Section I.—Inarticulata.
Order 1.—Atremata. Family Lingulidæ.—The Lingulas possess emerald green or golden brown duck-bill-shaped shells. Having no hinge, the dead valves of dried shells easily fall apart. The peduncle, which is sometimes over six inches in length, passes between the pointed posterior borders of the valves. Dr. François gives a very interesting account of the habits of Lingula anatina which he found living in the sand at Noumea, New Hebrides. The sole evidence of the animal’s existence is the presence, on the surface of the sand or mud, of a small, three-lobed slit (Fig. 4, upper figure). The tube (Fig. 4) in which the Lingula lives is about four inches deep, flat in the upper half, rounded below. The walls of the upper flat portion simply consist of the sand with a surface coating of mucous secretion; but in the lower end the sand grains are agglutinated so as to form a distinct tube.
Fig. 4.
Lingula anatina in tubes in the sand; upper figure shows trilobed opening on surface of sand. Dotted line in lower figure indicates position in retraction. (After François.)
The edges of the mantle-folds are provided with setæ (bristles), which form three funnels protruding through the three lobes of the slit-like mouth of the sand-tube; currents enter by the lateral funnels and leave by the central.
On the least alarm the animal is rapidly withdrawn as far as the centre of the tube (see the dotted line of the shell in the figure), the surface slit and upper part of the tube being obliterated. Each of the arms forms a spiral with several coils (Fig. 5). The Lingulidæ are of exceptional interest, in that they furnish a very remarkable example of “persistence of type.”
Shells of Lingula occur in the earliest Palæozoic strata, and so closely resemble those of the present day, that often no difference can be observed either in the shape of the valves or in the muscular impressions on their inner surface (Fig. 6).
Lingula occurs in the Indo-Pacific, Australia, China, Japan, and the Pacific Islands. Glottidia, a smaller form, with two small curved plates on the brachial and a ridge on the peduncle valve, is found on the American coast of the Pacific, and in the Atlantic.
Fig. 5.
Lingula anatina, removed from shell, mantle reflected, coiled arms separated slightly; a, mouth. (Marginal setæ omitted.) Ventral aspect, three-quarter face.
Fig. 16.
Lingula anatina. Interior of valves showing muscle scars.
V. Peduncle valve, D. Brachial valve.
Order 2.—Neotremata. The Discinidæ includes two genera, Discina and Discinisca (Fig. 7), with orbicular conical shells, of horny calcareous composition; both valves are conical in the former genus, but in the latter the peduncle valve is flattened. Sometimes the embryos settle down on the parent shells, and we see a mass of shells in various stages of growth, as in the specimen of Discinisca lamellosa from Peru.
Fig. 7.
Discinisca lamellosa. Peru. (After G. Sowerby.)
A group of old and young specimens; largest showing foramen in peduncle valve, the rest showing brachial valves.
Fig. 8.
Three specimens of Crania anomala on a stone. Loch Fyne.
The Craniidæ form small limpet-like shells (Fig. 8) closely adherent to the rocks by the whole surface of the peduncle valve; although this valve is so named, no peduncle or foramen is found in this family. The Neotremata, like the Lingulidæ, are remarkable examples of persistence of type, since forms very similar to the present day Discinas and Cranias occur in the Palæozoic, Ordovician and Silurian strata.
A piece of rock, with several specimens of Crania anomala attached, is exhibited.