1916.] 



NATURAL SCIENCES OF PHILADELPHIA. 



1 13 



The curves in fig. 2 show the rate of growth in three different 

 environments. These are but approximations. Further study may 

 show that the first year may be really the second. 



The oldest snail found was estimated as being seven years old. 

 The great majority are about two or three years old. 



Cooke (1895) estimates that each female snail lays two hundred 

 egg capsules a year. Each capsule contains about four hundred 



Fig. 2. 



eggs. Food seems to be in most places abundant. The snails have 

 been observed to be limited in number by the following factors : 



(1) Cannibalism within the egg capsule, only ten to twelve hatch- 

 ing out of 300-400 eggs. 



(2) Limit to the number that can take refuge in cracks in rocks 

 and yet get enough to eat. 



(3) Fish, such as pollack, will gobble up any stragglers. 



(4) Half-grown snails, ones too big for the pollack, are used by 

 herring gulls as food, the empty shells being regurgitated. 



■ ■ 



