Natural History, Mollusca/Brachiopoda

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(Arm-footed Mollusks.)

The animals bearing the above title are inhabitants of bivalve shells, one valve of which is frequently perforated, to give exit to a fleshy peduncle or stem, by which the animal is affixed to the rocks. When we open the two valves of the shell, we find each of them lined with a broad membrane, very thin, delicate, and semi-transparent, which together constitute the mantle. The edges of these membranes are thickened, and fringed with organs, already several times mentioned in these pages, and which we shall see assuming great importance as we investigate the economy of the lower tribes of animals. These organs are called cilia and consist of very subtile and microscopically minute hairs, arranged in close series, and capable of moving in unison, with vibratile waves, and of thus producing rapid currents in the water.

At the bottom of the cleft formed by the two leaves of the mantle, the mouth is placed, on each side of which is a long fleshy process, fringed all along one side with delicate hairs. In some species, these arms (which give name to the Class) are enormously developed; they are free for their whole length, and are capable, at the will of the animal, of being coiled up in many spiral folds, or of being protruded from the shell to a distance equal to thrice its length. The mechanism by which they are unfolded is simple and beautiful. The stem of the process is hollow, and partially filled with a fluid, which being forcibly injected towards the extremity, by the contraction of a double series of muscles behind, the whole of the lengthened organ is straightened and projected.

Most of the species have a shelly frame-work within one of the valves, consisting of slender loops and arches, variously arranged, and more or less complex. This is intended to support the fringed arms, and to keep the valves open, or even to assist in opening them; for there is in this class nothing corresponding to the hinge-cartilage, which performs the latter function in the Conchifera.

Respiration in these animals seems to be performed by the mantle itself; the long fringed arms having apparently nothing to do with this office, notwithstanding their gill-like structure.

Some of the species are found in the shallows of sandy shores; but others inhabit the darkness and solitude of the deep sea; some of the Terebratulœ dwelling in water from sixty to ninety fathoms deep; while Crania personata has been dredged up from a depth of 255 fathoms. The respiration and nutrition of animals that can subsist beneath a pressure so enormous, are subjects, as Professor Owen remarks, "suggestive of interesting reflections, and lead one to contemplate with less surprise the great strength and complexity of some of the minutest parts of the frame of these diminutive creatures. In the unbroken stillness which must pervade those abysses, their existence must depend upon their power of exciting a perpetual current around them, in order to dissipate the water already laden with their effete particles, and to bring within the reach of their prehensile organs the animalcules adapted for their sustenance."[1]

Genus Terebratula.

In this genus, which I select to represent the Class, the valves are unequal; and the lower one, more prominent than the other, is perforated to admit the passage of a short fleshy stem, by which the animal is firmly attached to rocks, and other foreign bodies. The shell is delicate in texture, more or less triangular, and symmetrical. The shelly frame-work of the arms, sometimes called by collectors the carriage-spring, is attached to the inner surface of the upper valve, or that which is not perforated.

Most of the species of this genus, as of the whole Class, are known only in a fossil state; those which are recent are widely diffused, and flourish in extremely warm and extremely cold climates, as well as those which are intermediate. Thus some are found in the Indian ocean, beneath the equator; while the T. psittacea, brought home from the late Arctic expedition, was dredged at Boothia in the Polar sea.

  1. Comp. Anat. i, 279.