The Distribution of Senescence 



thoroughly-studied, at least from the descriptive point of view. 

 The life-span varies in different species from a few days to 

 several months, and each species tends to exhibit very sharp 

 specific age. After a period of growth, which takes place by 

 increase in cell size, the nuclear number being fixed, and adult 

 vigour, rotifers enter a period of senescence, with conspicuous 

 loss of activity, degeneration of cells, deposition of pigment, and 

 ultimate death in extension. In some forms the senescent phase 

 is genuinely post-reproductive, but in the majority it occurs 

 while egg-laying is still occurring at a diminished rate, and may 

 be accompanied by the production of malformed eggs, or eggs 

 of varying size. 



The external appearances of rotifer senescence have been 

 vividly described in several forms (Callidina, Plate 1886; Pleur- 

 otrocha, Metchnikoff, 1907; Proales, Noyes, 1922; Jennings and 

 Lynch, 1928; Hydatina, Plate, 1886; Lecane, Szabo, 1935; Miller, 

 1931; Rotifer vulgaris, Spemann, 1924). The animal becomes 

 sluggish in behaviour and reaction to jarring, the tissues and 

 cuticle shrink and become opaque or granular in appearance, 

 swimming is replaced by creeping, pigment accumulates in the 

 gut, digestive gland and mastax. The movements of the pharyn- 

 geal cilia are the last signs of life to persist. 



It seems clear that this is an endogenous process of degenera- 

 tion. A number of attempts have been made to correlate it with 

 other features of rotifer organization. Plate (1886) considered 

 that senescence in Hydatina occurred typically when the activity 

 of the ovary, and the supply of germ cells, failed. This is not the 

 case in all rotifers, however. In Lecane inermis Miller (1931) 

 found that the mictic females cease egg laying early in life and 

 have a relatively prolonged post-reproductive period, while 

 amictic females show signs of age before the last egg is produced, 

 and all are dead within two days thereafter; the life-span of 

 males is even shorter (Fig. 20). In this species, fertilization of 

 the mictic females does not appear to influence longevity. 

 Miller attributes the difference in life-span between mictic and 

 amictic females directly to the difference in fertility, but this is 

 not fully borne out by her life-tables, the chief difference being 

 in the longer post-reproductive period of the mictic females. In 

 Hydatina senta it is the amictic females which are the longer 



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