“SHELL-FISH.”
One of the greatest hindrances to the unscientific, in the way of a
proper understanding of the true nature and relative position of many
forms of life, is to be found in our misuse of words--our poverty of
language, which compels us to make one word serve for quite dissimilar
and unrelated things. This unfortunate term, “Shell-fish,” which we
have felt compelled to put at the head of this chapter, in place of
the more accurate “Bivalve Mollusks,” is a case in point. I really
want a name that only includes these; but in order to be strictly
popular in my chapter-heads, I must use this very general term. Just
now I turned to a popular and portable dictionary to see what was a
familiar definition of the compound, and I read there, “Shell-fish,
testaceous mollusks,” but even for a popular explanation that does not
go far enough, for Shell-fish also includes crabs and lobsters, which
are not mollusks, but crustaceans. I daresay, too, that in a fishery
suit, if it served their purpose, lawyers would show plainly that it
embraced tortoises and turtles, which are chelonian reptiles. We are
all aware that in popular and legal language everything that comes out
of the sea is a fish, excepting the coral-polyp which everybody, except
naturalists, knows is an insect!
What I really wish to make clear, after this little growl, is that the
present chapter will deal only with such creatures as are (like oysters
and cockles) sandwiched or boxed between two valves or half-shells, and
will not even glance at those mollusks that are contented with a shell
all in one piece; these are relegated to the next chapter.
[Illustration:
SPINY COCKLE. BANDED VENUS.
]
The Mollusca that actually live between tide-marks, though numerous as
individuals, do not represent many species; but those of which we may
find the recently-vacated shells, thrown up by the tide from greater
depths, will total up to a considerable number. The bivalves must be
sought for on sandy beaches and mud flats, especially at the mouths of
rivers. Most of them are burrowers, excavating a way by means of the
powerful foot with which they are provided. This instrument is well
seen in the Razor-shells (_Solen_), or the Cockles (_Cardium_), where
it reaches extraordinary development. Even where the animal lives far
beyond our limits in deep water we can, by a little thought, get some
notion of their habits by examining the empty shells that are cast
up within the littoral zone by heavy seas. Those that are fresh and
clean externally, though without any signs of wear from long washing
among the shingle, may be safely regarded as burrowers that habitually
lie beneath the sand or mud. These, too, will be found to have both
_valves_ of the shell almost, if not quite, equal in size and shape;
whilst those which, like the Oyster and the Scallop, lie upon the
sea-bottom, have very unequal valves, the under one being deeper and
concave, whilst the upper valve is flat and more brightly coloured, to
harmonise with its surroundings. Often, too, this exposed upper valve
will be crusted with acorn-shells, _Serpulæ_, _Sertulariæ_, or seaweeds.
It may prevent confusion further on if we now say a few words by way
of defining the parts of a bivalve shell, its latitude and longitude,
and its relation to the animal whose vital activities produced the
valves. The Spiny Cockle, or Red Nose (_Cardium aculeatum_) of our
illustration, is on its back. If we were to take it, or any other
bivalve shell, and turn it the other way, so that the hinge connecting
the two valves was uppermost, we should have it in the natural position.
A bivalve mollusk is an inferior creature to a limpet or a winkle,
because these have heads with eyes, but the bivalve has not. In the
larval condition it has eyes, but by a retrograde movement like that of
the cirripedes, it gets rid of these as useless in the life it is to
live henceforth. But in spite of its want of a head, we know which is
its anterior and its posterior end, its dorsal and its ventral surface;
and with our knowledge of the relation of animal and shell, we are not
troubled to open the valve to look at the creature, when we wish to
describe the parts of a shell. It will be noticed that each valve curls
over near the hinge and takes a form not greatly unlike a beak. This
is more strongly marked in some species than in others; anyhow, it is
popularly known as the beak, though it is technically distinguished as
the _umbone_, or boss.
If these beaks have the slightest tendency to either end of the shell,
it will be to the front, where we should expect the creature’s head to
be, if it had one. This point made clear, by reference to the shell we
have just picked up, we can say which is the right and which the left
valve. The valves are hinged by a band of a substance that looks much
like catgut. It is elastic in character, and is always pulling at both
valves, so that the natural tendency of the shell is to gape open. But
inside the shell there are, in most bivalves, two much more powerful
bands of muscular fibres (the oyster has but one), which, by their
tension, can slowly or suddenly bring the edges of both valves closely
and tightly together, and hold them so for an indefinite period. You
can see the marks where these muscles were attached, one at each end of
the valve. Between these two marks (“muscular impressions”) there runs
a colourless line marking the area to which the _mantle_ was attached
(“pallial impression”), but this line is often interrupted, towards the
hinder end of the shell, by a bay or sinus (the “pallial sinus”).
