CEPHALOPODA. 445 



(486.) In order to understand the manner in which the remark- 

 ably constructed camerated shells, such as those of Nautilus, are 

 produced, it is not necessary to imagine any deviation from the 

 simple mode of procedure adopted in all the cases we have as yet 

 considered. The continual elongation of the spiral cone is, as is 

 evident from the lines of growth visible upon its outer surface, 

 effected by the addition of successive layers to the margin of the 

 aperture of the last-formed chamber, wherein the animal resides ; 

 and as the production of the calcareous secretion whereby the shell 

 is enlarged is most rapidly effected upon that side of the body 

 where the funnel (Jig. 205, i) is situated, the gradually expand- 

 ing shell naturally revolves around an eccentric axis. While the 

 growth of the shell continues, the animal is constantly advancing 

 forwards, and thus leaves the first-formed portions of the shell un- 

 occupied. At intervals, as the Nautilus thus removes itself fur- 

 ther and further from the bottom of its abode, that portion of its 

 mantle which covers the general surface of its visceral sac (fig. 

 205, d) secretes floors of shelly substance behind it ; and thus the 

 septa, s, 5, are formed whereby the shell is separated into chambers, 

 every chamber having in turn been occupied by the body of the 

 Nautilus. The gradual prolongation of the fleshy siphon (h) is 

 easily understood, because it naturally increases in length with the 

 growth of the animal : but how the two muscles (fig. 205, g), 

 that fix the body to the shell, progressively advance their points of 

 attachment as the shell enlarges, is not so readily explained ; nei- 

 ther are we prepared to account satisfactorily for the accomplish- 

 ment of this part of the process. 



(487.) It has been already stated that in all Cephalopods the 

 aperture of the mouth is situated in the centre of the disc formed 

 by the union of the origins of the feet (Jigs. 210, 214). The 

 oral orifice is generally surrounded by a broad circular lip (Jig. 

 208, A, a), which being not unfrequently fringed or papillose, 

 there is little doubt of its possessing sufficient sensibility to render 

 it of material assistance in manducation. 



The circular lip partially conceals a pair of strong horny man- 

 dibles, not unlike the beak of a parrot, but differing in this parti- 

 cular, that in the Cephalopod the upper mandible is the shorter 

 of the two, and is overlapped by the lower jaw. - The mandibles 

 detached from the soft part are represented in fig. 208, B, a, b. 

 There is likewise another important difference between the struc- 

 ture of the beak of the Cuttle-fish and that of the bird, inasmuch 



