56 THE DURATION OF LIFE. 



case of old animals takes place in the spring, as soon as the winter 

 sleep is over ; while in the two-year-old animals it also happens 

 later in the summer.' 



(5) ' The Hyalineae are mostly biennial : they seldom live three 

 years, and even in the largest species such an age is probably 

 exceptional. The smallest Hyalineae and Helicidae live at most two 

 years. The length of life is dependent upon the time at which the 

 parents are fertilized, for this decides whether the young begin to 

 shift for themselves early in the summer or later in the autumn, 

 and so whether the first year's growth is large or small.' 



(6) ' The species of Limnaeus, Planorlis, and Ancylus live two 

 to three years, that is they take two to three years to attain the full 

 size. L. aiiricularis is mostly biennial, L. palustris and L. pereger 

 two to three years : I have found that the latter, in the mountains 

 at Oberstorf in the Bavarian Alps, may exceptionally attain the 

 age of four years, that is, it may possess three clearly defined annual 

 markings, whilst the specimens from the plain never showed more 

 than two.' 



(7) ' The Paludinidae attain an age of three or four years.' 



(8) ' The smaller bivalves, Pisidium and Cyclas, do not often live 

 for more than two years : the larger Najadae, on the other hand, 

 often live for more than ten years, and indeed they are not full 

 grown until they possess ten to fourteen annual markings. It is 

 possible that habitat may have great influence upon the length of 

 life in this order.' 



' Unio and Anodonta become sexually mature in the third to the 

 fifth year.' 



As far as I am aware but few statements exist upon the length 

 of life in marine mollusca, and these are for the most part very 

 inexact. The giant bivalve Tridacna gigas must attain an age of 

 60 to 100 years 1 . All Cephalopods live for at least over a year, 

 and most of them well over ten years ; and the giant forms, 

 sometimes mistaken for ' sea-serpents,' must require many decades 

 in which to attain such a remarkable size. L. Agassiz has deter- 

 mined the length of life in a large sea snail, Natica fieros, by 

 sorting a great number of individuals according to their sizes : he 

 places it at 30 years 2 . 



1 Bronn, ' Klassen und Ordnungen des Thierreichs,' Bd. III. p. 466 ; Leipzig. 

 3 Bronn, 1. c. 



