300 CLASS V. CEPHALOPODA. ORDER I. POLYTHALAMIA. 
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 Gas- 
tropods. 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 enve- 
loped 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 fast- 
ened 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 cham- 
ber: the tentacles are consequently protruded over the lateral margins of 
the aperture, and the body is retained within the shell by the adhesion of 
the membrane 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 formation deposits a sim- 
ple hollow shell, out of which it necessarily 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 posi- 
tion (a consideration of the habits of this pelagic mollusk will show the 
necessity for this membrane). As the soft parts increase in bulk, the 
muscular girdle which binds them to the shell would naturaily be forced 
