A Conchological Manual, George Brettingham Sowerby [little readers .TXT] 📗

commercially valuable, by the zeal and emulation manifested by

_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