The Distribution of Senescence 



hormonal control over growth, and possibly over the mainten- 

 ance of life-processes generally. 



The chief senile changes in Daphnia appear to be in the fat 

 body, intestinal epithelium and musculature (Schulze-Robbecke, 

 1951). This author found no evidence of the 'cerebral death* 

 which was once widely accepted as the general cause of inverte- 

 brate senescence (Harms, 1926; Muhlmann, 1900, 1911), and 

 which was described by Walter (1922) in Cyclops. Withdrawal 



20 



CO 

 LU 



h- 



Z 10 



10 



20 



DAYS 30 



Fig. 25 (c). — Growth of D. magna — third type. One growth-cycle only 



(Edlen, 1938). 



of ovarian activity does not seem to be the direct cause of 

 senescence in Daphnia. Schulze-Robbecke's oldest specimens 

 still showed considerable ovarian activity, and continued to lay 

 eggs, though in reduced numbers, up to the time of death. 

 Oocytes at all stages of maturation remained in the ovary to the 

 last. Schulze-Robbecke attributed the death of Daphnia in old 

 age to failure of nutrition following degenerative changes in 

 the gut. 



Walter's work on Cyclops (1922) dealt with C. viridis, which 

 has a life-span of about 9 months, 'senile' change in gut 

 epithelium and in the cerebral ganglia being evident from the 



95 



