where body-cells are not continuously and 

 'indeterminately' replaced. Some forms which 'degrow' under 

 adverse conditions appear to be capable, in all probability, of 

 unlimited alternate growth and degrowth, at least in the 

 laboratory, while in a few, such as actinians, the adult can 

 remain indefinitely in statu quo, though with a changing popula- 

 tion of cells. Senescence is most striking in forms such as rotifers 

 where determinacy of cell number is very highly-developed and 

 the power of regeneration is usually negligible. There do not 

 appear to be any invertebrate cells (except possibly pelecypod 

 neurones, of whose longevity and renewability we know little) 

 which are called upon to remain for 100 or more years in active 

 function, like a human neurone, or for still longer, like the 

 neurones of the tortoise. The distribution of senescence in 

 invertebrates suggests that in spite of the general argument 

 against the selection of long-lived forms, relatively great longev- 

 ity is sometimes an evolved adaptation, and that if some cold- 

 blooded vertebrates are in fact immune to senile change, that, 

 too, is likewise a specialized mechanism and not a primitive or 

 an 'inherent' mechanism which has been lost with increasing 

 somatic complexity. 



2-5-1 PORIFERA 



Bidder infelicitously cited 'the sea anemone, the bath sponge 

 and the water-vole' as three organisms insusceptible to sen- 

 escence. The only serious study of senescence in Porifera appears 

 to be that of Arndt (1928) who concludes that it does not occur, 

 although some sponges are fatally disrupted by their own larvae. 

 Aquarium specimens have an effectively limited life, as in so 



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