ACEPHALOUS MOLLUSCA.
"Sigiliatim mortales, cunctum perpetul."
APULEIUS.
The Mollusca proper were divided by Cuvier into five great classes:--
I. Lamellibranchiata, or Acephalous Mollusca, often called Conchifera.
II. Brachiopoda. III. Gasteropoda. IV. Pteropoda. V. Cephalopoda.
The name Mollusca indicates the characters which most struck the
ancients: they are soft--in Latin, _mollis_: their flesh is cold, humid,
and viscous. In consequence of their very softness, they are generally
furnished with an apparatus of defence or protection, in the shape of a
calcareous cuirass, called a _shell_. According to the species this test
is a coat of mail, a buckler, or a tower. The mollusc is thus armed and
defended against all attacks from without, nearly after the manner of a
knight of the middle ages; only the knight was not quite shut up in his
armour, while the mollusc is attached to it by indissoluble organic
bonds. "Such a life and such a habitation!" says Michelet. "In no other
creature is there the same identity between the inhabitant and the nest.
Drawn from its own substance, the edifice is the continuation of its
fleshy mantle. It follows its form and tints. The architect has
communicated its own substance to the edifice."
The shell of the Mollusca has been variously appreciated by naturalists.
"We might regard the shell as the bone of the animal which occupies it,"
says a celebrated French naturalist; and then gives expression to a very
different view. "We may say as a general thesis that testaceous
molluscs are animals with whom ossification is thrown out on the
external surface in place of the interior, as in the Mammals, birds,
reptiles, and fishes. In the case of the superior animals the bones lie
in the depths of the body; in the shelled Mollusca the bones are placed
on the superficies. It is the same system reversed."
Other zoologists reject as altogether untenable this assimilative
theory. "The shell which serves as a dwelling and a shelter cannot," say
these authors, "be considered as a skeleton, because it does not assume
the external form of the animal; because it does not attach itself to
the organs of locomotion; and, finally, because it is the product of
secretion, which increases in proportion to the development of the body
itself." This last opinion appears to us to be the most acceptable.
However that may be, from the immense variety of form and size, from the
beauty and brilliancy of their colours, the shells of the molluscs are
among the most attractive objects of natural history. Nor is it from
their beauty alone that a fine collection of shells becomes interesting:
a living creature has inhabited the shell, a creature which in its
organization and its life, above all, by its habits, excites in a high
degree our interest, curiosity, and admiration. It has been said that
the shell "is like a medal struck by the hand of Nature to commemorate
climates." In short, the waters of different regions of the globe,
whether fresh or salt, are characterized by the presence of particular
shells; moreover, the comparison of living shells with those which lie
in a fossilized state buried in the depths of the soil is a most
important element of our knowledge touching the origin of the different
beds out of which our globe is constituted.
Thus, we must not shut our eyes to these beings, in appearance so
miserable and obscure, if we would possess a general knowledge of the
animal kingdom. The Creator has endowed them with many wonderful gifts
to embellish their lives, and who would dare to disregard them? Who
could examine and compare their structure without being charmed with the
study? Man, who descends into the depths of the earth in search of the
precious metals--who dives into the deep in pursuit of the treasures it
conceals--who stoops his head over works of art--would surely not refuse
to bend himself for a moment to the sand of the sea, to gather in his
hand, to bring nearer to his eyes, these marvellous works of the Divine
Creator!
The true molluscs are divisible into two great classes: the Acephalous,
or Headless, and those having a head of structure more or less perfect,
which are called Cephalous Molluscs.
The Acephalous or Headless Molluscs are so called from the Greek ἀ,
privative, and κεϕαλὴ, head. They have no head; the body is
surrounded by the folds of the skin; the shell consists of two valves.
Such is a summary description of all the Acephalous Molluscs. They are
sometimes naked, and sometimes enclosed in a shell, whence they are
known as Testaceous Molluscs. They are called _bivalves_, because their
shell consists of two halves, or valves united by a hinge. They are
sheltered in this double carapace as a book is in its cover.
Although they have no head, they can feed themselves, and they reproduce
their kind. They have friendships and enmities, perhaps even passions;
probably these are not very lively, for most of them scarcely ever
change their place, even to make the least movement. Many of them remain
fixed to the rock on which they were hatched, and tumultuous sensations
are not quite compatible with immobility.
The bivalves[8] are found in every sea. The shell of the bivalve is
ovoid, globulous, trigonal, heart-shaped, elongated like a pea-pod, or
flat like the leaves of a tree, having an opening down the ventral side.
In some one valve is flat, the other round and swelling in the centre.
The shell is thus an outer envelope, consisting of two pieces, more or
less corresponding to each other in size and shape (of which the oyster
is an example), formed of carbonate of lime deposited in membranous
cells in its outer layers, the inner layers being composed of thin
coatings of lime deposited in the outer surface of the tissue, called
the mantle leaves. The valves are united to the animal by the insertion
of certain muscles, and by the horny epidermis of the mantle, which
stretches over the edge of the valves. The hinge and ligament which
unite the two valves consist of a dense elastic substance, somewhat
resembling india-rubber; the hinge is formed of teeth, and cavities into
which the teeth fit. The ligament acts in opposition to certain
contractile muscles within, which draw them together, and is placed
either within or without the hinge, or partly both. On separating the
valves, the two leaves of the mantle present themselves. These are thin
delicate leaves, furnished at the margin with sensitive tentacles and
other organs of sense, and with glands sometimes highly coloured. The
use of these organs is thus described by Mr. Rymer Jones:--
"When the animal is engaged in increasing the dimensions of its abode,
the margin of the mantle is protruded and firmly adherent all round to
the circumference of the valve with which it corresponds. Thus
circumstanced, it secretes calcareous matter and deposits it upon the
extreme edge of the shell, when the secretion hardens and becomes
converted into a layer of solid testaceous substance. At intervals this
process is repeated, and every newly-formed layer enlarges the diameter
of the valve. The concentric strata thus deposited remain
distinguishable externally, and thus the lines of growth marking the
progressive increase of size may easily be traced."
