SCIENCE-GOSSIP. 



291 



lis, four ; Planorbis albus, one ( ; ), and he tells 

 me tha: " the May-fly was in season, and the trout 

 were well gorged with May-fly." As the bodies 

 in many of the shells were comparatively quite 

 fresh when I examined them the day after capture. 

 it is clear that the trout really like this food, 

 and will eat mollusca even when such tempting 

 viands as the May-fly are present in abundance. 

 The fact that salmon take practically no food 

 during their residence in fresh water is also in 

 favour of mollusca. The lobster, which will 

 break open even a whelk's shell ( s ), is replaced by 

 Astacns, at once less numerous and less powerful ; 

 it will, however, eat molluscs, shells and all ( !l ). 

 Birds no doubt destroy a certain number. Ducks 

 seem, in fact, to thoroughly clear out any pond 

 they go to frequently, and it is a matter of 

 congratulation that there are still some ponds left 

 undried which have not been spoilt by ducks ( 10 ). 

 Small fishes, such as gold-fish and sticklebacks, 

 are known to destroy some molluscs, but as it 

 seems to take a gold-fish a day or so( u ) to extract 

 the snail, the ravages they commit are probably 

 not very extensive. Rats are well-known to have 

 a partiality for mussels ( l -), and frogs must be 

 included among their enemies. Hydrophihis piceus 

 and Dyliscus marginalis eat snails, and the latter is 

 said to have a nice taste in them('), and leeches 

 are, perhaps, very destructive ("). It has been 

 recorded that bats will eat Paludina ( u ), Planorbis and 

 Anodon( x,i ). Adding to all this the ravages of para- 

 sites, the number of freshwater mollusca destroyed 

 by all their enemies must be far less, both absolutely 

 and relatively, than in the sea. 



With the advantages which have been pointed 

 out surrounding a freshwater life, why have not 

 more marine mollusca taken advantage of them ? 

 It is not that marine organisms cannot fairly 

 readily stand a change, especially if gradual, from 

 salt water to fresh This was shown long ago 

 experimentally by F S. Beaudant ( l7 J, who found 

 that marine mollusca could, by gradually changing 

 the water, become accustomed to a freshwater 

 life, and net virsa . though a sudden change is 

 attended with fatal results. From the table given 



mi which clip specie* occur 

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by Semper we learn that after 151 days in sea- 

 water eighty-five per cent of the 391 specimens of 

 fifteen marine species were alive. In half sea half 

 fresh water, seventy-four per cent, were alive. After 

 27S days in sea-water sixty-one per cent, survived, 

 and after half-and-half for fifteen days, and then 

 273 days in quite fresh water, forty-two per cent, 

 lived. The common mussel (Mytilus edulis) showed 

 the greatest power of resistance, 100 per cent, 

 surviving in each of the four cases. This 

 is interesting in view of its connection with 

 Dreissena ( Is ), pointed out elsewhere, and the exist- 

 ence of freshwater Mytilus. It is also noticeable 

 that, as might have been expected, shore species 

 bear the change better than deep-sea ones. 

 The real explanation is probably threefold : (1) 

 because free-swimming larvae get swept out to sea 

 again, for undoubtedly Sollas' objection to direct 

 migration does carry some weight with it ; (2) 

 because fresh water is often somewhat impure and 

 contaminated with sewage, etc. ; and (3) because 

 the climatic conditions in fresh water are so much 

 more severe than in the sea. The inhabitants of 

 our streams and ponds, owing to the small volume 

 of the waters, are exposed to great variations of 

 temperature, and are liable — as has been especially 

 noticeable during the last few years, in this 

 district at any rate — to complete desiccation. In 

 this sense freshwater animals lead a far more 

 dangerous life than marine ones, which live in a 

 fairly constant volume of water of a fairly constant 

 temperature. This must, however, be carefully 

 distinguished from a struggle for existence with 

 other animals. Such hardships do not normally 

 lead to the development of any kind of cryptic, 

 epigamic, or sematic coloration. This is produced 

 by the interrelation of organism with organism, 

 not of organism with inorganic nature ( ln ). 



Certain forms of Limnaca, noticeably the very 

 abundant L. peregra {">), show an extraordinarily 

 wide range in variation. This, perhaps, is a 

 further indication of the comparative ease of 

 the conditions under which they live, as mentioned 

 under Tachia above. It is noticeable, especially as 

 the immediate origin of freshwater pulmonales is 

 so obscure, that the typical freshwater genera, 

 Limnaca, I'hysa, Planorbis, are of world-wide distri- 

 bution. 



Now since I.imnata and other shells partake in 

 no way of the characters of marine shells, and 

 since their life is mm h easier and more peai elul, 

 it is noi unreasonable to regard their thin uni- 

 colorous covering as an example of shell degencra- 



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