SUB-KINGDOM VI.
Mollusca (soft). General characteristics—Mollusks, a numerous branch of the animal kingdom, are so called from the softness of their bodies, which usually have no internal skeleton or framework to support them. They are covered with a tough, muscular skin, and generally protected by a shell. They have a gangliated nervous system, in some cases well developed, the medullary[6] mass not enclosed in a cranium or spinal column—of which they are mostly destitute—but distributed more or less irregularly through the body. They have hearts, and an imperfect circulative system, the blood being pale or blueish. Some breathe in air, some in fresh, more in sea water. Some—both of those on land and those in water—are naked, others have a calcareous covering or shell. The larger marine mollusks are often guarded by very strong, heavy shells. Some are viviparous, others oviparous, and multiply rapidly.
SNAIL.
Three general, and many subdivisions are recognized. The total number of living species is said to exceed twenty thousand, of which only a few can be mentioned. The classes under this division are Lamellibranchiata, Gasteropoda, and Cephalopoda. The chief representatives of the first class are all ordinary bivalves.
Ostrea (oysters) are well known bivalve mollusks. The shells are so irregular in surface and shape that it would be impossible, as it is unnecessary, to describe them. The animal itself is very simple in structure, proverbially stupid, low in the scale of animal life, but highly esteemed as a delicious article of food. They are found in almost all seas, and in water of from two to six fathoms, but never very far from some shore. They multiply so rapidly that, though the consumption increases with the increase of population and the facilities for distributing them, the supply is equal to the demand. Boston is mostly supplied from artificial beds, and the flats or shallow bays in the vicinity of our maritime cities abound in such “plants.” Baltimore and New York have each an immense local trade, and the oysters exported from the Chesapeake Bay fisheries amount to about $25,000,000 annually.
Class II.—Gasteropoda (stomach-footed). This class, including the great snail family, is a very large division of terrestrial, air-breathing mollusks. Their light shells vary much in form; when spiral and fully developed they have as many as five or six whorls, symmetrically arranged. The shell can contain the whole body, but the animal often partially crawls out and carries the shell on its back. The locomotion is slow and accomplished by the contractile muscle of the ventral foot.
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.