28 



Shell Life 



of both sexes, but as a rule in bivalves tlie sexes are 

 in separate individuals. 



We have here several points of difference in struc- 

 ture and habit be- 

 tween the Garden 

 Snail and the Swan 

 Mussel ; there is 

 also a want of 

 uniformity in the 

 length of their 

 lives, for our pres- 

 ent subject attains 

 a respectable old 

 age varying from 

 twenty to thirty 

 years, and does 

 not begin to pro- 

 duce young until 

 it is five years old. 

 This difference of 

 age may be at 

 least partly ac- 

 counted for by the 

 quiet sedentary 

 life lived by the 

 Swan Mussel, which has also probably brought about 

 a condition of degeneration from a primitive form ; 

 whilst the active Snail has no doubt advanced from 

 that primitive condition. The evidences of this 

 degeneration are to be found in the loss of head, 

 eyes, masticatory apparatus, and reduced means of 

 locomotion. 



One other dissimilarity between these two types 



Soft parts of Swan Mussel (^AnodothUi) 



a foot ; h, mouth ; e, lips; d, c, gills; /, vent; 

 g, mantle ; ]i, siphon 



