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 