"While the margin of the mantle is thus the sole agent in enlarging the
circumference of the shell," the professor continues farther on, "its
growth in thickness is accomplished by a secretion of a kind of
calcareous varnish derived from the external surface of the mantle
generally, which, being deposited layer by layer over the whole interior
of the previously existing shell, progressively adds to its weight and
solidity. There is, however, a remarkable difference in character
between the material secreted by the marginal fringe and that furnished
by the general surface of the mantle membrane. The former we have found
more or less covered by glands appointed for the purpose, situated in
the circumference of the mantle; but as these glands do not exist
elsewhere, no colouring matter is ever mixed with the layers that
increase the thickness of the shell, so that the latter always remain of
a delicate whitish hue, and form the well known iridescent material
usually distinguished by the name of nacre or mother-of-pearl."
(_General Outline_, p. 385.)
The process by which shells attain their beautiful markings is thus
described by Mr. Jones:--"The external surface is exclusively deposited
by the margin of the mantle, which contains in its substance certain
coloured spots, which are found to be of a glandular character, and to
owe their peculiar character to a pigment they secrete, which is mixed
with the calcareous matter; coloured lines are therefore found on the
exterior of the shell wherever these glandular organs exist. Where the
deposition of colour is kept up throughout the process of enlargement,
the lines are unbroken and perfect; but where the coloured matter is
furnished only at intervals, spots and patches of irregular form and
increasing in size with the enlargement of the mantle are the
consequence."
Bivalves move about and change from place to place by means of an
extensible fleshy organ called, from some of its functions, a foot; in
fact, it has less resemblance to a foot than to a large tongue. It is a
muscular mass, capable of being pushed out from between the mantle and
the valves, and varies much in form; it is in turn a hatchet, a
ventilator, a pole, an awl, a finger, and a sort of whip. This foot is
simple, forked, or fringed. In some species the tissues are spongy, and
capable of receiving considerable quantities of water. When the organ
swells, it is elongated and stiff; on the other hand, by suddenly
expelling all the water, it gets small and pliable, and can now return
to its shell. This organ is represented in Fig. 128 (_Donax trunculus_,
Linn.), in which it is singularly developed. This bivalve is found on
the sea shore in shallow water; it buries itself almost perpendicularly
in the sands. They are so abundant on the French side of the Channel and
on the shores of the Mediterranean, that they form a considerable
portion of the people's food. These bivalves have the singular power of
leaping to a considerable height and then throwing themselves to a
distance of ten or twelve inches--a spectacle which may be witnessed any
day at low water. When abandoned by the retreating tide, they try to
regain the sea. If seized by the hand, in order to drag them out of the
sand, aided by their compressed, branched, and angular feet, they give
to their shell the sudden and energetic movement under which the
bounding action takes place.
[Illustration: Fig. 128. Donax trunculus (Linnæus).]
The shell of the Donax is slightly triangular and compressed; its length
exceeds its height; it is regular, univalve, unequally lateral, and its
hinge bears three or four teeth on each valve. The action of these feet
is very simple, and is compared by Réaumur to that of a man placed on
his belly, who, stretching out one hand, seizes upon some fixed object,
and draws himself towards it. There is just this difference, that the
movement of the member in the mollusc is altogether contractile.
Authors have described more than 30,000 species of molluscs, so that our
space only permits us to describe a few families, or rather types of
families.
The arrangement of bivalves now most generally adopted in England is
that of Woodward, as developed in the last edition of his manual of the
mollusca; it is greatly based on that of Lamarck. We have adopted his
arrangement altered from a descending to an ascending scale of
organisation.
The Conchifera are divisible into two sections, Siphonida, from the
animals having respiratory siphons, and Asiphonida, destitute of them.
The solen may be taken as a type of the first, and the oyster of the
second. The division Siphonida is divided into two sub-sections, those
without and those with a pallial line sinuated. The first family of this
section is the Pholadidæ, which includes Teredo, Xylophaga, and Pholas,
animals which possess extraordinary powers of boring; not merely as the
Solens do, through sand, but through the hardest rocks.
The _Teredos_ are marine animals having a special and irresistible
inclination for submerged wood; for while wood exposed to the air
becomes a prey to terrestrial animals, so submerged wood is subject to
invasion by aquatic animals, of which the Teredo is by far the most
formidable. The Teredos in the bosom of the ocean perforate the hardest
timbers, whatever be their essence. The galleries bored by these
imperceptible miners riddle the whole interior of a piece of wood,
destroying it entirely, without the slightest external indication of its
ravages. The galleries sometimes follow the grain of the wood; sometimes
they cut it at right angles; the miners, in fact, change their route the
moment they meet in their way either the furrows hollowed out by one of
their congeners, or some ancient and abandoned gallery. By a strange
kind of instinct, however multiplied may be their furrows or tubes in
the same piece of wood, they never mingle--there is never any
communication between them. The wood is thus attacked at a thousand
diverse points, until it is invaded and its entire substance destroyed.
It is by secret ravages of this kind that the piles and other submarine
constructions upon which bridges are built are often riddled and
perforated. They appear to all outward examination as solid and perfect
as at the moment they were first driven; but they yield to the least
effort, bringing ruin and destruction on the edifices they support.
Ships have been thus silently and secretly mined, until the planks
crumbled into dust under the feet of the sailors. Others have gone down
with their crews, entirely caused by the ravages of these relentless
enemies, which are terrible from their unapproachable littleness.
M. Quatrefages, who has minutely studied the organization and habits of
the Teredos in the Port of Saint Sebastian, reports the following fact,
which will give the reader some idea of the rapidity with which these
dangerous molluscs pursue their ravages:--
"A boat, which served as a passage-boat between two villages on the
coast, went down in consequence of an accident at the commencement of
spring. Four months after some fishermen, hoping to turn her materials
to advantage, raised the boat. But in that short space of time the
Teredos had committed such ravages that the planks and timbers were
riddled and worm-eaten so as to be totally useless."
