22 MOLLUSCA. 



that all these animals die immediately, if we suddenly 

 change their place of abode ; but that, if we gradually in- 

 crease the proportion of salt in the water for the one set, 

 and diminish it for the other set, we can, in general, accus- 

 tom them to live in a kind of water which is not natural to 

 them. He found, however, some species which resisted 

 these attempts, and which could not bear any alteration in 

 the quality of the water in which they reside. Before much 

 confidence can be placed in the accuracy of these results, 

 it would be desirable that the experiments were repeated 

 and varied by other observers. There are, indeed, many 

 sources of error to be guarded against. When we change 

 animals from fresh to salt water, or from salt water to fresh, 

 we must necessarily derange their motions, by compelling 

 them to reside in a medium of a different degree of density 

 from the one in which they have been accustomed to dwell, 

 and to which the arrangement of the different parts of the 

 body is adapted. By such a change of place, it would be 

 difficult for those which breathe air to come to the surface, 

 and descend again in their new situation. In those with 

 gills, the application of a new kind of fluid to the surface of 

 such delicate organs, would considerably influence the func- 

 tion of respiration. The change of situation would like- 

 wise be accompanied by a corresponding change of food, 

 and consequently, not merely the organs of locomotion and 

 respiration, but likewise those of digestion, would suffer a 

 derangement in their operations. We know that the power 

 of suffering in the animals of this class is very great, and 

 that they survive, though sadly mutilated. Some of the 

 snails will live in a quiescent state for years, without food, 

 and almost without air. Unless, therefore, the animals 

 subjected to these experiments of a change of situation, have 



