CEPHALOPODA. 593 



cess of the mantle, which retains its primitive attachment, the calcifi- 

 cation is continued backwards for a short distance around that pro- 

 cess, which now forms the commencement of the membranous siphon, 

 and acquires the partial protection of the calcareous tube. An air- 

 tight chamber is thus formed, traversed by the siphon, which per- 

 forates its anterior wall or septum ; by a repetition of the same 

 processes, a second chamber is formed, included within two perforated 

 septa ; and similar, but wider partitions continue to be added, con- 

 currently with the formation of the new layers which extend and 

 expand the mouth of the shell, until the animal acquires its full growth, 

 which is indicated by the body having receded for a less distance 

 from the penultimate septum before the formation of the last septum 

 is begun. 



The periodical formation of these septa in the progress of growth 

 is analogous to that of the projecting external plates in the Wen- 

 dletrap, and of the rows of spines in the Murex*; but these ex- 

 ternal processes consist of the opake calcareous layer of the shell, 

 whilst the internal processes in the Nautilus consist of the nacreous 

 layer like the septa in the Turritella. Thus the embryo Nautilus 

 at first inhabits a simple shell Uke that of most univalve MoUusca, 

 and manifests, according to the usual law, the general type at the 

 early stage of its existence ; although it soon begins, and apparently 

 before having quitted the ovum, to take on the special form. 



In acquiring the camerated structure of the shell the Nautilus 

 gains the power of rising from the bottom and the requisite condition 

 for swimming ; by the exhalation of some light gas into the deserted 

 chambers it attaches to its otherwise too heavy body a contrivance 

 for ascending in its atmosphere, as we ascend in ours by the aid of 

 a balloon. But the Nautilus, superior to the human aeronaut, 

 combines with the power of elevating and suspending itself in the 

 aqueous medium, that of opposing its currents and propelling itself 

 at will in any direction. It possesses the latter essential adjunct 

 to the utility of the balloon as a locomotive organ, by virtue of the 

 muscular funnel, through which it ejects into the surrounding water, 

 doubtless with considerable force, the respiratory currents. 



It appears that the proportion of the air-chambers to the dwelling 

 chamber of the Nautilus and its contents, is such, as to render 

 it of nearly the same specific gravity as the surrounding water. 

 The siphon, which traverses the air-chambers, communicates with 

 the pericardium, and is most probably filled with fluid from that 

 aquiferous cavity. 



Dr. Buckland* conjectures that the Nautilus may possess the power 

 ♦ CCCXCn. p. 828. 

 QQ 