The mantle is a delicate membrane on each side of the mollusk’s body,
which has the power of forming the shell, to which it is attached save
at the margins. The “pallial sinus” is caused by the syphons which
are protruded at that end of the shell. At the other end, as shown
in the figure of the Banded Venus, is the “foot.” The “syphons” are
two delicate tubes, and if you were to put a living Venus, or other
syphon-bearing mollusk into a glass of clear sea-water, and drop
a little finely-divided indigo, or other colouring matter, in the
immediate neighbourhood of these syphons, you would observe a stream
of the minute colour-particles rushing into one of these tubes, and
a stream of clear water issuing from the other. The inflowing stream
passes between the leaf-like gills, or respiratory organs (“branchiæ”),
where it is effectually strained, all solid matter being retained and
passed on to the stomach, whilst the filtered water passes out through
the second syphon. The length and form of these syphons differ in
distinct species, but each kind is pretty true to its own type, and,
consequently, the impression that it makes on the interior of the
shell, taken in conjunction with the muscular and pallial impressions
and the hinge-teeth, are a certain guide to the discrimination of
species.
These are matters that are essential to one’s knowledge of the
mollusca, and they must be learnt; but the few species we shall be
able to mention in this chapter will be indicated more by their
external shape, marks, and colouring. When so identified, the reader
should strengthen his knowledge by a practical study of these internal
impressions, and the characters of hinge and teeth.
This Spiny Cockle, or Red Nose (_Cardium aculeatum_), is not the
Vulgar Cockle (_C. edule_), although it is much sought for food on
its native Devonshire coasts. It is a very much larger species than
the last-named, and gets its name of Red Nose from the brilliant hue
of its long strong foot, which is at once a burrowing instrument and
a leaping pole. By pushing its pointed end down into the sand, and
then bending it into a hook, it can, by contracting the foot, pull the
thick prickly shell down after it. On the other hand, by pressing its
bent tip against some unyielding substance, it can use it as a spring,
which shall suddenly send the shell flying through the water to some
considerable distance. The Spiny Cockle is a creature of clean, sandy
beaches, where it may be found at low-water, but only on the Devonshire
coast.
The Common Cockle (_C. edule_) is very much smaller, its shell free
from prickles, and marked merely with bold rounded ridges. It is more
likely to be found where the sands are not wholly of sand, but contain
a liberal admixture of mud. On some of our coasts it is exceedingly
abundant, and in times of famine has saved populations from starvation.
It is certainly on record that the people of the Isle of Barra, in the
Hebrides, have been thus preserved many years ago, when all the people
sought the Cockle on the great expanse of sands at the northern end of
the island. “It was computed that for a couple of summers, at the time
alluded to, no less than from one to two hundred horse-loads were taken
at low-water, every day of the spring-tides, during the months of May,
June, July, and August.”
The Cockles have gained their name of _Cardium_ and Cardiaceæ from
the fact that if the shell is viewed “end on”--the curving beaks,
of course, uppermost--it will present the conventional heart-shape
(_Kardia_, Greek--heart). Some nearly allied genera, exhibit a similar
form, but narrower, and therefore not so suggestive of hearts; but
the Heart Cockle (_Isocardia cor_) is more truly heart-shaped than
the Cockles of the genus _Cardium_. It is about three inches across
its longest diameter, very thick and heavy, and the beaks are so
greatly curled that no one will be disposed to quarrel with the name,
either of the genus or the species. It is a deep-water species, but in
suitable localities the empty shell may be found washed in by gales.
It is chiefly found on the west coast, and it is probable that its
headquarters, in British waters, is around the Isle of Man.
Several of our most familiar bivalves are not very distantly related to
the heart shells. There are, for instance, the Venus shells of which we
have already given a figure of one species, the Banded Venus (_Venus
fasciata_). It is a solid, heavy little shell, of some shade of brown,
with broad bands of a lighter hue radiating from the beak. A series of
strong ridges run parallel with the margins, or, as usually expressed,
the ridges are concentric. The various species of the genus inhabit
sand and gravel from low-water mark to a hundred and forty fathoms. The
animal must be obtained by the dredge, but the empty shells are thrown
up freely after storms. A much larger species is:--
The Warted Venus (_V. verrucosa_), a drab-coloured shell, with very
rough and unequal ridges. In some specimens these ridges are so broken
by radiating lines, that the ornamentation has the appearance of being
warty. The various species of Venus have three strong hinge-teeth on
each valve, and the inner edge, though at first sight smooth, is very
finely “milled.”
