I.] APPENDIX. 57 



(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 w^hich 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, Planorbis, and Ancylus live 

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

 the full size. L. auricularis 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 excep- 

 tionally 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 Pahidinidae attain an age of three or four years.' 



(8) ' The smaller bivalves, Pisidimn 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 mark- 

 ings. It is possible that habitat may have great influence upon 

 the length of life in this order.' 



' Unto 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 ^ 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 determined the length of life in a large sea snail, 

 Natica heros, by sorting a great number of individuals according 

 to their sizes : he places it at 30 years '. 



I am glad to be able to communicate an observation made at 

 the Zoological Station at Naples upon the length of life in 



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

 Leipzig. 



2 Bronn, 1. c. 



