﻿BRITISH FOSSIL CEPHALOPODA. 



Anatomy of the Nautilus. 



The animal of the Nautilus takes its shape from the last chamber of its well, 

 known spiral shell, which in ordinary states it appears almost exactly to fill. As 

 has been already pointed out, we must place the axis of the commencement of the 

 alimentary canal horizontal ; the portion of the animal resting against the last 

 septum will then be posterior, the outer or convex side of the shell will lie below, or 

 ventrally, while the upper, or dorsal, part of the body will abut against the previous 

 whorl. (See PI. I., fig. 1.) 



The exterior covering, or mantle, is divided into two parts (fig. 1, 0, /); the 

 anterior portion is the thicker and more fibrous ; it consists of two layers adhering 

 to each other, and containing within them muscular fibres and glandular organs. 

 These latter are, first, the nidamental glands in the female (fig. 1, A), to be presently 

 described ; and, secondly, numerous little crypts developed at its outer folded edge 

 for the secretion of the outer layer of the shell. On the dorsal side, the mantle fold 

 leaves but a shallow interval between itself and the inner mass of the body, and it 

 is almost immediately reflected on to the convexity of the previous whorl, which it 

 covers with a black deposit. On the ventral side, the junction of the mantle fold 

 with the body takes place much farther back, and the former extends as a covering 

 to the latter as far as the convex margin of the shell, whose shape it defines. There 

 is thus left between the two a deep and wide cavity known as the mantle-cavity. 

 The exterior surface of this ventral mantle fold is somewhat longitudinally plicated 

 (fig. 1, e), but otherwise nothing has been observed to produce the " normal Kne " 

 observed upon the shell in this position. The mantle fold fits over the body within 

 by simple apposition, and covers, but is not perforated by, the funnel. The posterior 

 portion of the mantle (fig. 1,/) covers the convexity of the body behind, and has 

 the shape of the last septum. It is of greater tenuity, consisting of one membrane 

 only, and allows the viscera to be seen through it. According to all observers, the 

 membranous siphon is a continuation of this membrane backwards through the 

 various chambers. 1 It is the exterior of this part of the mantle that has more 

 especially the function of secreting the inner nacreous layer of the shell. On the 

 dorsal side of this part are three longitudinal bands — one leading to the siphon, 

 and one on each side parallel to it, which pass behind the siphon and join one 

 another. 



The junction of these two parts of the mantle is along a band (fig. 1, g) which 

 has a sigmoid course not far from parallel to the chord of the last septum (as 

 drawn in section), and therefore much more forward on the ventral side. Along this 



1 The relations, however, of the septa, which must be deposited by the mantle, to the siphonal 

 covering, as observed in the shell, seem to render this doubtful, as they appear to be quite independent 

 and to be differently secreted. 



