Class III.—The Cephalopoda (head-footed) have distinctly formed heads, large, staring eyes, mouths surrounded with tentacles or feelers, symmetrically formed bodies wrapped in a muscular covering; they have all the five senses, and are carnivorous. This class is entirely marine, and breathes through gills on the side of the body. The naked Cephalopoda are numerous, and furnish food for many other animals. Those living in chambered shells, once numerous, are now known principally by their fossils, the pearly Nautilus (sailor) being their only living representative. This has a smooth, pearly shell, and is much prized as an ornament. It is a native of the Indian Ocean, dwelling in the deep places of the sea, and at the bottom. The shell is too well known to need description, and but few specimens of the living animal have been obtained.
NAUTILUS.
The following facts as to the physical organization and habits of this interesting species of univalves are gleaned from the American Cyclopædia, and given for those who have not access to any extensive work on the subject; they were mostly copied for the Cyclopædia from Professor Owen’s celebrated memoir, and the “Proceedings of the Linnæan Society of London:”
The posterior portion of the body containing the viscera is soft, smooth and adapted to the anterior chamber of the shell; the anterior portion is muscular, including the organs of sense and locomotion, and can be drawn within the shell. The mantle is very thin behind, and prolonged through the calcareous tube of the occupied chamber as a membranous siphon, and through all the divisions of the shell to the central nucleus; on the upper part of the head is a broad, triangular, muscular hood, protecting the head when retracted and used as a foot for creeping at the bottom of the sea, with the shell uppermost. … The mouth has two horny mandibles, like the beak of a parrot reversed, the lower overlapping the upper, moving vertically, and implanted in thick, muscular walls. … There are ninety tentacles about the labial processes and head. The internal cartilaginous skeleton is confined to the lower surface of the head, a part of the cephalic nervous system being protected in a groove on its upper surface, and the two great muscles which fasten the body to the shell are attached to it. The funnel is very muscular and is the principal organ of free locomotion, the animal being propelled backward by the reaction of the ejected respiratory current against the water before it. … The nautilus, though the lowest of the Cephalopods, offers a nearer approach to the vertebrate animals than does any other invertebrate, in the perfect symmetry of the organs, the larger proportion of muscle, the increased bulk and concentration of the nervous centers in and near the head, and in the cartilaginous cephalic skeleton. The mature nautilus occupies but a small part of the shell, the parts progressively vacated during its growth are one after another partitioned off by their smooth plates into air tight chambers, the plates growing from the circumference toward the center, and pierced by the membranous siphon. The young animal before the formation of these chambers can not rise from the bottom of the sea, but the older ones come to the surface by the expansion and protrusion of their bodies producing a slight vacuum in the posterior part of the chamber unoccupied, and, some say, by the exhalation of some light gas into the other deserted chambers. They rise in the water as a balloon does in the air, because lighter than the element surrounding them. They float on the surface with the shell upward, and sink quickly by reversing it. By a nice adjustment, in the completed structure, between the air chambers and the dwelling chamber, the house and its inhabitant are nearly of the same specific gravity as water. … In parts of the Southern Pacific, at certain seasons of the year, fleets of these little ships are carried by the winds and currents to the island shores, where they are captured and used for food.
ARGONAUT.
The Paper Nautilus, or argonaut, secretes a thin, unchambered shell in which its eggs are carried; has tentacles or arms with which it crawls on the bottom, and swims backward, usually with the back down, squirting water through its breathing funnel. The argonaut differs from the true nautilus in having larger arms of more complicated structure, with sucker discs, and partially connected by a membrane at the base. It has an ink gland and sac, for its secretion. It has a great number of little cells, containing pigment matter of different colors, whose contractions and expansions give it a remarkable power of rapidly changing its tints. There is no internal shell, and it is ascertained that the external shell is peculiar to the female, and is only an incubating and protective nest for the eggs. The eggs are attached to the involuted spire of the shell, behind and beneath the body of the female. From the fact that the animal has no muscular or other attachment to its shell, and has been known, after quitting it, to survive sometime without attempting to return, the argonaut has been supposed to be a parasitic occupant of the cast off shell of another, but is now pretty clearly proved to be the architect of its own shell, which it also repairs when broken by the agency of its palmated arms. It is said the argonaut rises to the surface, with the shell upward, turning it downward when it floats on the water; by drawing the six arms within the shell and placing the palmated ones on the outside, it can quickly sink. This explains why the animal is so seldom taken with the shell. The shell is flexible when in water, but very fragile when dry. The largest known specimen is in the collection of the Boston Society of Natural History; it is 10 by 6½ inches. For a full account of this animal see “Proceedings of the Boston Society of Natural History,” vol. v, pp. 369-381.
End of Required Reading for April.