Fig. 320.—Stages in the development of the larva of Terebratulina septentrionalis. (After Morse.) The youngest larva has two segments, a third then appears, the larva then fixes itself, and the second segment folds over the first and develops bristles round its edge.
Habits
There is little to be said about the habits and natural history of the Brachiopoda. When once the larva has settled down, the animal never moves from the spot selected; occasionally it may rotate slightly from side to side on its stalk, and from time to time it opens its shell. As so frequently is the case with sessile animals, the sense organs are reduced to a minimum, the eyes of the larva disappear, and the only communication which the animal has with the world around it is by means of the currents set up by the cilia on the tentacles.
In spite of the absence of any definite eyes, Thecidium, according to Lacaze-Duthiers, is sensitive to light; he noticed for instance that, when his shadow fell across a number of these animals he was watching in a vessel, their shells, which had been previously gaping, shut up at once.
In Cistella the tentacles can be protruded from the open shell, and in Rhynchonella the spirally-coiled arms can be unrolled and extruded from the shell, but this does not seem to have been observed in other genera, with the possible exception of Lingula. The food of these animals consists of minute fragments of animal and vegetable matter, a very large proportion of it being diatoms and other small algae.
Fig. 321.—Figures illustrating the tubes in which Lingula anatifera lives. The upper figure is a view of the trilobed opening of the tube. The lower figure shows the tube in the sand laid open and the animal exposed. The dotted line indicates the position of the body when retracted. The darker portion is the tube of sand agglutinated by the secretion of the stalk. (After François.)
Lingula differs markedly from the other members of the group, inasmuch as it is not firmly fixed to a rock or some such body by a stalk or by one of its valves, but lives in a tube in the sand. Some recent observations of Mons. P. François[427] on living specimens of Lingula anatifera which he found living in great numbers on the sea-shore at Nouméa in New Caledonia may be mentioned. The presence of the animal is shown by a number of elongated trilobed orifices which lead into the tube in which the Lingula lives. The animals, like most other Brachiopods, live well in captivity, and he was able to watch their habits in the aquaria of his laboratory. The Lingula place themselves vertically; the anterior end of the body just reaches the level of the sand; the three lobes into which the orifice of the tube is divided corresponding with the three brushes of setae which project from the anterior rim of the mantles. These setae are described by Morse as projecting in the form of three funnels; currents of water are seen continually passing in at the side orifices and out through the central. The tube consists of two portions: an upper part, which is flattened to correspond with the flat shape of the body, and a lower part, in which the stalk lies. The upper part is lined with a layer of mucus, but the sand is not glued together to form a definite tube. The lower part of the stalk, or the whole when the animal is contracted, is lodged in a definite tube composed of grains of sand agglutinated by mucus, probably secreted from the walls of the stalk. At the least sign of danger the stalk is contracted violently, and the body is withdrawn to the bottom of the upper portion of the tube. The rapid retreat of the animal is followed by the collapse of the sand at the mouth of the tube, and all trace of the presence of the Lingula is lost.
The shells of this species are frequently rotated through a small angle upon one another, a movement which is prevented in the Testicardines by the hinge. In very young transparent specimens François was able to observe the movements of the fluid in the system of tubules which penetrate the mantle; these tubules are figured by him, and Fig. [315] is taken from his illustration.