T.HE PEARLY NAUTILUS. 177 



forcing of the water through the siphon ; while the 

 protrusion of the head and tentacula, by relieving the 

 pericardium from pressure, permits the air of the cham- 

 bers to expand, and drive back the water — the buoyancy 

 of the animal accordingly returning. Surely no com- 

 ments are needed to enforce upon the attention of the 

 reader the beauty and fitness of such a contrivance — 

 a contrivance which enables the pearly nautilus to float 

 on the surface of the deep, enjoying the light and 

 warmth of the sun, and to luxuriate in its rays ; and 

 then, in a moment, when danger threatens, to sink to 

 the bottom, and there find a shelter and a hiding place. 



The mantle of ordinary cephalopods is thick and 

 tough ; in this shell-covered species, however, it is very 

 delicate, and is fixed by a lateral muscle on each side 

 to the inner aspect of the shell, over the external sur- 

 face of which, to a certain distance, an expansion is 

 reflected : a tougher and firmer membrane covers the 

 head, and forms, when the animal is withdrawn into its 

 shell, a sort of leathery operculum ; the funnel, as in 

 other cephalopods, is large and distinct, and gives 

 passage to the water discharged from the branchial 

 cavities, and also to the digested aliment. To the eyes 

 of this animal reference has already been made. 



So difi'erent in structure are the arms or tentacula of 



