OPINIONS OF HOOK AND PARKINSON. .3.31 
vertical position, with little or no muscular exer- 
tion, and leave the creature at ease to regulate 
the movements of its tentaciila (p) in crawding 
3^nd seizing its prey.* 
Dr. Hook considered (Hook’s Experiments, 
Svo. 1720, page 308) that the air chambers 
'vere jilled alternately ivitli air or water; and 
Darkinson (Organic Remains, vol. iii. p. 102), 
admitting that these chambers were not ac- 
<^essible to water, thinks that the act of rising 
or sinking depends on the alternate introduction 
air or tvatcr 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 
iiake it sink. As the shell, when attached to the living animal, 
"'as probably a quarter of an ounce heavier than these dry dead 
shells, and the specific gravity of the body of the animal 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. 
* If the chambers were filled with water, the shell could not 
he 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 
jttacks from its enemies. Rumphius states, that at the bottom, 
e creeps with his boat above him, and with his head and 
^ros (tentacula) on the ground, making a tolerably quick pro- 
S'‘ess. I have observed that a similar vertical position is main- 
tained by the shell of the Planorbis corneus, whilst in the act of 
crawlijig at the bottom. 
