THE NAUTILUS. 121 



The testaceous Cephalopods, of which I have already told 

 you there are only a few living representatives, appear to 

 reside habitually at great depths in the ocean, whence they 

 have the capability of ascending from time to time to the 

 surface; yet their navigation there, so far from being a natural 

 portraiture of a ship driven with sails and oars, is in all pro- 

 bability of a passive kind, or influenced only by the reaction 

 of the respiratory currents when expelled by the funnel 

 upon the surrounding medium. The specimen of Nautilus 

 pompilius, brought home to this country by Mr. Bennet, 

 and which has afforded Mr. Owen the means of preparing 

 one of the best and most beautiful monographs in compara- 

 tive anatomy, was taken on the coast of the New Hebrides 

 floating on the surface, but just in the act of again sinking 

 to the bottom,* — where lies its proper scene of action, for 

 the chief locomotive organ is a flattened muscular disk that 

 surmounts the head, analogous to the plane foot of the Gas- 

 teropods, which the Nautilus must also resemble in its man- 

 ner of creeping. The description of Rumphius is very 

 graphic. " When he thus floats on the water he puts out 

 his head and all his barbs (tentacles), and spreads them upon 

 the water, wdth the poop (of the shell) above water ; but at 

 the bottom he creeps in the reverse position, with his boat 

 above him, and with his head and barbs upon the ground, 

 making a tolerably quick progress. He keeps himself chiefly 

 upon the ground, creeping sometimes also into the nets of 

 the fishermen ; but after a storm, as the weather becomes 

 calm, they are seen in troops floating on the water, being- 

 driven up by the agitation of the waves. Wlience one 

 may infer, that they congregate in troops at the bottom. 

 This sailing, however, is not of long continuance ; for having 

 taken in all their tentacles they upset their boat, and so re- 

 turn to the bottom." -j- — By what mechanism the Nautilus 

 effects his ascent and descent is still conjectural. Dr. Hook 

 supposed that it had the power of generating air into, and 

 expelling it from, the deserted chambers, thus regulating its 

 specific gravity in the same manner as fish by means of their 

 air-bladders. Mr. Parkinson, in adopting this theory, as- 

 sumes that the seat of the accumulation of the gaseous fluid 

 is the membranous tube which runs through the siphuncular 

 apertures of the septa and traverses all the chambers ; and 

 he believes that this tube has a corresponding power of dila- 



* Mr. Bennet says that the Nautilu.*, when at rest, cillicr afloat or on the 

 ground, covers its shell with the mantle, which is like that of the Cypra'a, 

 —Neiv South Wales, ii. 409, 



t Owen's Memoir, 53. 



