The Distribution of Senescence 



Lilly, 1953; Fossaria — van Cleave, 1935; Sphaerium — Foster, 

 1932; Teredo navalis — Grave, 1928). Specimens of Limnaea 

 columella kept in captivity under good conditions give a life- 

 table showing a typical senile increase in force of mortality 

 similar to that in Drosophila (Winsor and Winsor 1935; Baily 

 1931). The growth of Limnaea has been studied by Baily (1931) 

 and Crabb (1929). Among rather longer-lived forms, Pelseneer 

 (1934) found a complete cessation of shell growth and a decline 



200 



100 - 



i 



i 



I 2 3 4 5 6 

 YEARS OF LIFE 

 Fig. 27. — Egg production of Eulota fruticum (after Kunkel, 1928). 



in fertility with age in Gibbula umbilicalis, complete infertility 

 being general at about 54 months, and the extreme life-span 

 4J-5 years. In Eulota fruticum, the relation of egg-laying to age 

 has been determined. According to Kunkel, death takes place 

 in captivity 'when the germinal glands are exhausted' (Kunkel, 

 1928). (Fig. 27.) In Physagyrina DeWitt (1954) found a distinct 

 post-reproductive period, amounting to as much as 49 per cent 

 of the total life-span in mass culture, or 36 per cent in isolation 

 culture, the overall mean life-spans being 211 and 143 days 

 respectively. Szabo and Szabo (1929-36 passim) published a 



103 



