of certain littoral Mollusca. 389 
inconclusiveness of those experiments for proving the respiratory organs 
of Melampus, Pedipes, &c. to be pectinated, by showing that the 
animals of Limnea, with respiratory organs well known not to be so, 
are equally capable of supporting life under similar circumstances, it 
will establish at least the fact, that a class of animals exists, which, 
with respiratory organs originally formed for breathing atmospherick 
air, have yet the power either of accommodating these very same 
organs, (not of developing or employing different ones, as certain Rep- 
tilia do in the converse case*), to the abstraction of oxygen from 
water, or alse, perliaps, even of supporting life solely by the action of 
the water on the integuments or mantle: by, in short, asort of conver-= 
sion of the whole exposed surface of the body into a breathing apparatus, 
without employing the aerial breathing organs at all. It must indeed be 
admitted, that in animals like these, in which the influence of oxygen 
on the blood is tending fast to its minimum, it is not difficult to imagine 
that the system of the animal functions may with much greater theo- 
retical plausibility be conceived capable of accommodating itself to such 
a change, than in higher races of animals and types of organization : in 
which the oxygenous fluid exercises a much more powerful influence, 
and playsa far more important part in the conditions of vitality. 
The solution of this problem is equally interesting in a geological 
point of view as in others. It will tend to demonstrate the right or wrong 
collocation of many fossil shells; a question of so much consequence in 
the discrimination of various strata. It will go to prove whether certain 
genera which have been heretofore referred to the land or fresh-water 
Pulmonifera do or do not belong to the marine or at least littoral Pecti- 
nibranchiata, J am therefore proportionately interested in its right 
determination ; and no farther anxious for the verification of my former 
inferences, than as far as regards the establishment of the truth, They 
will stand, at all events, as useful starting points for the researches of 
others : and the conclusions there drawn (some of them certainly too 
positively) will also, if proved erroneous, serve to display the necessity of 
extreme caution in all inductive reasoning to Naturalists in general, 
* Sce Note +, page 387, 
