There is a mechanical uniformity observable in the description of
shells of the same species which at once suggests the probability that
the generating figure of each increases, and that the spiral chamber of
each expands itself, according to some simple geometrical law common
to all. To the determination of this law the operculum lends itself,
in certain classes of shells, with remarkable facility. Continually
enlarged by the animal, as the construction of its shell advances so as
to fill up its mouth, the operculum measures the progressive widening
of the spiral chamber by the progressive stages of its growth.
* * * * *
The animal, as he advances in the construction of his shell, increases
continually his operculum, so as to adjust it to his mouth. He
increases it, however, not by additions made at the same time all round
its margin, but by additions made only on one side of it at once. One
edge of the operculum thus remains unaltered as it is advanced into
each new position, and placed in a newly-formed section of the chamber
similar to the last but greater than it.
That the same edge which fitted a portion of the first less section
should be capable of adjustment so as to fit a portion of the next
similar but greater section, supposes a geometrical provision in the
curved form of the chamber of great complication and difficulty. But
God hath bestowed upon this humble architect the practical skill of
the learned geometrician; and he makes this provision with admirable
precision in that curvature of the logarithmic spiral which he gives to
the section of the shell. This curvature obtaining, he has only to turn
his operculum slightly round in its own place, as he advances it into
each newly-formed portion of his chamber, to adapt one margin of it to
a new and larger surface and a different curvature, leaving the space
to be filled up by increasing the operculum wholly on the outer margin.
* * * * *
Why the Mollusks, who inhabit turbinated and discoid shells, should, in
the progressive increase of their spiral dwellings, affect the peculiar
law of the logarithmic spiral, is easily to be understood. Providence
has subjected the instinct which shapes out each to a rigid uniformity
of operation.--_Professor Mosely_: _Philos. Trans._ 1838.