At the beginning of the eighteenth century, half the coast of Holland
was threatened with annihilation because the piles which support its
dikes and sea-walls were attacked by the Teredo; and it proved no
contemptible foe. Many hundreds of thousands of pounds were expended in
order to avert the threatened danger. Fortunately, a closer attention to
the habits of the mollusc has brought a remedy to a most formidable
evil; the mollusc has an inveterate antipathy to rust, and timber
impregnated by the oxide of iron is safe from its ravages. This taste of
the Teredo being known, it is only necessary, in order to scatter this
dangerous host, to sink the timber which is to be submerged in a tank of
prepared oxide of iron--clothed, in short, in a thick cuirass of that
antipathy of the Teredo, iron rust. Ships' timbers are also served with
the same protecting coating; but the copper in which ships' bottoms are
usually sheathed serves the same purpose.
The singular Acephalous Mollusc known to naturalists as the _Teredo
navalis_, and popularly as the Ship Worm (Fig. 129), has the appearance
of a long worm without articulations. Between the valves of a little
shell, with which it is provided anteriorly, may be seen a sort of
smooth truncature, which surrounds a swelling projecting pad or cushion.
This cushion is the only part of the body of the animal which can be
regarded as a foot. Starting from this point, all the body of the Teredo
is enveloped by the shell and mantle, which form a sort of sheath
communicating by two syphons with the exterior.
The mantle adheres to the circumference of the shell. Above, it forms
two great folds, which may both be swollen by the afflux of the blood,
and acquire considerable size. One of these folds placed in advance,
which is called the _cephalic hood_, is worthy of attention. The tissue
of the mantle is of a greyish tint, very light, and transparent enough,
especially in the young, to permit of the mass of liver, the ovary, the
branchiæ, and the heart being distinguished in the interior, even to
counting its pulsations. The syphons are extensible, and attached the
one to the other for about two-thirds of their length, the upper part
being longer and thinner than the lower. It is by these tubes that the
aërated water enters which feeds and enables the animal to breathe. It
is discharged by the second tube, when deprived of its oxygen, and no
longer respirable, carrying with it the useless products of digestion.
This movement is continuous; but from time to time the animal shuts at
once the orifices of both tubes, and slightly contracts itself.
The shell, seen on the side, presents an irregularly triangular form; it
is nearly as broad as it is long; its two valves are solidly attached
the one to the other, above and below, by the mantle in such a manner as
only to permit of very slight movements. It is coloured in yellow and
brown lines; sometimes it is quite plain. On the upper edge of the
anterior truncature of the body of the animal is the mouth, a sort of
funnel, flat and slightly bell-shaped, furnished with four labial palpi,
a stomach without any peculiar feature, and a well-developed intestine.
[Illustration: Fig. 129. Teredo navalis (Linnæus).]
The heart consists of two auricles and a ventricle, which beat at very
irregular intervals, four or five in the minute. The blood is
colourless, transparent, and charged with small irregular corpuscles.
The act of breathing is accomplished in the branchiæ, or gills, and
mantle. Nevertheless, the one half of the blood returns to the heart
without passing through these branchiæ.
The nervous system is well developed, and consists of a brain, nervous
filaments, and of ganglions, which are distributed in the mantle, the
branchiæ, and the syphon tubes.
The adult animal is surrounded by a sort of sheath, consisting of a
solid mucus, which has sometimes been described erroneously as forming
part of the animal. The Teredo, shut up in this tube, is limited in its
movements; when observed in a vase, its motions are slow and
deliberate--movements of extension and contraction, by the aid of which
it contrives with difficulty to change its place; but nothing indicates
a true creeping movement. In a state of nature, according to M.
Quatrefages, the body of the animal is stretched out to three times its
length without diminishing in any respect its proportional thickness;
the afflux of water penetrating under the mantle, and of the blood which
accumulates in the interior vessels, sufficiently accounting for a
phenomenon which at the first glance is very singular.
The Teredo deposits a spherical greenish-yellow egg. Shortly after
fecundation, these eggs are transformed into larvæ. At first naked and
motionless, these larvæ are soon covered with vibratile cilia, when they
begin to move, at first by a revolving pirouette, afterwards swimming
about freely in the water. When one of these larvæ has found a piece of
submerged wood, without which it probably could not live, the curious
spectacle is observed of a being which fabricates, step by step, and as
it requires them, the organs necessary for the performance of its
functions. It begins by creeping along the surface of the wood by means
of the very long feet with which it is furnished. Then it is observed
from time to time to open and shut the valves of the little embryo shell
which partly envelopes it. As soon as it has found a part of the wood
sufficiently soft and porous for its purpose, it pauses, attacks the
ligneous substance, and soon produces a little pore, or cell, which will
be the entrance to the future canal.
Once fairly lodged in this little cell, the young Teredo is rapidly
developed; it covers itself with a coating of mucous matter, which,
condensing by degrees, assumes a brownish tint, forming a solid
covering, with two small holes for the passage of the syphon tubes. At
the end of three days this covering has become quite solid; it is the
commencement of the organized tube, in which the animal is to be
developed. When secured beneath this opaque screen, the little miner is
no longer exposed to observation; but if his cell is opened at the end
of a few days, it is found that it has secreted a new shell, larger and
more solid than the original one; it is the shell of the adult animal.
The young Teredo, which feeds on the raspings of the wood, increases
rapidly; it passes first from a spheroid form to an elongated shape, and
when its body can no longer be contained in the shell, it projects
beyond the edge, and would find itself naked were it not protected by
its membranous sheath, which adheres to the walls of the ligneous
channel, now the dwelling-place of the animal.
The process by which a creature soft and naked like the Teredo should
break into a solid piece of the hardest wood so quickly, and destroy it
with so much facility, was long a mystery. Until very recently, the
shell was looked on as the implement of perforation. But in that case
the shell should preserve certain traces of its action upon surfaces so
resistant as oak and fir; but the shell, on the contrary, is perfect,
with no signs of friction. On the other hand, the muscular apparatus of
the Teredo is not calculated to put the shell into rotatory action, were
the process a boring one. It does not seem therefore possible to
attribute these perforations to a simple physical action.
