The Distribution of Senescence 



longer life in captivity. Oldham (1930) kept Planorbis corneus 

 in active reproduction up to 6 years of age. Many Helicidae, 

 especially the smaller forms, have been regarded as annuals 

 in the wild (Lamy, 1933; Pelseneer, 1935): the potential life 

 of helicids and medium-sized land snails in captivity (exclud- 

 ing diapause, aestivation and so on), may, however, reach or 

 exceed 10 years (Rumina decollata, 12 years, 'Helix spiriplana\ 

 15 years — Vignal 1919; Helix pomatia, 6-7 years — Kiinkel 1916; 



25 



50 75 



Fig. 28. 



100 J25 150 175 200 225 

 AGE IN DA\S 



-Life-span of the pulmonate Limnaea columella at two different 

 population densities (from Winsor and Winsor, 1935). 



6-8 years — Cuenot, 1911; H. hortetisis, 6 years, hybrid H. hor- 

 tensis x nemoralis 10 years — Cuenot, 1911) while Oxystyla capax 

 has been revived from diapause after 23 years (Baker, 1934). 



Even less is known about the longevity and liability to senes- 

 cence of most marine gastropods. In Acmaea dorsuosa Abe (1932) 

 found that growth continued in 15-year-old specimens from 

 some localities, while in other localities an apparent specific 

 size was reached at 5 years. Apparent specific size in certain 



105 



