The Biology of Senescence 



number of studies upon the 'senescent' changes observable in 

 the digestive gland and nervous system of Agriolimax: these, 

 however, were inconstant from species to species, and a life- 

 table (Fig. 6c, p. 20) constructed from their data by Pearl & 

 Miner (1935) shows a steady high mortality: it is probable that 

 none of these slugs reached their maximum potential age. 



Very little evidence exists to relate the apparent senescence of 

 short-lived molluscs in the wild to their growth pattern, or to 

 their potential life-span in isolation. Van Cleave (1935) and 

 Hoff (1937) considered that the snails Fossaria and Viviparus con- 

 tinue to grow throughout life; in these forms, according to van 

 Cleave, the maximum size which is characteristic of the facies 

 of any colony is secured by a combination of environmental 

 effects on the growth rate and an endogenous process of sen- 

 escence which kills the animal after the completion of its life- 

 cycle, irrespective of its general somatic growth. In the large 

 Trochus niloticus Rao (1937) found no evidence of senile mortal- 

 ity, the upper limit of age being about 12 years in a wild 

 population, and growth continuing at a decreasing rate through- 

 out life. On the other hand, in spiral gastropods with an 

 elaborate lip armature, growth must be effectively determinate 

 so far as shell size is concerned. It has been suggested that in 

 Polygyra growth in size ceases at lip-formation, but body weight 

 and shell thickness continue to increase (Foster, 1936). Przi- 

 bram (1909) quoted observations by Taylor and de Villepoix 

 that the gland-cells of the mantle disappear in fully-grown 

 specimens of Helix aspersa, H. nemoralis and Clausilia perversa. 

 The growth of molluscs is seasonal, and the development of the 

 gonad appears in some forms to compete with, or inhibit, body 

 growth — in oysters, the periods of shell growth occur in each 

 year before and after spawning (Orton, 1928) while in 

 Hydrobia ulvae parasitic castration leads to gigantism (Roths- 

 child, 1935). The life-span of such giants was unfortunately not 

 recorded. On a small series of Limnaea columella Baily (1931) 

 found that shell growth ceased at or soon after sexual maturity, 

 and that the shortest-lived individuals were those with the 

 highest growth rates. A life- table was constructed for this 

 species by Winsor and Winsor (1935) (Fig. 28). Species whose 

 life-cycle rarely exceeds two years may be capable of much 



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