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.
[Illustration:
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On the least alarm the animal is rapidly withdrawn as far as the centre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.
[Illustration:
Fig. 5.
_Lingula anatina_, removed from shell, mantle reflected, coiled arms
separated slightly; _a_, mouth. (Marginal setæ omitted.) Ventral
aspect, three-quarter face.
]
[Illustration:
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.
[Illustration:
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.
]
[Illustration:
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.