566 CEPHALOPODA. 



subject of a monograph by Professor Owen, who has most completely 

 investigated its general organization and relations with other families 

 of the Cephalopoda. The shell of the Pearly Nautilus (N. Pompilius) 

 is extremely common, and may be met with in every conchological 

 collection, notwithstanding the extreme rarity of the mollusk that 

 inhabits it a circumstance, perhaps, to be explained by the fact that 

 the living animal dwells in deep water, and when it comes to the surface 

 is so vigilant against surprise, that at the slightest alarm it sinks to the 

 bottom. On making a section of the shell its cavity is found to be 

 partitioned off by numerous shelly septa into various chambers (fig. 

 284, s s), in the last of which the body of the animal is situated. A 

 long tube or siphuncle (h h), partly calcareous and partly membranous, 

 passes through all the compartments quite to the end of the series. 

 The membranous siphuncle is continued into the animal, and terminates 

 in a cavity contained within its body, hereafter to be described, which 

 is in free communication with the exterior. 



(1518.) Various conjectures have been indulged in concerning the 

 end answered by the camerated condition of the shell in these Mollusca. 

 Dr. Hooke* suggested the idea that the chambers might be filled with 

 air generated by the Nautilus, and thus made so buoyant that the 

 specific gravity of the animal and its shell should correspond with that 

 of the surrounding medium, and that, acting in the same manner as the 

 swimming-bladder of a fish, the creature would float or sink, as the air 

 in its shell was alternately compressed or rarefied. Should this suppo- 

 sition be correct, it would seem probable, as Dr. Buckland has pointed 

 out, that the simple retraction of the head, by injecting water from the 

 chamber within its body (pericardium) into the membranous siphuncle, 

 would cause the needful condensation of the air contained in this 

 singular float, and allow the Nautilus to sink to the bottom ; while the 

 protrusion of its arms, by taking off the pressure, and thus allowing of 

 the expansion of the confined air, would give every needful degree of 

 buoyancy, even sufficient to permit the mollusk to rise like a balloon to 

 the top of the sea. 



(1519.) The body of this Cephalopod is covered with a thin mantle 

 (a a), of which a large fold (b) is reflected on the exterior of the shell. 

 It is securely fixed to its residence by two lateral muscles, the insertion 

 of one of which is seen at g, A large coriaceous hood (n) covers the 

 head, and, when the creature retreats into its habitation, closes the 

 entrance like a door, while through the infundibulum (i) the ova and 

 excrementitious matters are expelled from the body. The most re- 



not far distant from the ship, and resembling, as the sailors expressed it, a dead 

 tortoiseshell cat in the water. It was captured, but not before the upper part of 

 the shell had been broken by the boat-hook in the eagerness to take it, as the animal 

 was sinking when caught." (Dr. Bennett's Journal.) 



* Philosophical Experiments and Observations. 8vo, 1726. 



