172 FRESHWATER FAUNA 



sea floods on the Norfolk Broads the first fish to suffer 

 are the tench, pike, bream, and roach : perch bear a 

 strong admixture of salt water, eels still more, while the 

 stickleback will continue to live on in pure sea water. 



In studying the effects of the change of medium, time 

 must be recognised as an important element. If a body 

 of sea water be slowly freshened, so that the fish living 

 in it have time to grow accustomed to the change, many 

 will be found to survive which would otherwise perish. 

 The Romans took advantage of this fact, and so were able 

 to breed marine fish in freshwater ponds, a practice lately 

 revived in Guernsey and Yorkshire.* 



The same is true of the Invertebrata, few survive a 

 sudden transference from salt to fresh water, but many 

 can be brought to endure a gradual change. This was 

 first shown by the careful experiments of Beudant, made 

 on fifteen different species of marine invertebrates, viz., 

 the common limpet, whelk, oyster, mussel, the common 

 cockle, Purpura lapillus, Turbo neritoides, Area barbata, 

 Venus maculata, Fissurella uncibosa, Haliotis tuberculata, 

 Tellina incarnata, Chama lazarus, Pecten varius, and the 

 acorn shell (Balanus striatus), none of which are capable 

 of resisting a sudden change of medium. They were placed 

 in a tank of sea water, which was very slowly replaced by 

 fresh ; the experiment was commenced on January 1st, 

 and by June 1st one half of the sea water had been 

 replaced, by September 1st the change was complete ; a 

 fortnight later the number of surviving individuals of each 

 species was counted, the limpets, mussels, Turbo purpura 

 and A rca, had lost none of their numbers; the cockles, 

 oysters, Balanus and Venus, very few ; but of Fissurella, 

 Haliotis, Tellina, Pecten, Chama, and the whelks, not a 

 single example was left alive. The positive result of this 

 experiment is to show that many marine invertebrates 

 * Day, " Fishes of Great Britain and Ireland," p. 84. 



