OPINIONS OF HOOK AND PARKINSON. 331 
tendency of the shell to rise to the surface would 
be counteracted by the strong muscular disk 
(PL 31, n.), with which the creature crawls, and 
adheres to the bottom, using freely its tentacula 
to seize its prey.* 
Dr. Hook considered (Hook's Experiments, 
8vo. 1726, page 308) that the air chambers 
were Jillecl alternaieli/ ivit/i air or ivater ; '\ and 
Parkinson (Organic Remains, vol. iii. p. 102), 
admitting that these chambers were not ac- 
cessible to water, thinks that the act of rising 
or sinking depends on the alieniate introductioit 
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 
shells, and the specific gravity of the body of the animal, when 
contracted into the shell, may have exceeded that of water to the 
amount of another quarter of an ounce, there remains about half 
an ounce for the weight of fluid, which being introduced into the 
siphuncle, would cause the shell to sink ; and this quantity seems 
well proportioned to the capacity both of the pericardium, and 
of the distended siphuncle. 
* See Sup. Note. 
t If the chambers were filled with water, the shell could not 
be thus suspended without muscular exertion, and instead of 
being poised vertically over the body, in a position of ease and 
safety, would be continually tending to fall flat upon its side ; 
thus exposing itself to injury by friction, and the animal to 
attacks from its enemies. Rumphius states, that at the bottom, 
He creeps with his boat above him, and with his head and 
barbs (tentacula) on the ground, making a tolerably quick pro- 
gress. The author has observed that a similar vertical position 
is maintainetl by the shell of the Planorbis corneus, whilst the 
animal is in the act of crawling at the bottom. 
