The Cambridge natural history, Vol. 03 (of 10)

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.

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.

Davidson in his Monograph on the British Fossil Brachiopoda states that the largest “recent Brachiopod which has come under my notice is a specimen of Waldheimia venosa Solander, measuring 3 inches 2 lines in length, by 2 inches in breadth, and 1 inch 11 lines in depth.” It was found in the outer harbour of Fort William, Falkland Islands, in 1843. A specimen of Terebratula grandis from the Tertiary deposits, however, exceeds this in all its dimensions. Its length was 4½ inches, its breadth 3 inches 2 lines, and its depth 2 inches 2 lines.

Distribution in Space

Brachiopods are very localised; they live in but few places, but when they are found they usually occur in great numbers. During the cruise of the Challenger, dredging was conducted at 361 stations; at only 38 or 39 of these were Brachiopoda brought up. Mr. Cuming, quoted by Davidson, records that after a great storm in the year 1836, he collected as many as 20 bushels of Lingula anatifera on the sea-shore at Manilla, where, he relates, they are used as an article of food. It has been suggested above that their abundance in certain localities is due to their limited powers of locomotion, which are effective but for a few hours, the larva being, moreover, so minute that unless borne by a current it could not travel far from its parent. When once settled down it has little to fear from the attacks of other animals. The size of its shell relative to its body would deter most animals from regarding it as a desirable article of food, and as far as is known at present the Brachiopoda suffer but little from internal parasites, the only case I know being a minute parasitic Copepod belonging to a new and as yet unnamed genus which I found within the mantle cavity of Cistella (Argiope) neapolitana in Naples. Their slight value as an article of diet has doubtless helped to preserve them through the long periods of geological time, through which they have existed apparently unchanged.

Two of the recent genera of the family Lingulidae, Lingula and Glottidia, are usually found between tide-marks or in shallow water not exceeding 17 fathoms. Discina is also found about the low-tide level, but one species at any rate, Discinisca atlantica, has been dredged, according to Davidson, “at depths ranging from 690 to nearly 2425 fathoms.” Their larvae frequently settle on the shells of their parents, and thus numbers of overlapping shells are found clustered together. Crania is usually dredged from moderate depths down to 808 fathoms, adhering to rocks, lumps of coral, stones, and shells.

Of the Testicardines, Terebratula Wyvillei has probably been found at the greatest depth, i.e. 2900 fathoms, in the North Pacific. It is interesting to note that its shell is glassy and extremely thin. The Brachiopoda are, however, as a rule, found in shallower water; they abound up to a depth of 500 or 600 fathoms, after which they rapidly diminish with increasing depth. About one-half the named species occur at a depth of less than 100 fathoms.

The vertical range of depth of certain species is great; Terebratula vitrea is recorded from 5 to 1456 fathoms, T. Wyvillei from 1035 to 2900 fathoms. This is to some extent explicable since, after a certain depth has been reached, many of the external conditions, such as absence of temperature and light, must remain constant even to the greatest depths of the ocean.

The area of the ocean explored by dredging forms such an infinitesimal fraction of the whole, that it seems superfluous to consider the horizontal distribution of Brachiopods. A few facts may, however, be mentioned. Certain species, as Terebratula vitrea, T. caput serpentis, Waldheimia cranium, Megerlia truncata, and Discinisca atlantica, have a very wide if not cosmopolitan distribution. The second of the above named extends as far north as Spitzbergen, and as far south as Kerguelen Island. Many species are, on the other hand, very localised, and have hitherto only been found in one place. A very considerable number of these have been dredged off Japan and Korea, and this region may be to some extent regarded as the headquarters of the group.

The following species have been obtained within the limits of the British Area, as defined by Canon Norman, who has been good enough to revise the list, which is founded on that drawn up by Davidson in his Challenger Report. Their range of bathymetric distribution is given in the column on the left.

Classification

The table of classification here appended is that suggested by Mr. Davidson in his Monograph on the Recent Brachiopoda.

It is impossible to come to any satisfactory conclusion as to the position of the group Brachiopoda with relation to the rest of the animal kingdom. They have, in accordance with the views of various investigators, been placed in close connexion with many of the large groups into which the Invertebrates are split up. The Mollusca, the Tunicata, the Polyzoa, the Chaetopoda, the Gephyrea, and of recent times such isolated forms as Phoronis and Sagitta, have all in turn had their claims advanced of relationship to this most ancient group. As far as I am in a position to judge, their affinities seem to be perhaps more closely with the Gephyrea and with Phoronis than with any of the other claimants; but I think even these are too remote to justify any system of classification which would bring them together under a common name. Investigation into the details of the embryology of the group, more especially into that of the Ecardines, might throw some light on this subject, and it is much to be desired that this should be undertaken without delay. That the group is a most ancient one, extending from the oldest geological formations, we know, that the existing members of it have changed but little during the vast lapse of time since their earliest fossil ancestors flourished, we believe; but we are in almost total ignorance of the origin or affinities of the group, and we can hardly hope for any light on the subject except through embryological research.