[Illustration: Fig. 130. Pholas dactylus having hollowed out a shelter
in a block of gneiss.]
Some naturalists have suggested, in explanation of this phenomenon, that
the animal is furnished with the means of secreting a liquid capable of
dissolving the woody fibre. This has been met by the statement that, in
whatever way the wood is attacked, whether the gallery is excavated with
or across the fibre of the wood, the groove is as exactly and neatly cut
as if it had been perforated by the sharpest tool, and that a corroding
dissolvent could not act with this regularity, but would attack the
harder and more tender parts unequally. This objection, which M.
Quatrefages opposes to the idea of a chemical solvent, appears to us to
admit of no reply. But, while opposing unassailable reasons against two
theories, the learned author does not leave us without a very reasonable
explanation of a very puzzling phenomenon. "Let us not forget," he says,
"that the interior of the gallery is constantly saturated with water;
consequently all the points of the walls which are not protected by the
tube are subjected to constant maceration. In this state a mechanical
action, even very inconsiderable, would suffice to clear away the bed of
fibre thus softened, and, if this action is in any degree continuous, it
suffices to explain the excavation of the galleries, however extensive
their ramifications. Again, the upper cutaneous folds, especially the
cephalic hood already mentioned, having the power of expanding at will
by an afflux of blood, covered with a thick coriaceous epidermis, and
moved by four strong muscles, seems to me very capable of performing the
operation. It appears very probable that it is this hood which is
charged with the removal of the woody fibre, rendering it incapable of
resistance by previous maceration, which may also be assisted by some
secretion from the animal." That the fleshy parts of the mollusc, acting
upon the surface, softened by long maceration in water, is the true
boring implement employed by the Teredo, is, probably, the only
explanation the case admits of; at all events, in the present state of
our knowledge, the explanation of the learned naturalist is the most
reasonable which can be given.
The engraving (Fig. 130) represents _P. dactylus_, which has hollowed
itself a home out of a block of gneiss. This dwelling is a cell just
deep enough to contain the animal and its shell, as represented in Fig.
131. To excavate its cell at the bottom of one of these gloomy retreats
seems to be all that the animal lives for. To ascend to the summit or
sink to the bottom of their narrow house makes up all the accidents of
existence to these strange creatures: the hole they dig is at once their
dwelling and their grave; which is attested both by the rocks of the
past and the present.
In its structure the shell differs notably from other Acephalous
Molluscs, which led Linnæus to place it with the multivalve shells.
Between the two ordinary valves, in short, this shell presents certain
accessary pieces, smaller than the true valves, and placed near the
hinge, as represented in _Pholas dactylus_ (Fig. 131), pieces which
would not be there without a purpose.
[Illustration: Fig. 131. Pholas dactylus (Linnæus).]
The shell is equivalve, gaping on each side, swelling below, very thin,
transparent, and white. The animal is a thick, white, elongated, fleshy
body; its mouth opening anteriorly, throws out a long tube traversed by
two canals or syphons, through one of which the water necessary for the
respiration of the animal is absorbed, and ejected through the other.
Through another opening in the mantle a very thick and short foot is
protruded.
There are three ways of accounting for this creature's method of
boring--the mechanical, the chemical, and the electric; the first being
the one generally held. In this case the animal uses its foot as a
boring tool. The second presumes on the Pholas secreting an acid which
corrodes the rock; the third that it possesses a galvanic battery with
similar powers. It is possible that all these three theories may have a
measure of truth. That the foot of the borer is used is clear. The
luminosity which is so characteristic of the animal is in favour of an
electric current, which is almost always accompanied by chemical
decomposition, which would set free the hydrochloric acid of the sea
water. The small size of the entrance to the chambers of the Pholas is
accounted for by the increase of its size during its residence there. De
Blainville thought that a simple movement of the shell incessantly
repeated would suffice to pierce the stone, macerated by the sea water
which passed through the breathing apparatus.
Mr. Robertson, of Brighton, exhibited the living Pholas in the act of
boring through masses of chalk, and thinks the process entirely effected
by the simple mechanical action of the "hydraulic apparatus, rasp, and
syringe."
"If you examine these living shells," says Gosse, "you will see that the
fore part, where the foot protrudes, is set with stony points arranged
in transverse and longitudinal rows, the former being the result of
elevated ridges, radiating from the hinge, the latter that of the edges
of successive growths of the shell. These points have the most accurate
resemblance to those set on a steel rasp in a blacksmith's shop. It is
interesting to know that the shell is preserved from being itself
prematurely worn away by the fact that it is composed of aragonite, a
substance much harder than those rocks in which the Pholas burrows. The
animal," Gosse adds, "turns in its burrow from side to side when at
work, adhering to the interior by the foot, and therefore only partially
rotating to and fro. The substance is abraded in the form of a fine
powder, which is gradually ejected from the mouth of the hole by
contraction of the bronchial syphon."
The Pholades are met with on every sea shore, and are plentiful in the
Channel; on the French coast they are called _Dails_, and sought for
their fine flavour. As examples of the genus, we may quote _Pholas
dadylus_ (Fig. 131); _Pholas candida_, found in the Channel and in the
Atlantic Ocean, which lives buried in the mud or in decayed wood;
_Pholas crispata_ (Fig. 132), also found in the Channel; _Pholas
papyracea_ (Fig. 133); and _Pholas melanoura_ (Fig. 134).
The bodies of many genera of Mollusca have the property of shining in
the dark, but none emit a light more brilliant than that of the
Pholades. Those who eat the Pholades in an uncooked state (which is by
no means rare, for the flavour of the mollusc does not require the aid
of cooking to render it palatable) would appear in the dark as if they
had swallowed phosphorus; and the fisherman who, in a spirit of economy,
supped on this mollusc in the dark, would give to his little ones the
spectacle of a fire-eater on a small scale.
[Illustration: Fig. 132. Pholas crispata (Linnæus).]
The perforations produced in stone by the Pholades have become important
evidence in a geological sense. In many countries there were evident
signs of a considerable sinking of the earth. But in no place is the
evidence of this so clear as in the monument of high antiquity on the
Pozzuolan coast, known as the Temple of Serapis.
