OF THE GULF OF MANAAR. 
101 
mind that the shell is no fortuity, has no separate growth or 
existence of its own, but dies when the animal and epidermis 
die ; bearing in mind that it is formed by the mantle, each 
layer of it having once been a portion of the mantle suc- 
cessively calcified (or hardened with carbonate of lime) and 
thrown off by the mantle to unite with the layers previ- 
ously formed ; remembering in murex the marked projections 
{varices) are the lips of former mouths, the very presence on 
the shell of the young pearl oyster of the flattened spines 
above mentioned at once refers one to the mantle as calcu- 
lated to have a similar formation. Accordingly we find Dr. 
Kelaart writing of the mantle as having tentacles consisting 
of " a series of long and short flat filaments, the long ones 
having lateral filamentous projections," and I observe myself, 
under the microscope, that the mantle is thus digitated. 
According to the law of formation of shell, then, these flat 
projections of the mantle must have formed the flattened 
spines; and the question arises whether the spines were 
secreted for the protection of the digitations of the mantle, 
or the digitations existed for the purpose of secreting the 
spatulated spines. 
Dr. Kelaart suggests the theory that these flat projec- 
tions of the mantle serve as tactile organs so keenly sensitive 
as to be equivalent to sight. It may be that they do, but 
still it is left unaccounted for why they should differ in form 
from the equally sensory mantle margin of the edible oyster 
in being flat and projecting instead of uniform in contour. 
And, if no special reason can be assigned for this their 
peculiar deviation in form, one may be permitted to infer 
that the peculiar formation of mantle exists for the purpose 
of forming the spines, rather than the spines for the sake of 
protectmg the mantle. Again, if the spines existed only for 
protecting the flat projections of mantle, one would expect to 
see them disappear after they had discharged their office, as 
*'in most cases marginal characters are absorbed away by 
