48 



during its capture. With respect to the siphon, there appears to he nothing in 

 its structure or mode of connexion with the body calculated to prevent its being 

 filled with such fluid matter as the pericardium may contain ; and as the peri- 

 cardium communicates with the branchial cavity, sea-water may in this way 

 pass into its cavity, and the relative situations of the gas and water be thus the 

 reverse of what Mr. Parkinson supposes them to be. 



The quantity of gas which according to this view must be accumulated in 

 the camerated portion of the shell, might be supposed to be incompatible with 

 the habits of the Pearly Nautilus as a ground-dwelling animal, and that the 

 bottom of the sea is its principal sphere of action, is proved by the nature of its 

 food. But the soft parts of this animal weighed, in the specimen here described, 

 fifteen ounces avoirdupoise, and little more than half that weight suffices to sink 

 the shell with all its closed chambers full of air. Although, therefore, the specific 

 gravity of the shell is greatly diminished and is thus rendered less cumbersome to 

 the inhabitant ; yet to rise with it to the surface must require some exertion on 

 the part of the inhabitant : while in order to float there at ease, an additional 

 A^olume of air is probably taken into the dwelling-chamber, in which case the 

 act of sinking would be accomplished by simply reversing the shell. 



On a retrospect of the details of the organization of Nautilus Pompilius, it ap- 

 pears that the plan of formation of all the principal systems or collections of 

 organs for individual functions is strictly Cephalopodic, the variations depending 

 either on excess or defect of development. 



Thus in both orders the principal masses of the muscular system take their 

 origin from an internal cartilaginous skeleton, and extend in opposite directions 

 before and behind their point of attachment ; the anterior mass being subservient 

 to locomotion and prehension, the posterior to respiration and attachment to a 

 more or less developed shell. In the former division of the muscular system 

 the Pearly Nautilus presents us with a peculiar arrangement of one of its parts, 

 adapting the animal to a mode of progression unknown to any of the higher 

 Cephalopods, and in which it participates with some of the inferior forms of 

 Mollusks. The part alluded to is the flattened plate or disk which surmounts 

 the head ; but, generally considered, the anterior masses present an inferiority 

 of development corresponding to the diminutive nature of the parts which repre- 

 sent the cephalic arms of the Dibranchiata. The posterior masses, on the con- 



