122 Mr. Lovell Reeve on the Pearly Nautilus 



parts. The anterior or outer part incloses a well-developed 

 head, with a pair of strong, horny mandibles, a mass of some 

 thirty or forty tentacles, and a number of delicate structures, 

 including the organs of smelling, hearing, seeing, &c. ; and 

 over all these parts is a capacious fleshy hood, which may be 

 considered as the analogue of the operculum in the Gastro- 

 pods. The inner or posterior part of the body contains the 

 viscera, with a funnel or vent-hole extending from beneath the 

 tentacles, and the entire abdominal mass, together with the 

 breathing apparatus, is enveloped by a large sack-like mantle 

 fitting into the hollow of the shell. The anterior portion of 

 the mantle, or that which is attached to the back part of the 

 head, is produced into a considerable fold, which overlaps the 

 involuted convexity of the shell, and from the lower extremity 

 of the entire body extends a central membranous tubiform 

 process, which, by passing the siphonic apertures of the septa, 

 extends completely through the convolutions of the shell, 

 from chamber to chamber, until it is fastened to the nucleus 

 or parietal wall of the central or first-formed chamber. Around 

 the circumference of this abdominal sack there is a thin layer 

 of horny matter, called the belt, expanding on each side into 

 a kind of broad patch, and becoming the medium of muscular 

 attachment. 



The natural position, then, of the Nautilus in its shell, is 

 with the back of the head and concavity of the hood against 

 the camerated convexity of the spire, and the funnel resting 

 on the outer concave wall of the chamber : the tentacles are 

 consequently protruded over the lateral margins of the aper- 

 ture, and the body is retained within the shell by the adhesion 

 of the m.embrane and the horny girdle. 



The following appears to us to be the manner in which the 

 Nautilus constructs its shell. The animal in its embryo for- 

 mation deposits a simple hollow shell, out of which it neces- 

 sarily advances as it increases in bulk ; and in order to assist 

 its specific gravity at the bottom of the ocean, the vacated 

 portion of the shell is chambered in by the secretion of trans- 

 verse septa, the animal having first taken the precaution to 

 secure a strong tubiform membrane to the inner wall, in order 

 to adjust its position (a consideration of the habits of this pe- 

 lagic moliusk will show the necessity for this membrane). As 

 the soft parts increase in bulk, the muscular girdle which 

 binds them to the shell would naturally be forced from any 

 adhesion ; but from its being furnished with a certain degree 

 of elasticity, it advances by a series of periodical slips, the 

 suddenness of which is undoubtedly counteracte by the at- 

 tachment of the central membrane. The growth of the shell 



