Page:Natural History, Mollusca.djvu/322
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