[Illustration: Fig. 133. Pholas papyracea (Solander).]
[Illustration: Fig. 134. Pholas melanoura (Sowerby).]
In speaking of the culture of oysters by the Romans, we shall have
occasion to mention the disappearance of the Lucrin Lake, and its
replacement by an enormous mountain, the Monte Nuovo. Now, Pozzuolo is
situated at the foot of Monte Nuovo. We need not add that the whole
neighbourhood is volcanic. Pozzuolo touches on the Solfaterra, on the
Lake Avernus, and is not far from Vesuvius; and in the bay is the
monument of other days, erroneously called the Temple of Serapis. In
reality it was most probably a thermal establishment, established for
its mineral waters, although the world has agreed to call it a temple.
However that may be, the building has been nearly levelled by the hand
of time, aided by the hand of man; and the ruins now consist of three
magnificent marble columns of about forty feet high. But the curious and
important fact is, that these three columns, at about ten feet above the
surface, are riddled with holes, and full of cavities bored deeply into
the marble, and these borings occupy the space of three feet on each
column. The cause of these perforations is no longer doubtful. In some
of the cavities the shell of the operator is still found, and it seems
settled among naturalists that it belongs to a species of Pholas,
although M. Pouchet, a naturalist of Rouen, denies this. "As far," he
says, "as I have been able to judge from the fragment which I extracted
from this temple, which is destitute of the hinge, it is infinitely more
probable that this mollusc is a species of the genus _Corallisphaga_."
In spite, however, of M. Pouchet's scepticism, the mass of evidence is
opposed to his theory.
There are two modes of explaining the fact to which we have called
attention. To enable the stone-boring molluscs which live only in the
sea to excavate this marble, the temple and columns must have been
buried several fathoms deep in sea-water. It is only in these conditions
that the borers could have made an incision, and laboured at their ease,
in the marble column.
But since the same traces of perforation are now visible ten feet above
the surface, it follows that, after being long immersed under water, the
columns have been elevated to their present position. The temple has
been restored to its primitive state, carrying with it, engraved in
marble, ineffaceable proofs of its immersion. Sir Charles Lyell has
consecrated a long chapter to the successive sinking and elevation of
this temple, which proves the fact most conclusively.
* * * * *
Family two, the Gastrochænidæ, is a somewhat heterogeneous one, as it
contains Saxicava and Aspergillum. We have only space for a short
account of the latter, the animal which has received the strange name
of the Watering Pot, and is represented in Fig. 135. It inhabits a
calcareous tube, thick, solid, of considerable length, and nearly
cylindrical, presenting at one extremity an opening fringed with one or
many foliaceous folds in the form of frills, and at the other extremity
a convex disk, pierced with holes like a watering-pot: whence its name.
The animal is attached by certain muscles to the interior of the tube.
Chenu, to whom we are indebted for our information respecting this
curious mollusc, tells us "that the animal which inhabits this curious
shell was first described by Russell, whose account of it is deficient
in the anatomical details, which might explain the utility of the holes
in the disk of the central fissure, and of the spiriform tubes found
there." We suppose that this arrangement is necessary in order to
facilitate respiration; and M. De Blainville thinks the small tubes are
intended for the passage of the fillets which are necessary to fix the
animal to the body on which it is to live, and in such a manner as to
admit of its movements round a fixed point.
[Illustration: Plate XX.--Temple of Serapis at Pozzuolo.]
[Illustration: Fig. 135. Aspergillum vaginiferum (Lamarck).]
The animal which inhabits the _Aspergillum_ is elongated, contractile,
and only occupies the upper part of the tube, but it can stretch itself
out sufficiently for all its wants. Shells of this genus are very rare,
although a great number of species are known. They are found in the Red
Sea, and in the seas of Australia and Java. The shells are generally of
a white or yellowish tint; some have the tube covered with a glutinated
sand, mixed with small fragments of shells of diverse colours. We know
nothing of their habits, and their singular forms have left naturalists
in doubt as to the place which should be assigned to them in the method
of arrangement. It is only after having recognized the existence of two
valves, which was detected with great difficulty just under the disk,
and forming part of the sheath in which the animal is encased, that it
has been decided to range them with the _Tubicola_, and with the shells
presenting an arrangement analogous or equally singular. These molluscs
are, as M. Chenu says, little known, rare, and hence much sought for by
collectors. They are exclusively exotic, the most common species being
from Java. It is imported into Europe by the Dutch. Our third family,
the Anatinidæ, includes Myochama, Pandora, Lyonsia, Myacites,
Pholadomya, Thracia, and Anatina, genera which were more important in
the former than in the present seas; some, in fact, being wholly
extinct, or represented, as in Pholadomya, by one or two living species.
Our fourth family, the Myacidæ, including Gycimeris, is found only in
America; Panopaæ, now principally extinct; Thetis, Neæra, Corbula, and
Mya, or Gaper.
Our fifth family, Solenidæ, contains the Solens, which under the name of
"razor-fish" are so abundant on the sandy shores of all parts of the
globe. These molluscs live buried vertically in the sand, a short
distance from the shore; the hole which they have hollowed, and which
they never quit, sometimes attains as much as two yards in depth; by
means of their foot, which is large, conical, swollen in the middle, and
pointed at its extremity, they raise themselves with great agility to
the entrance of their hole. They bury themselves rapidly, and disappear
on the slightest approach of danger.
When the sea retires, the presence of the Solen is indicated by a small
orifice in the sand, whence escape at intervals bubbles of air. In order
to attract them to the surface, the fishermen throw into the hole a
pinch of salt; immediately the sand becomes stirred, and the animal
presents itself just above the point of its shell. It must be seized at
once, for it disappears again very quickly, and no renewed efforts will
bring it to the surface a second time. Its retreat is commonly cut short
by a knife being passed below it; for it burrows into the ground with
such velocity that it is difficult to capture it with the hands alone.
