354 ORDER TETRABRANCHIATA ; NAUTILUS. 



the number of chambers would vary in different specimens, 

 according to their respective ages, and the consequent number of 

 additions they have made to their shells ; and this is found to be 

 the fact. 



967. The general structure of the animal is intermediate 

 between that of the Dibranchiate Cephalopods and that of the 

 Gasteropods. The tentacles are numerous, amounting to more 

 than a hundred ; they are slender, tapering, and retractile ; and 

 instead of being furnished with suckers, as in the Octopods and 

 Cuttle-fishes, are more or less distinctly annulated. The inner 

 tentacles are called labial tentacles, as they are attached to a 

 membrane surrounding the mouth ; the sheaths into which the 

 outer tentacles are retractile, are considered to be analagous to 

 the arms of the dibranchiate Cephalopods ; and the dorsal pair 

 of these are united and greatly dilated, so as to form a broad 

 hood, which serves to close the aperture of the shell when the 

 animal is retracted. Of the habits of the Nautili, little is known. 

 They are said to creep along the bottom of the sea, and probably 

 feed upon the Crustacea which they find there, as the specimen 

 dissected by Professor Owen contained portions of a small crab. 

 According to Rumphius, they are sometimes driven up from the 

 bottom by storms, and are then seen, after the weather has again 

 become calm, floating in troops at the surface, with the mouth of 

 the shell upwards. The power of rising and sinking in the 

 water at pleasure which has been attributed to the Nautilus, 

 but its possession of which appears to be very doubtful, has been 

 accounted for by the chambered structure of its shell ; and by a 

 power which it has been supposed to possess, of diminishing its 

 bulk, by forcing water from the. sac which surrounds the heart 

 into the siphuncle, or allowing it to be expelled from that tube 

 by the elasticity of the air in the chambers, when it desires to 

 increase the bulk of the soft parts of its body. If such increase 

 and decrease in bulk could be effected without any change in 

 the weight of the whole mass, it would be caused (according to 

 well-known principles of Hydrostatics) to ascend or descend in 

 water ; the animal with its shell being altogether of so nearly 

 the same specific gravity with that fluid, that a very slight 



