The Distribution of Senescence 



many other groups, but sponges seem ideally able to conform 

 to Bidder's expectations of them. 



2-5-2 COELENTERATES 



In hydromedusae, Child (1918) observed a progressive 

 decrease in metabolism and pulsation rate with increasing size, 

 which he regarded as evidence of senescence. His work on the 

 processes of ageing and rejuvenation in hydroids (1915) depends 

 on the criterion of resistance to cyanide as evidence of 'physio- 

 logical age' — one which is hardly acceptable in this context. 

 Child's results with Pennaria, using this test of age, were in any 

 case less consistent than those he obtained with planarians, 

 where cyanide resistance rose steadily throughout life (Child, 

 1915). 



Evidence that the life-span of sea anemones is 'indeterminate* 

 is probably stronger than for any other metazoan group. 

 Dalyell's (1848) celebrated specimens of Actinia lived for 70 

 years in captivity without any sign of deterioration. An even 

 more famous batch of sea anemones were collected 'some years 

 prior to 1862', and were first identified as Sagartia troglodytes 

 by Ash worth and Annandale (1904), later by Stephenson as 

 Cereus pedunculatus (1935). They remained in the aquarium of 

 Edinburgh University Department of Zoology until 1940 or 

 1942, when they were all simultaneously found dead. Budding 

 continued freely throughout life, and the animals underwent 

 no obvious change during eighty to ninety years of continu- 

 ous observation (Warwick, 1954, personal communication). 

 Whether gametogenesis likewise continued throughout life is 

 not known. 



Hydra, The long-standing controversy over the senescence of 

 Hydra illustrates some of the difficulties of placing a geronto- 

 logical interpretation on life-tables and histological appear- 

 ances. Hydra was a favourite organism, earlier in the century, in 

 the argument over the 'potentielle Unsterblichkeit' of metazoa. 

 Differences in culture conditions almost certainly account for 

 the very irregular results obtained. 



Early workers (Hertwig, 1906; Boecker, 1914; Berninger, 

 1910) on this question found it impossible to keep Hydra for long 

 periods without the onset of 'depression', evidenced by cloudy 



81 