This shell has by some been compared to a knife-handle; by others to a
razor, which has become its popular name. It is a thin, transparent,
long, and slender equivalved bivalve, with parallel edges, gaping and
truncated at both extremities. The tints are rose-coloured, bluish-grey,
and violet; the valves slightly covered with an epidermis of a greenish
brown.
The animal which lives in this elegant dwelling has the form of an
elongated cylinder. Its mantle is closed in its whole length, and only
open at the ends at one side for the passage of the food, and at the
other for the passage of a tube formed of two syphons united together.
This curious shell, various species of which are represented in PL.
XIX., are known as razor-fish, sabre-fish, and other names, which in
some respects indicate the peculiar form of the shell, as well as its
sharpness.
[Illustration: PLATE XIX.--Razor-fish. Solenidæ.
I. Solen siliqua. (Linn.)
II. Solen vagina. (Linn.)
III. Solen ensis. (Linn.)
IV. Solen ensis major. (Lamarck.)
V. Solen ambiguus. (Lamarck.)
VI. Solen legumen.
]
The Tellinidæ, the sixth family in our table, is very important, as
including a vast number of genera and species, of which, as types, we
will particularise Tellina and Donax; but Galatea, Mesodesma, Semele,
Sanguinolaria, Psammobia, and Capsula, are important genera.
Along the shores of the Channel and in the Mediterranean there are few
bivalves more abundant than the several species of the genus _Donax_.
They live near the shore in shallow water, burying themselves
perpendicularly in the sand. They have the very singular habit,
considering their apparent helplessness, of being able to leap to a
certain height and then project themselves ten or twelve inches. This
may often be witnessed in the case of individuals left by the retreating
tide. If seized by the hand, and attempts are made to disengage them
from the sand, they continue to impress on their shell a sudden and
energetic movement, aided by the elasticity of their foot, which is at
once decisive and angular.
[Illustration: Fig. 136. Donax rugosus (Linnæus).]
[Illustration: Fig. 137. Donax denticulatus.]
The shell of the _Donax_ is nearly triangular in shape, compressed,
longer than it is high, regular, equivalve, not equilateral; the hinge
with three or four teeth on each valve.
The animal is slightly compressed, and more or less triangular. Its
mantle, which forms two symmetrical lobes enveloping the body, is open
pretty nearly in all its extent, but it is united posteriorly, and
terminates in two syphons or nearly equal tubes, as in Fig. 130, p. 326.
One of these tubes serves the purpose of respiration: it is the
_bronchial syphon_. The other, serving the purpose of ejecting the
products of digestion, is termed the _anal_ tube. The tentacles of the
bronchial tube seem to be possessed of exquisite sensibility. When
touched, the animal draws in its syphon, and only puts it forth anew
when the danger has passed. The species of _Donax_ are very numerous,
especially in the Asiatic and American seas. Among the European species
we may mention _Donax rugosus_ (Fig. 136) and _Donax denticulatus_ (Fig.
137).
[Illustration: Fig. 138. Tellina radiata (Linnæus).]
[Illustration: Fig. 139. Tellina virgata (Linnæus).]
[Illustration: Fig. 140. Tellina sulphurea (Lamarck).]
[Illustration: Fig. 141. Tellina donacina (Linnæus).]
Next to _Donax_ naturalists rank the genus _Tellina_, which includes
many species of very minute shells, all remarkable for their beauty of
form, and for their brilliant and varied colours. One of these, called
the Rising Sun (_Tellina radiata_), is represented in Fig. 138. The
Tellinas are found in every sea; the French coast furnishes many
species: examples, _Tellina virgata_ (Fig. 139) and _Tellina sulphurea_
(Lamarck) (Fig. 140). In Fig. 141 _Tellina donacina_ is represented with
its two vital tubes, or syphons.
The seventh family, or Mactridæ, include Lutraria and Mactra, or the
otter and kneading-trough shells. They are widely distributed; there are
several British species of both.
The eighth family, Veneridæ, includes Venus, Cytherea, Meroe, and
Artemis; beautiful genera, and as such called by Linnæus and his
followers after heroines of Greek mythology. Petricola, Venerupis,
Tapes, Lucinopsis, and Trigona, also belong to the family. These
acephalæ of size so small, like their congeners, inhabit every sea; they
are found in every region of the globe, more than a hundred and fifty
species being known. The shell is elliptic in form, the valves smooth,
striated, spiny, and lamellous, like those of Cardium and Donax. Like
these, they bury themselves in the sand.
[Illustration: Fig. 142. Venus verrucosa (Linnæus).]
[Illustration: Fig. 143. Cytherea geographica (Chemnitz).]
Among the vast number of species, many of them are extremely rare, and
much sought after by collectors in consequence of their great beauty. In
the principal ports of France, _Venus verrucosa_ (Fig. 142), and another
species known in the south of France under the name of Clovisse, are
eaten there like oysters. Prepared with fine herbs, the Clovisse, we
have M. Figuier's authority for saying, is not to be despised. "We may
be believed also," he says, "if we add that nothing is more delicious
than to eat the living Clovisse torn from the rock of the Phara of Lake
Thau, when the Mediterranean sun of a day in winter is shining down upon
us, the heart rejoicing in manhood's strength." In PL. XVIII. some of
the principal species are represented, along with some of the more
remarkable species of _Cytherea_. In Fig. 143 we have the elegantly
pencilled shell of _Cytherea geographica_, together with the animal in
its natural connection.
The sub-section we shall now treat of is without the pallial line
sinuated. The Cyprinidæ form the ninth family of our arrangement of the
Conchifera, and contain, Cardia, Cypricardia, Isocardia, Crassatella,
Astarte, Circe, and Cyprina, which amount together to some hundred
species.
The Cycladidæ are our tenth family, and include Cyrenoides, Cyrena,
Pisidium, and Cyclas.
The Lucinidæ is the eleventh family, containing Galeomma, Lepton,
Montacuta, Kelia, Diplodonta, Corbis, and Lucina.
