CH. VII] SNAILS AND SLUGS 209 



depths of colour. These are usually regarded as distin- 

 guishing one year's growth from that of another. This 

 explanation is very probably correct, but I am not aware 

 of any exact observations on the matter. The length of 

 life of H. aspersa is said to be about five years, and that of 

 H. pomatia from six to eight years. It is however highly 

 probable that growth ceases some time before natural 

 death ensues ; hence shells could hardly be expected to 

 exhibit a number of " annual lines " exactly corresponding 

 to the years of life of an old snail. I have never found 

 more than three " annual lines " on shells of H. aspersa. 



The thickness of the shell is dependent upon the 

 nature of the soil in any given locality. Thus Taylor 

 states that shells of H. aspersa collected in Guernsey where 

 calcareous strata are absent are very thin, whereas those 

 found on the limestone strata at Swanage and Tenby are 

 unusually thick. 



Semper's 1 researches on the phenomena of growth have 

 shown that in the freshwater snail Limncea stagnalis the 

 size attained by the shell is influenced both by the 

 temperature and by the volume of the water in which the 

 specimens are reared. The optimum temperature for this 

 species is between 68 F. and 77 F., while growth ceases 

 alike if the temperature exceed 90 F. or fall below 53 F. 

 The volume of water required per individual is about 

 5000 cubic centimetres. Specimens reared in 100 c.c. of 

 water attained a length of only 6 mm. in 65 days, others, 

 members of the same family, reared in 250 c.c. of water 

 became 9 mm. long, while yet others in 2000 c.c. of water 



1 " Animal Life," Internal. Sci. Ser. vol. xxxi. 

 L. H 



