A Guide to the Shell and Starfish Galleries

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On the least alarm the animal is rapidly withdrawn as far as the centre of the tube (see the dotted line of the shell in the figure), the surface slit and upper part of the tube being obliterated.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.

Section Ii.—Articulata.

Order 3. PROTREMATA.—This group, formerly very abundant, is now almost extinct, the Family Thecidiidæ representing the Order at the present day. Thecidium mediterraneum (Fig. 9) forms little oval boxes about a third of an inch in length, shaped somewhat like a pear cut in half (peduncle valve), and with a semicircular lid (brachial valve) working on a hinge on the upper flat surface. The foramen and peduncle are absent; but between the pointed end of the peduncle valve and the hinge is an area filled in by a calcareous plate characteristic of the Protremata.

Fig. 9.

Thecidium mediterraneum. A, natural size. B, section through shell. Magnified. ]

The brachial valve opens like the lid of a snuff-box, and shuts down on the least alarm with the rapidity of lightning. The peduncle valve is fixed on the rocks by its convex surface. The species is common in the Mediterranean in from 30 to 300 fathoms, and is also found in the West Indies.

Order 4. TELOTREMATA.—This group, which at the present day contains the largest number of species, includes the Lamp shells, so called from their resemblance to an ancient lamp. The valves are joined by a well-marked hinge, the peduncle passes through the peduncle valve through a foramen completed by two plates secreted by the mantle edges, and the brachial valve has attached to it a calcareous scaffolding of processes or loops for the support of the “arms.”

The shells in this group are frequently ridged. Their colour is usually white, but sometimes red or yellow; deep-sea forms are generally vitreous.

Rhynchonella psittacea has a black shell with a pointed incurved beak; each of the arms forms a many coiled spiral and can be protruded beyond the shell; the brachial skeleton is comparatively small and simple, consisting of two separate processes.

In Terebratulina the brachial skeleton forms a simple loop; in Magellania the loop is reflected on itself (Fig. 2).

The beautiful and unique specimen of Dyscolia wyvillii, from 390 fathoms W. Indies, is remarkable for its size, being over two inches in length. The small vitreous specimens of Terebratula wyvillii were obtained off Chili from a depth of 2160 fathoms; specimens of the same species were obtained also from a depth of 2900 fathoms in the North Pacific.

TUNICATA.

The Tunicata are marine animals, the majority of which live, in their adult stage, a stationary life, fixed to the rocks or sea-bottom, but a comparatively small number are free-swimming.

Fig. 1.

Ascidia mentula from the right side. at, atrial aperture; br, branchial aperture; t, test.

They occur in the form of cartilaginous or leathery sacs, fleshy incrustations, solid fleshy masses, free-swimming, barrel-shaped animals, solitary or united into chains or hollow cylinders; or, lastly, of minute free-swimming tadpole-shaped organisms. To explain briefly the structure of a Tunicate, Ascidia mentula (Fig. 1), is selected. The animal, which lives on a muddy bottom, in from five to twenty fathoms, resembles a conical sac fixed by the broader end, of grayish green colour and about 4 inches in height. At the narrower end are two orifices, one terminal—the branchial orifice or mouth, and the other a little lower—the atrial orifice: the former has eight lobes and the latter six.

When the Ascidian is undisturbed, the orifices are wide open, and currents enter by the branchial and leave by the atrial orifice. On the least alarm, the orifices close, jets of water being at the same time squirted out; hence the popular name “Sea-squirts” given to these animals.

Fig. 2.

Diagrammatic section of Ascidia representing the three sacs, and the branchial sac as the pharynx or throat.

a, branchial; and b, atrial orifice; c, tunic or test; d, mantle; e, branchial sac; f, gullet; g, stomach; h, anal orifice; i, dorsal lamina; dotted line indicates the endostyle. ]

The Ascidian is orientated as follows: hold the animal with the branchial orifice pointing forwards and the atrial upwards; the branchial orifice will be anterior and the opposite end posterior; the atrial orifice will lie on the upper or dorsal aspect, the opposite aspect being lower or ventral, and the sides right and left. The aspects, in fact, correspond with those of a vertebrate animal. A vertical section roughly shows the animal to be formed of three concentric sacs (Figs. 2, 3). The outermost, which is tough and membranous, is called the Test or Tunic, the whole group owing its name to the presence of this protective covering.

Fig. 3.

Diagrammatic dissection of A. mentula.

at, atrial orifice; br, branchial orifice; a, anal orifice; brs, branchial sac; dl, dorsal lamina; end, endostyle; m, mantle; ng, nerve ganglion; oea, orifice of gullet; pbr, peribranchial cavity; st, stomach; t, test; tn, tentacles.

(After Herdman: Tunicata, Encyc. Britannica.) ]

The middle sac, termed the Mantle, which almost corresponds in shape to the outer, is composed of connective tissue, muscle-fibres, blood-vessels, etc.; in spirit specimens, the mantle is shrunk away from the test except at the orifices and at a point behind, where vessels enter the test.

The innermost or Branchial Sac is attached behind the branchial orifice and along the ventral edge, but otherwise hangs free in the interior, the space around and outside of the sac being termed the atrial or peribranchial cavity.

The delicate walls of the branchial sac, which resemble fine muslin, are perforated by innumerable vertical slits, termed stigmata, arranged in transverse rows (Fig. 4).

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