The Distribution of Senescence 



place strictly in order of age, each hydranth having a life of 

 4 days at 21° G. and 7 at 17° G. When regression is accelerated 

 by starvation or adverse culture conditions, the age order is 

 still preserved. 



2-5-3 SUNDRY INVERTEBRATES 



Existing observations are scattered rather thinly over a num- 

 ber of groups. Child (1911, 1913, 1914, 1915, 1918) carried out 

 exhaustive studies upon the regeneration of planarians, and 

 upon their capacity for de-differentiation, to which subsequent 

 research has been able to add little or nothing. Here again, as 

 in Pennaria, he employed the increase in resistance to dilute 

 cyanide solutions as a criterion of senescence, on the assump- 

 tion that this change reflected a decrease in metabolic rate. 

 While susceptibility decreased as a function of age in the grow- 

 ing animal, planarians kept for several months at a constant 

 size showed no such increase, and planarians undergoing shrink- 

 age under adverse food conditions showed a decrease in sus- 

 ceptibility. Child also demonstrated the 'rejuvenation', partial 

 or entire, of regenerating fragments of planarians. This further 

 observation, using the same criterion of resistance to toxicity 

 (1915), that a gradient of 'rejuvenation' exists in Stenostomum 

 (Rhabdocoela) during the production of new zooids has been 

 confirmed by Sonneborn (1930) using direct life- table studies. 

 Sonneborn's experiments showed that the regenerative effects 

 of fission were markedly unequal in the two halves, since the 

 head portions, which required only to regenerate tails, under- 

 went typical senescence, and died after a limited number of 

 divisions, while tails, which required to regenerate most of the 

 body and nervous system, could be propagated indefinitely. 



In Aeolosoma (Gligochaeta), Haemmerling (1924) found that 

 the anterior end of the body appeared to undergo eventual 

 senescence, new worms being produced from the posterior end. 

 Stole (1902) had already given a circumstantial histopatho- 

 logical account of 'senile' death in Aeolosoma as a whole, but the 

 appearances observed might have resulted from almost any 

 environmental cause. In JVais (Annelida), Stolte (1924, 1927) 

 found extensive histological changes with age, with disappear- 

 ance of the normal zones, degeneration of the visceral ganglia, 

 G 83 



