58 THE DURATION OF LIFE. [I. 



Ascidians. The beautiful white Cionea intesHnalis has settled in 

 great numbers in an aquarium at the Station, and Professor 

 Dohrn tells me that it produces three generations annually, 

 and that each individual lives for about five months, and then 

 reproduces itself and dies. External conditions accounting for 

 this early death have not been discovered. 



It is known that the freshwater Polyzoa are annual, but it is 

 not known whether the first individuals produced from a 

 colony in the spring, live for the whole summer. The length 

 of life is also unknown in single individuals of any marine 

 Polyzoon. 



Clessin's accurate statements upon the freshwater Mollusca, 

 previously quoted, show that a surprisingly short length of life 

 is the general rule. Only those forms of which the large size 

 requires that many years shall elapse before the attainment of 

 sexual maturity, live ten years or over [Unio, Anodonfa) ; 

 indeed, our largest native snail {Helix pomatia) only lives for 

 four years, and many small species only one year, or two 3rears 

 if the former time is insufficient to render them sexually mature. 

 These facts seem to indicate, as I think, that these molluscs are 

 exposed to great destruction in the adult state, indeed to a 

 greater extent than when they are young, or, at any rate, to an 

 equal extent. The facts appear to be the reverse of those found 

 among birds. The fertility is enormous ; a single mussel con- 

 tains several hundred thousand eggs ; the destruction of young 

 as compared with the number of eggs produced is distinctly 

 smaller than in birds, therefore a much shorter duration of the 

 life of each mature individual is rendered possible, and further 

 becomes advantageous because the mature individuals are ex- 

 posed to severe destruction. 



However it can only be vaguely suggested that this is the 

 case, for positive proofs are entirely absent. Perhaps the 

 destruction of single mature individuals does not play so impor- 

 tant a part as the destruction of their generative organs. The 

 ravages of parasitic animals {Trematodes) in the internal organs 

 of snails and bivalves are well known to zoologists. The 

 ovaries of the latter are often entirely filled with parasites, and 

 such animals are then incapable of reproduction. 



Besides, molluscs have many enemies, which destroy them 

 both on land and in water. In the water, — fish, frogs, newts, 



