Page:Popular Science Monthly Volume 31.djvu/768

748 oxygen that is held in the water. The stream also whirls minute bodies, microscopic animals and plants, to the mouth, which swallows them. Our terebratula has no other means of getting food. As the oyster and the rotifer are fed by the streams and whirlpools produced by the ciliæ, so also is the terebratula. When the shell is closed, only the exchange of gases takes place; but when it is opened, or when the animal is gaping, it is eating, for these larger bodies are drawn in through the whirling. That this really takes place is shown by the contents of the stomach, in which we may observe undigested remains, the silicious shells of plants, diatoms, or Radiolariæ, and the needles of fungoids. The organic substance is digested and dissolved in the stomach, while the undigested remains are expelled through the mouth. Sea-water, especially near the ground, to which the terebratulæ are attached, swarms with shell-protected and naked matter of this kind. The terebratula is, moreover, in the happy situation of having nothing to do but spit out the shells, for the meat itself flies into its mouth.

Every fixed animal produces moving young. Were this not the case, the animals could not be distributed into spaces beyond their immediate abode. Very curious young are produced from the eggs of our arm-foots, which do not at first resemble their parents at all; larvæ that swim around in freedom, having eyes, and armed with bristles. They so much resemble the larva of some of the ringed worms, that one would be apt to suppose at first sight that they were of that kind. But after a time of wandering they settle themselves down, and there then takes place, with the formation of the shell, a retrograde metamorphosis, by which the animal is gradually brought back to its definite form. It is therefore easy to conceive that our arm-foot leads a very safe life, and that, protected as much as possible against enemies and other dangers, it can spin out its existence as long as the sea does not dry up. Of course, in their younger days, the wandering larvæ may be swallowed up in numbers by other animals, but when the young brachiopod is once fixed, the shell, the mantle-edge with its bristles, and the ciliary apparatus protect it so well, that not even parasites can attack it. Although fungoids, other animals, and occasionally fellow-beings of its own race may establish themselves on the outside of its shell, and load it to a certain extent, but otherwise do it no harm, no parasite has ever been seen within. The little wart that appears on the glass terebratula in the picture is one of their young. There have always been fishes which, after a fashion, eat corals and crush them with their hard teeth, to digest the polyps which the corals contain, and expel the lime-substance; but I hardly think they would take in the arm-foots, whose bodily substance is so stringy and yields so little nourishment. In the deep sea, also, the animals are thoroughly protected against the sudden changes of temperature which animals living in shallow waters and near the shores have to encounter.