A Conchological Manual by George Brettingham Sowerby (early reader books .txt) 📖
- Author: George Brettingham Sowerby
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_mere_ Conchologists for the possession of rare specimens, few travelling
merchants and sea captains would have thought them worthy of a corner in
their cabins. In this case, few specimens being brought to the country, the
more Philosophical Naturalist would have been left without the means of
obtaining materials to work upon, or of attracting public attention to his
favourite pursuit.
On account of these and other considerations, it has been thought advisable
that the present undertaking should bear a purely conchological character.
The peculiarities of the shells alone being detailed for the assistance of
those who collect and study them, while at the same time, in deciding upon
their affinities and places, in the arrangement, it will be necessary to
take advantage of the conclusion to which those have arrived, who have
studied the animal in all its parts. And the conviction must be expressed,
that if ever a complete Natural System shall be formed it will result from
the labours of the last mentioned class of naturalists.
DEFINITION OF A SHELL.
Before entering minutely into the description of shells, it will be
necessary to distinguish from the true testaceous Mollusca two kinds of
animals which have formerly been associated with them. Of these, the first
is the class of CRUSTACEA, consisting of crabs, crayfish, &c. These differ
from shell-fish, not only in structure and chemical composition, but also
in the fact that the animal has jointed limbs, and that the substance of
the flesh is inseparable from the hard external covering, which invests
each particular joint as with a sheath; whereas the Molluscous animal is
but partially attached to its shell, from which it possesses the power of
partly withdrawing and returning. The second class is that to which the
sea-urchin, or Echinus, belongs, of which there are many genera and
species. The testaceous covering of Echini is composed of a number of small
pieces, placed edge to edge, forming a more or less globular external
covering to the flesh, which is supported in the centre by a number of
bones leaning upon each other in a pyramidal form. The _test_ is of a
fibrous texture, guarded on the outside with moveable spines, which turn on
ball and socket joints.
A true shell is composed of one or more calcareous pieces, commonly called
valves, each piece formed by a series of layers, applied obliquely upon
each other, in such a manner that each new layer begins within, and
terminates a little in advance of the one before it.
STRUCTURE AND GROWTH.
We shall now endeavour to describe the manner in which the growth of each
separate valve, or each regularly formed shell, proceeds from the nucleus.
Before the young animal has left the egg, if it be an _oviparous_ species,
or the body of the parent if _viviparous_, the nucleus of the shell is
generally formed, and specimens are sometimes preserved in which the young
shell is seen within the egg, as in the cut, fig. 1, 2; or adhering to the
inner surface of the full-grown shell by the dried mucus of the animal, as
seen in fig. 3.
In both cases, the nucleus is generally of a more horny and transparent
composition than the parts subsequently produced. As soon as the animal is
hatched, or, in other words, leaves the egg or body of the parent, of
course it begins to increase in size, and to require a corresponding
enlargement in the shell. To effect this, a small quantity of mucus
substance, secreted by the mantle of the animal, is deposited on the edge
of the aperture. When this is dry and become sufficiently hard, it is lined
by a more calcareous secretion; and these together form a new layer, which
is followed by others in succession; each new layer being larger than the
one that preceded it until the whole being complete, the full-grown animal
is invested with a shell commensurate with its own proportions. Thus from
the apex or nucleus the formation proceeds, as it were, downwards, taking
the shape of the part which secretes it, on which it is in a manner
moulded.
The nucleus, or first formed portion, may for technical purposes be
considered, mathematically, as the apex of a spiral cone. And here it must
be observed, that whether the shell consist of one or several pieces, each
piece has a separate nucleus, and the process of formation is separately
repeated with each. The word cone is used for convenience, and its meaning
extended so as to include all those structures which commencing at a point
enlarge downwards.
From the apex, the next layer is deposited on its edge, and advancing
beyond it necessarily adds to its extent. Thus, suppose for the sake of
illustration, the part marked _a_ in the diagram, fig. 4, to represent a
nucleus, the cross lines (_l_) will shew the consecutive layers, which
enlarge their circle as they add to their numbers. This disposition of
shelly matter into layers is marked externally by concentric striæ, or
_lines of growth_, while on the inside the edges of the laminæ are
consolidated into a kind of enamel. If a perpendicular section of a solid
portion of a shell were magnified, it would present, in many instances, an
appearance resembling the diagram, fig. 5; _a_ may be taken to represent
the horny part of the layers which form the outer coating, named
"_Periostaca_," or "_Epidermis_;" the undulating line _b_, is formed by the
edges of the calcareous layers, and causes the striæ, or lines of growth,
which are often distinguishable on the surface of the shell; the space _c_
is the middle part of those layers, and at _d_ they are consolidated into
the enamel which lines the interior.
