In place of bearing simple suckers (Acetabula), like the last order of Cephalopods, this group is furnished with true organs of prehension, or tentacles. They differ from the first group chiefly in their more numerous arms, which are quite tentaculiferous, having neither suckers nor capsules, and by having an external shell. The number of living species is extremely limited; for this group of animals belongs peculiarly to the earlier ages of our globe, is gradually becoming extinct, and presents in our days only some rarer species, when we compare them with the prodigious numbers of these beings which animated the seas of the ancient world. In fact, the only living type of the order is the nautilus, which has a singular resemblance in form to the argonaut.
The shell of this mollusc has a regularly convoluted form, the last whorl being equal to all the others. It is divided internally into numerous cells, formed by transverse partitions, concave in front and perforated towards the centre, and forming a kind of funnel, which gives passage to a respiratory siphon.
In the last cell of the shell (Fig. 312) is the animal, covered by its mantle, which covers the walls of the cells. When it contracts itself it is protected by a sort of triangular and fleshy hood. Numerous contractile tentacles, re-entering into the sheath, some of them furnished with numerous lamellæ, surround the head, which is, besides, scarcely distinguished from the body. The head bears two great projecting eyes, planted upon a peduncle.
Fig. 312. Nautilus pompilius (Linnæus), showing the interior of the lower cell, to which the animal is fixed.
Like Sepia and Octopus, the mouth of the Nautilidæ is armed with mandibles, fashioned like the parrot's beak; the branchiæ are four in number. The circulating system consists of a ventricle and auricle, and the locomotive tube is protected in its whole length. The shell is secreted by the outer edge of the mantle, while its posterior extremity fashions the walls of the cells, which indicate the successive growth of the individual.
The siphon, which traverses all the chambers, receives and protects the ligament, by the aid of which the Cephalopod is retained in the last chamber of the shell.
Fig. 313 is the same section, with the last cell empty, and with the perforations through which the siphon passes.
Fig. 313. Nautilus pompilius (Linnæus) showing the lower cell and the partition giving passage to the siphon.