Dr. Hook considered (Hook's Experiments, 8vo. 1726, page 308) that the air chambers were filled alternately with air or water; and Parkinson (Organic Remains, vol. iii. p. 102,) admitting that these chambers were not accessible to water, thinks that the act of rising or sinking depends on the alternate introduction of air or water into the siphuncle; but he is at a loss to find the source from which this air could be obtained at the bottom of the sea, or to explain "in what manner the animal effected those modifications of the tube and its contained air, on which the variation of its buoyancy depended."[1] The theory which supposes the chambers of the shell to be permanently filled with air alone, and the siphuncle to be the organ which regulates the rising or sinking of the animal, by changing the place of the pericardia fluid, seems adequate to satisfy every hydraulic condition of a Problem that has hitherto received no satisfactory solution.
I have dwelt thus long upon this subject, on account of
its importance, in explaining the complex structure, and
hitherto imperfectly understood functions, of all the numerous
and widely disseminated families of fossil chambered
shells, that possessed siphunculi. If, in all these families, it
can be shown that the same principles of mechanism, under
various modifications, have prevailed from the first commencement
of organic life unto the present hour, we can
hardly avoid the conclusion which would refer such unity
of organizations to the will and agency of one and the same
intelligent First Cause, and lead us to regard them all as
"emanations of that Infinite Wisdom, that appears in the
shape and structure of all other created beings."[2]
have observed that a similar vertical position is maintained by the shell of the Planorbis corneus, whilst in the act of crawling at the bottom.