In some species the layers are irregularly grouped together, and their
edges overlap each other, so that they are easily separable, and advancing
beyond each other, give a leafy appearance to the external surface. This
structure is termed _foliaceous_. A very familiar instance of this may be
observed in the common oyster. If a specimen of this shell be broken, the
substance will be seen to exhibit a degree of looseness, and a magnifying
glass will enable the student to trace distinctly the laminæ of which it is
composed. The accompanying representation of a magnified section (fig. 6)
will shew at _a_, the external surface, with the foliations or leaves; at
_b_, the parcels of layers which form them; and at _c_, the pearly
structure produced by their consolidation, and by the subsequently
deposited enamel which covers their external surface.
CLASSIFICATION.
The classification of shells, that is, their systematic arrangement into
_classes_, _orders_, _families_, _genera_ and _species_, cannot be made to
depend entirely upon the characters observable in them, viewed by
themselves; for this reason, that many similarly formed shells form the
habitations of animals perfectly distinct, and that many molluscous animals
are found to agree with each other in every respect but in the form of
their testaceous support. There are, however, many important distinctions
to be observed in the shells themselves, leading to the establishment of
many of those very divisions, which would afterwards be confirmed by an
examination of the soft parts. It is necessary to attend, as far as means
and opportunity will allow, to _all_ the points of difference, both in the
shell and in the animal, in order to form, and in some instances even to
appreciate, a generic or larger distinction. It will therefore be our
endeavour to explain the general principles upon which those distinctions
are formed, and the manner in which they are applied and expressed in
detail by scientific writers.
NUMBER OF PIECES, OR INDEPENDENTLY FORMED PARTS.
The first, most simple and obvious division of shells, is that which
results from the number of separate pieces composing them. Hence the
distinction implied by the terms UNIVALVE, or consisting of a single piece;
BIVALVE, or composed of two pieces; and MULTIVALVE, or composed of more
than two. For an example of _univalve_, take a common whelk; for a
_bivalve_, take a muscle or a scallop; and for a _multivalve_, the
barnacle, or balanus, found adhering to the common oyster.
But although this arrangement may appear at first sight perfectly easy and
plain, some explanation will be necessary in order to guard the student
against understanding the above expressions in their strictest sense,
without qualification. Thus the univalves are said to consist of a single
piece, or spiral cone; but it would be more correct to speak of this piece
as forming either the whole or the principal part of the shell: for there
is in many instances, a much smaller flattened piece attached to the foot
of the animal, which being drawn in when it retires, closes the aperture as
with a kind of door, to which in fact the word valve might be very properly
applied; it is called however the OPERCULUM, and the little horny plate,
frequently drawn out by means of a pin from the aperture of a periwinkle,
will present a familiar example.
The same may be said respecting the bivalves; for besides the principal
portions or valves of which the shell is composed, there are in many
species, one or two smaller separate portions, named "_accessary plates_"
by some authors. They are fixed by means of cartilages, on the back of the
hinge.--The engraving, fig. 7, represents the accessary valves of a species
of Pholas, which was on this account arranged by Linnæus with the
Multivalves. Nearly allied to the Pholades is a set of shells to which De
Blainville has given the name "_Tubicolæ_," or inhabitants of tubes. In
this case, the bivalve shell is connected with a testaceous tube or pipe,
to which it is attached either by one or by both valves, or in which it
lies attached only by the cartilages of the animal. In the genus
Aspergillum, the two small valves are soldered into the sides of the tube
in such a manner as to constitute a part of it. One of these shells, called
the Water-spout, might be taken up by a person not aware of its real
nature, and regarded as a pipe or tube prettily fringed, and nothing more;
but upon a closer examination, he would find the two valves, the points of
which are visible from the outside of the tube.
HABITS--_Land, Fresh-water, or Marine Shells._
Another distinction, leading to important results in classification, is
that which is derived from the nature of the element breathed by the
Mollusc. And although this consideration belongs more especially to the
study of the animal itself, yet the habits of the animal materially
influence the structure of the shell.
The TERRESTRIAL or LAND Molluscs live on land, breathe air, and feed on
plants and trees.--Those who find pleasure in horticultural pursuits will
at once call to mind a too familiar example of these Molluscs in the common
garden snail. The Land-shells are all univalves, and constitute a family in
the Lamarckian system under the name "_Colimacea_," or snails,
corresponding with the Linnean genus Helix.--They are generally light in
structure and simple in form.
The AQUATIC, or Fresh-water Molluscs, such as the Planorbis, commonly
called the Fresh-water Snail; the Unio--known by the name of Fresh-water
Muscle, is found in ponds, ditches and rivers. The _epidermis_ of these is
generally of a thick, close-grained character, and they are subject
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