APPENDIX. 57 



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

 the Zoological Station at Naples upon the length of life in 

 Ascidians. The beautiful white Cionea intestinalis 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, pre- 

 viously 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, Anodonta) ; indeed, our largest 

 native snail (Helix pomatia) only lives for four years, and many 

 small species only one year, or two years if the former time is in- 

 sufficient to render them sexually mature. These facts seem to 

 indicate, as I think, that these molluscs are exposed to great de- 

 struction 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 contains several hundred 

 thousand eggs ; the destruction of young as compared with the 

 number of eggs produced is distinctly smaller than in birds, there- 

 fore a much shorter duration of the life of each mature individual 

 is rendered possible, and further becomes advantageous because the 

 mature individuals are exposed 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 important 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 



