The Biology of Senescence 



swelling and cytolysis. With better cultural methods Goetsch 

 (1922, 1925) kept individuals of Pelmatohydra oligactis, Hydra 

 attenuata, and Chlorohydra viridissima alive for 27 months. Goetsch 

 considered that like the actinians Hydra was capable of remain- 

 ing indefinitely in statu quo. Gross (1925) working with P. oligactis 

 failed to keep any individual alive for more than 349 days, 

 'senescence' being evidenced by irregular and hypertrophic 

 budding or by the animal becoming smaller and smaller in the 

 presence of abundant food. 'Senile' changes in Gross's material 

 began after the fourth month of life. A life- table, drawn from 

 Hase's (1909) data by Pearl and Miner (1935), extending over 

 only 148 days, indicates some increase in mortality with age, 

 but is closer to the log-linear than to the rectangular contour 

 (see Fig. 6c, p. 20). Hartlaub (1916) had already described 

 experiments on Syncorinae in which he concluded that the 

 power of producing gametes was lost relatively early in life, 

 while that of budding persisted. 



David (1925) kept isolation records in cultures of P. oligactis 

 and satisfied himself that in this form the individual animals 

 tended to die between 20 and 28 months in approximate order 

 of individual age — an important observation which has not been 

 repeated. According to Schlottke, however (Schlottke, 1930), 

 the material in David's histological sections was heavily parasit- 

 ized. Schlottke's own observations suggested that all the tissues 

 of Hydra are continuously replaced throughout life, from a sub- 

 jacent reserve of interstitial cells. This view is supported by the 

 work of Brien (1953), who showed by marking experiments that 

 there is continuous growth in Hydra from before backward, the 

 marked zone travelling down the animal and being ultimately 

 rejected at the base: a case, in other words, of 'indeterminate 

 growth' coexisting with a final specific size. 



In colonial hydroids, however, it seems to have been shown 

 beyond reasonable doubt that the life-span of each hydranth is 

 physiologically determinate. The resorption and involution of 

 hydranths was described in full by Huxley and de Beer (1923); 

 the hydranth shrinks, the gut becomes filled with cellular 

 debris, and the degenerating material is returned to the colony 

 by the contraction of the hydranth itself. In Obelia and Cam- 

 panularia Growell (1953) has now shown that regression takes 



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