In the small family of which we have made the Tridacna the
representative, as well as in some preceding families, the mantle of the
animal is more or less largely open, but never with such a prolongation
as to form tubes. In the _Cardiums_, now under consideration, as well as
_Donax_, _Tellina_, and _Venus_, the respiratory organs are somewhat
modified, so as to adapt them to the habits of the animal. All these
molluscs live buried in the mud or sand, and two great tubes issuing
from the interior of their bodies bring the atmospheric air into
communication with their respiratory organ--namely, the _branchial
leaves_.
The twelfth family, Cardiadæ, contains the familiar
cockles--Cardium--which is derived from καρδὶα, _a heart_, which
they are supposed to resemble in form, are amongst the most
widely-distributed of shells. The shell is convex, as we see in _C.
hians_ (Fig. 144), somewhat heart-shaped, equivalved, the edges dentate
or corrugated, the hinge furnished with four teeth upon each valve. The
accessary ornaments vary with the species, some being smooth, as in
_Cardium Greenlandicum_, Chemnitz (Fig. 145); others, and by far the
greater number, are furnished with regular sides, generally obtuse,
sometimes in ridges diverging from the point and armed with straight or
curved spines, arranged in the oddest manner, as in _Cardium aculeatum_
(Fig. 146).
[Illustration: PLATE XVIII.--Venus and Cytherea.
I. Venus plicata. (Gmel.)
II. Venus puerpera. (Linn.)
III. Venus reticulata. (Linn.)
IV. Venus Gnidia. (Broderip.)
V. Cytherea zonaria. (Lamarck.)
VI. Cytherea petechialis. (Lamarck.)
VII. Cytherea maculata. (Linn.)
]
In _C. hians_ (Fig. 144), the mantle has a large opening in front,
fringed anteriorly with papillæ in the form of tentacula; the
inhabitant of the shell has a very large foot, with a bend or knee
near the middle; its mouth is transverse and funnel-shaped, and
furnished with a triangular appendage. One of the peculiarities in the
organization of these molluscs is its direct connection with their mode
of life. In short, these molluscs, which most commonly live on the sea
shore, and bury themselves in the sand to the depth of four or five
inches, are enabled to breathe, to draw water for their nourishment, and
also to throw off the products of digestion, by having the mantle
prolonged into two tubes, the orifices of which reach to the surface of
the soil. By means of these feet and an extremely curious organ of
locomotion, the Cardiums can at will issue from their holes and re-enter
them. The fishermen of the shore easily recognize the presence of these
animals by the little jets of water which they throw up through the
sands.
[Illustration: Fig. 144. Cardium hians (Brocchi).]
[Illustration: Fig. 145. Cardium Greenlandicum (Chemnitz).]
[Illustration: Fig. 146. Cardium aculeatum (Linnæus).]
[Illustration: Fig. 147. Cardium edulis (Linnæus).]
These molluscs are found in every sea on the globe, and under all
latitudes. Many of them belong to our own and the French coasts, where
they are eagerly sought for by collectors, as well as for food. The
flesh of the animal, however, is coriaceous, and little esteemed. The
species most common on the littoral of the Atlantic is _Cardium edulis_
(Fig. 147), its white or fawn-coloured shell being hollowed out into six
and twenty furrows, forming so many corrugated ripples on its side.
[Illustration: Fig. 148. Cardium costatum (Linnæus).]
_Cardium costatum_ (Fig. 148) is an exotic species which inhabits the
coast of Guinea and the Senegal, the shell of which, white and fragile,
is much sought after by collectors.
The thirteenth family of our table, Tridacnidæ, consists of only eight
or ten species, but it contains the largest of all, the giant Tridacna.
The historian of the wars of Alexander the Great speaks of oysters
inhabiting the Indian Ocean which were more than a foot long; these were
probably _Tridacna_, the shells of which were most likely to be seen by
the Macedonian conquerors. The valves of _Tridacna gigas_ are sometimes
found a yard and a half in length, and weighing five hundred pounds.
Magnificent examples may be seen in the church of Saint Sulpice, Paris,
where they hold the holy water. These beautiful shells were the gift of
the Venetian Republic to Francis I. Under Louis XIV., the curé Languet
had them presented to the church of Saint Sulpice, where they are used
as fonts for holy water. Another pair are exhibited in the church of
Saint Eulala, at Montpelier, but much smaller in size. The shells of
_Tridacna_ are formed, as represented in PL. XVII., of three acute
angles, festooned on their edges by broad sides bristling with white
scales. The hinges have two teeth; the ligament is elongated and
external.
[Illustration: Plate XVII.--Tridacna gigantea, Holy Water Basin in the
Church of Saint Sulpice at Paris.]
The animal of _Tridacna_ is remarkable for its fine colours. _Tridacna
safrana_ is of a beautiful blue round the edges, rayed through a shade
of very pale blue. More in the interior is a row of small moons of a
yellowish green; the centre is a bright violet, with brownish
longitudinal punctured lines. "We have at this moment before our eyes,"
say the travellers Quoy and Gaimard, "one of the most charming
spectacles that can be seen, when at a little depth beneath the surface
a number of these animals display the brilliant velvety colours and
varying shades of their submarine parterres. As we can only perceive the
gaping opening of the valves, we may imagine to ourselves what is its
first aspect." The mantle of the animal is closed and ample; its edges
are swollen, and reunited in nearly its whole circumference in such a
manner as to leave only three very small openings--two in the upper
part; the one serves the purpose of discharging the products of
digestion, the other gives entrance and exit to the water necessary for
respiratory purposes. The third opening is in the lower part of the
body, and free; it leaves an opening for the passage of the foot, which
is enormous, and is surrounded with an ample tuft of byssoidal fibres.
[Illustration: Fig. 149. Tridacna squamosa (Lamarck).]
[Illustration: Fig. 150. Tridacna squamosa, on the inside (Lamarck).]
Aided by this silky tuft, the animal attaches itself to the rocks, and
suspends its weighty shell from them. If it is intended to remove those
attached to the sides of the rock, it is necessary to cut the cords of
the tendonous byssus, by which it is held suspended, with a hatchet.
All the species are inhabitants of the Tropical seas. The _Tridacna_
_gigas_ is a native of the Indian Ocean. The flesh, though coriaceous,
and by no means of an agreeable flavour, is a great resource to the poor
Indians. The accompanying representations of _Tridacna squamosa_ (Figs.
