MOLLUSCA. 417 



custom 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 qua- 

 lity of the water in which they reside. Before much con- 

 fidence 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 diffi- 

 cult 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 function 

 of respiration. The change of situation would hkewise be 

 accompanied by a corresponding change of food, and con- 

 sequently, not merely the organs of locomotion and respira- 

 tion, but likewise those of digestion, would suffer a derange- 

 ment in their operations. We know that the power of suf- 

 fering 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 subject- 

 ed to these experiments of a change of situation, have been 

 observed to thrive on the food which it spontaneously yields, 

 to execute their accustomed motions, and above all, to pro- 

 pagate their kind, we shall be disposed to conclude, that 

 patient suffering has been mistaken for health, and vivaci- 

 ousness for the power of accommodation. 



VOL. II. D d 



