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On the least alarm the animal is rapidly withdrawn as far as the centre of the tube (see the dotted line of the shell in the figure), the surface slit and upper part of the tube being obliterated.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