149 and 150) will convey a general idea of the genus.
Our fourteenth family, Hippuritidæ, is entirely fossil; but the
fifteenth, Chamidæ, of which the best example is the rugose genus
Charina, is widely distributed in tropical seas.
The very numerous division of shells called Asiphonidæ, possesses
animals without respiratory siphons. The shells we shall now describe
belong to the sixteenth family, Unionidæ, which contain Iridina, Anodon,
and Unio.
The pond mussels, _Anodon_, are found in lakes, rivers, and seas of
almost every region of the globe. Their shells are rounded or oval,
generally very thin, regular, and equivalve, not gaping, the hinges
without teeth, whence their name, from the Greek, ὀδότοϛ, _without
teeth_. These shells are nacred inside, and generally smooth.
The _Anodon cygnea_ (Fig. III., PL. XVI.) is broad, deep, and light, and
is sometimes employed for skimming the cream off milk. The genus is
divided into many groups, the principal forms of which are represented
in PL. XVI.
The river mussels, _Unio_, are, like the Anodon, found in the muddy
bottoms of all countries. The animal resembles the Anodon, but the shell
presents a toothed hinge. The lower face of the valve is nacrous, but
shaded with purplish violet, copreous, and iridescent; the anterior face
is of a green colour, which varies from tender to blackish green.
[Illustration: Fig. 151. Unio littoralis (Cuvier).]
Among the species found in European seas may be noted the Rhine mussel,
a large species, the nacre of which is employed for ornamental purposes.
_Unio littoralis_ (Cuvier), represented in Fig. 151, and the painter's
mussel, _Unio_ _pictorum_ (Fig. 152), employed in the arts to contain
certain colours. Those known as the river mussels are leathery, of an
insipid taste, and scarcely eatable: the finest species are found in the
great American rivers.
[Illustration: PLATE XVI.--Anodonta.
I. Anodonta angulata. (Lea.)
II. Anodonta ensiformis. (Spix.)
III. Anodonta cygnea. (Linn.)
IV. Anodonta magnifica. (Lea.)
V. Anodonta anserina. (Spix.)
VI. Anodonta latomarginata. (Lea.)
]
[Illustration: Fig. 152. Unio pictorum (Linnæus).]
Mussels, as we have seen, produce pearls of moderate value. Linnæus, who
was aware of the origin of the Pintadine pearls, and of pearls in
general, was also aware of the possibility of producing them
artificially from various molluscs. He suggested bringing together a
number of mussels, piercing holes in their shells with an augur in order
to produce a wound, and afterwards leave them for five or six years, to
give the pearl time to form. The Swedish Government consented to try the
experiment, and long did so in secret; pearls were produced, but they
were of no value, and the enterprise was abandoned as unsuccessful.
Scottish pearls were much celebrated in the middle ages, and between the
years 1761 and 1784 pearls to the value of £10,000 were sent to London
from the rivers Tay and Isla; "and the trade carried on in the
corresponding years in the present century," says Mr. Bertram, "is far
more than double that amount." The pearl, according to Mr. Bertram, is
found in a variety of the mussel, which is characterised by the valves
being united by a broad hinge, and having a strong fibrous byssus, with
which it attaches itself to other shells, to rocks, and other solid
substances. "The pearl fisheries of Scotland," he adds, "may become a
source of wealth to the people living on the large rivers, if prudently
conducted." Mr. Unger, a dealer in gems in Edinburgh, having discerned
the capabilities of the Scotch pearl as a gem of value, has established
a scale of prices which he gives for them, according to their size and
quality; and it is now a fact that the beautiful pearls of our Scottish
streams are admired beyond the orient pearl. Empresses and queens, and
royal and noble ladies, have made large purchases of these gems; and Mr.
Unger estimates the sum paid to pearl-finders in the summer of 1864 at
£10,000. The localities successfully fished have been the classic Doon,
the Forth, the Tay, the Don, the Spey, the Isla, and most of the
Highland rivers of note. Scottish pearls are much whiter in colour than
oriental. What tint they have is bluish, while those of the East are
yellowish. Pink pearls are produced by several exotic species of Unio.
Our seventeenth family are the Trigoniadæ, affording Trigonia, of which
so many occurred in the Jurassic period of Geologic History in the
strata of Europe, but of which two or three are alone left alive in the
seas of Australia.
The eighteenth family, the Arcadæ, affords between 200 and 300 species
of the families of Leda, Nucula, Pectunculus, and Arca.
Of the eighteenth family, Arcadæ, we shall only at present instance
Pectunculus.
[Illustration: Fig. 153. Pectunculus aureflua (Reeve).]
[Illustration: Fig. 154. Pectunculus delessertii (Reeve).]
The genus _Pectunculus_ are abundant on the shores of the Mediterranean
and along the Atlantic coast. If we take up at hazard a handful of
shells on any part of the French coast, one-third will consist of
_Pectunculus_. They are found mixed with Cardium, Venus, Razor-fish, and
Pectens. Their round and robust frame attracts much attention. They form
the first of those charming infantile collections which are gathered at
the mother's feet.
The animal which inhabits this pretty shell is moulded on its curvature;
like the shell, it is round and squat; it is furnished with a mouth,
large, and thick for its size, and with double branchiæ. When the animal
is taken alive, it sometimes exudes a thick mucous liquid over the
shell, which has disgusted many a young collector with his capture.
Among numerous species of _Pectunculus_ we note as worthy of
representation: _P. aureflua_, Reeve (Fig. 153); _P. delessertii_, Reeve
(Fig. 154); _P. pectiniformis_, Lamarck (Fig. 155); and _P. scriptus_,
Born (Fig. 156).
[Illustration: Fig. 155. Pectunculus pectiniformis (Lamarck).]
[Illustration: Fig. 156. Pectunculus scriptus (Born).]
FOOTNOTES:
[Footnote 8: The term bivalve as constituting a class must be taken in a
limited sense, for several genera, as pholas for example, have six or
more valves.]