346 Natural History of Molluscous Animats : — 
as the experiments of Dr. Brewster have proved, to the me- 
chanical disposition of the component particles. This sur- 
face, however, is, besides, often tainted yellow, red, or brown ; 
and these spots probably derive their colour from being near 
to, or in contact with, an organ which secretes a coloured 
fluid. Thus the yellow or brown spots observable in some 
univalves are produced, according to Blainville, by the proxi- 
mity of the liver ; and the shell of the Janthina is stained of a 
uniform blue by the excrementitious fluid of the animal. 
The painting of the external surface is in general super- 
ficial, and its tracing is dependent on the arrangement of the 
glands which secrete the colouring matter, and which are 
situated on the margins of the cloak or collar. If you exa- 
mine a banded snail, you will perceive as many coloured spots 
on the edge of the collar as there are zones on the shell; and 
if a part of the margin of the shell be cut away, the piece re- 
produced is brown opposite to the dark portion of the collar, 
but in other parts yellow. In those cases where the colours 
are disposed in continuous bands, there is no difficulty in 
understanding the mode of their formation; but no experi- 
ments or observations yet enable us to explain how some are 
painted in spots disposed in every varied form, some in lines 
and spots intermixed, and some in bands placed at intervals 
in the direction of the striz of growth. What Reaumur and 
Bosc have said on the subject is so entirely hypothetical, that 
it is unnecessary to repeat it. You can, I doubt not, create 
for yourself a theory which shall be as satisfactory as theirs ; 
but, remember, it is not worth the trouble, unless it is based 
on experiment. 
Beneath the superficial and visible colouring of the full- 
grown cowries, and of some olives, there is concealed a deeper 
painting, different from the first both in colour and in charac- 
ter. ‘This is accounted for when we remember the peculiar 
manner in which these shells are perfected. The first thin 
shell is coloured like other shells, and in the same manner 
from the margins of the cloak; but when the dilated cuta- 
neous lobes of the animal have attained their full size, and 
deposit the highly polished testaceous coat which strengthens 
the shell, they deposit at the same time a colouring matter 
which always assumes a form different from the first; the 
painting deriving its peculiar character from the manner in 
which the colouring glands are arranged in the lobes. 
The colour of shells is a good deal influenced by the 
operation of light; so much so, that, in general, the liveliness 
of the one bears some proportion to the intensity of the other. 
How varied, vivid, and beautiful are the testaceous Mollisca 
