The Biology of Senescence 



and the cessation of reserve-cell production from the embryonic 

 tissue persisting in the posterior end. The significance of these 

 changes is again obscure, and no attempt was made to deter- 

 mine actuarially the mortality rates at different ages. Rhab- 

 docoelians have (Bresslau, 1928-33) been observed to be in- 

 creasingly susceptible to protozoan parasites the longer they live. 



How far the capacity for 'degrowth', which is found in 

 planarians, is evidence of a potentially indeterminate life-span 

 is not evident, but it seems likely that forms such as Lineus 

 (Nemertinea), which revert on starvation over a period of years 

 to a mass of cells resembling an embryo (DawidofT, 1924), might 

 be maintained indefinitely in alternate growth and degrowth 

 until the patience of the investigator was exhausted. 



The evidence in fissile worms at present suggests that non- 

 senescence depends upon fairly active replacement of cells, and 

 that any organ which fails to take part in the regenerative pro- 

 cess is liable to undergo senile change. Harms (1949) considered 

 that the senescence of Serpulids was due primarily to changes 

 in the nervous system, and rejuvenated old specimens oiProtula 

 by grafting young heads. Some further work on this subject as 

 careful as that of Child and Sonneborn would probably be well 

 worth undertaking. 



Morphogenetic loss throughout life of the power of regenera- 

 tion in a nematode of determinate cell-number was actually 

 demonstrated by Pai (1928) in Anguillula aceti. Amputation of 

 the tail with nuclear removal kills the animal at any age. In 

 young individuals, provided the nucleus is left intact, wound 

 healing and cytoplasmic regeneration can take place. In mature 

 animals there is wound closure but no cytoplasmic regenera- 

 tion, while in senile animals amputation is fatal. In Anguillula 

 senescence follows a pattern very similar to that of rotifers 

 (see below) and occurs at about 44 days. The degenerative 

 cellular changes in ovaries, gut and nerve cells have been des- 

 cribed: these appear in the two or three days preceding death 

 Pai, 1928; Burger, 1954). 



2-5-4 ROTIFERS 



The ageing of rotifers is one of the most spectacular examples 

 of endogenous senescence in animals. It is also one of the most 



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