250 ACTION AT THE BOTTOM. 



Thirdly, it remains to consider the effect of the air, sup- 

 posing it to be retained continually within the chambers, 

 at the bottom of the sea. Here, if the position of the moving 

 animal be beneath the mouth of the shell, like that of a 

 snail as it crawls along the ground, the air within the 

 chambers would maintain the shell, buoyant, and floating 

 at ease above the body; and the 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 ad- 

 heres 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 filled alternately ivith 

 air or water ;f and Parkinson (Organic Remains, vol. iii. p 



The place of tlie pericardial fluid, therefore, will be always in the peri- 

 cardium, excepting when it is forced into and retained in the siphuncle, by 

 muscular pressure, during the contraction of the arms and body closed up 

 within the shell. When these are expanded, either on the surface, or at the 

 bottom of the sea, the water will have free access to the branchiae, and the 

 movements of the heart will proceed freely in the distended pericardium ; 

 which will be emptied of its fluid at those times only, when the body is 

 closed, and the access of water to the branchiae consequently impeded. 



The following experiments show that the weight of fluid requisite to 

 be added to the shell of a Nautilus, in order to make it sink, is about half an 

 ounce. 



I took two perfect shells of a Nautilus Pompilius, each weighing about 

 six onuces and a half in air, and measuring about seven inches across their 

 largest diameter; and having stopped the siphuncle with wax, I found that 

 each shell, when placed in fresh-water, required the weight of a few grains 

 more than an ounce to make it sink. As the shell, when attached to the 

 living animal, was 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 dis- 

 tended siphuncle, ^ 



* See Sup, Note, 



•}■ If the chambers were filled with water, the shell could not be thus 

 suspended without muscular exertion, and instead of being poised verti- 

 cally over the body, in a position of ease and safety, would be continually 



