186 MANUAL OF THE MOLLUSCA. 
Animal. In the recent nautilus, the mandibles are horny, 
but calcified to a considerable extent; they are surrounded by 
a circular fleshy lip, external to which are four groups of labial 
tentacles, twelve or thirteen in each group ; they appear to answer 
MDH yy 
—"npy MM 
Mi M yp 
Ny 
iy Ip" 
My 
rR 
LT) 
Fig. 50. Nautilus pomprlius in its shell.* 
to the buccal membrane of the calamary (Fig. 1). Beyond these, 
on each side of the head, is a double series of arms, or brachial 
tentacles, thirty-six in number; the dorsal pair are expanded, 
and united to form the hood, which closes the aperture of the 
shell, except for a small space on each side, which is filled by 
the second pair of arms. The tentacles are lamellated on their 
inner surface, and are retractile within sheaths, or ‘‘digita- 
tions,” which correspond to the eight ordinary arms of the 
* This woodcut and eighteen others illustrating the tetrabranchiata, are the property 
of Dr, Gray, to whom we are indebted for their use. Fig. 50 represents the recent 
nautilus, as it appears on the removal of part of the outer shell-wall (from the 
specimen in the British Museum). The eye is seen in the centre, covered by the hood 
(h); t, tentacles, nearly concealed in their sheaths; 7, funnel; m, margin of the 
mantle, very much contracted; nm, nidamental gland; a, c, air-cells and siphuncle; §, 
portion of the shell; a, shell-muscle. The internal organs are indicated by dotted 
lines; 5, branchie ; h, heart and renal glands; c, crop; g, gizzard; 2, liver; 0, ovary. 