[Illustration: SMOOTH VENUS.]
The finest of these shells is the large, heavy Smooth Venus (_Cytherea
chione_). It is a deep-water species, found chiefly on the southern
and western coasts, where, in spite of its great weight, it is
frequently washed up after storms. It is wonderfully smooth, inside
and out; even the lines of growth are not high enough or sharp enough
to take off this smoothness of the outside, which is coloured of
a pale pinkish-brown tint marked by concentric lines of a lighter
hue, and by much darker radiating bands. It is all very simple, but
very effective. The inside is coated with white, and the muscular
and pallial impressions are very strongly marked, though in no way
interfering with the general plan of entire smoothness. The edges,
too, are rounded and as smooth as the edge of a tea-cup. It is three
and a half inches across the longest diameter of the shell, and its
circumference, at right angles to the last measurement, is eight
inches. The hinge-ligament is an inch long, and the teeth are very
strong and prominent.
It is by no means a common shell outside the districts mentioned, but
I have frequently found it on sandy shores in Cornwall, thrown up by
storms, with the living animal still within. I believe most of the
entire shells found on beaches have been thrown up whilst the animal
is still in possession, and evidence upon this point may be obtained
by examining the ends of the shells. It will be found that those which
came to the surface with the animal are more or less chipped at the
ends, where a Puffin, or other sea-bird, has cracked off a portion to
enable it to prize the valves open; additional evidence will be found
in portions of the muscular bands still adhering to the valves.
[Illustration: RAYED ARTEMIS.]
On the same sand and pebble beaches we shall find in greater plenty
another of the Venus shells, the Rayed Artemis (_Artemis exoleta_).
We presume that Linnæus, in giving this species its name of _exoleta_
(Latin, worn-out), was struck by the fact that however fresh a specimen
may be, it has the appearance of having been knocking about with sand
and shingle for some time. The shells are white, with variable rays of
pinky-brown (sometimes entirely absent), and finely and evenly marked
with concentric grooves. In proportion to its size, it is a very thick
shell; very round in outline, except that a piece appears to have been
nicked out of the edge in front of the beak. When the shell is closed,
these marks on the two valves, coming together, form a heart-shaped
depression of a brown tint, and called the _lunule_.
The lunule is not peculiar to this species, but is shared by a large
number of bivalves. It is well-marked in the Smooth Venus, but
not so completely heart-shaped as in the Rayed Artemis. There is
a finely-developed set of hinge-teeth, and the pallial impression
is deeply sunk. A closely allied species, the Smooth Artemis (_A.
lincta_), is smaller, not banded, and the concentric ridges are finer
and less perceptible. It is this peculiar type of smoothness that
suggested the specific name of _lincta_ (Latin, sucked), its appearance
being as though a specimen of _exoleta_ had been sucked until smooth.
Both these have a hatchet-shaped foot for digging into the sand.
Great quantities of this bivalve are washed up in winter, and I have
frequently come across a piece of rock protruding through the sand,
around which there were dozens of these shells, broken or chipped,
giving evidence, from their fresh muscles, that they had but recently
been destroyed. It has reminded me of the favourite stone under the
hedge, whereto the Thrush brings her snails to be hammered until the
shell yields up its luscious contents. Artemis has met with a fate
similar to that of the hedgerow snails, for her fortress has been
broken by gulls, puffins, or even by ravens when winter has taught them
not to be too particular about their food.
[Illustration: CROSS-CUT CARPET-SHELL.]
There is a group of Venus shells whose exterior is ornamented with
concentric and rayed rounded ridges, in some cases strong, though
regular and even, whilst in others they are but slightly perceptible;
but their place is, to some extent, taken by colour. They bear the
generic name of _Tapes_ (Latin, tapestry) which is exceedingly
appropriate, for the patterns of some species is very suggestive
of tapestry and carpet. Especially is this so with the Cross-cut
Carpet-shell (_Tapes decussata_), whose exterior looks like the back
of a piece of tapestry carpet, both in texture and colour. The latter
is of a nondescript drab, with occasional tinges of red and stain-like
smears of bluish-grey. The ridges radiate from the beak, and they are
nicely rounded, but their lines are by no means straight. They are cut
across by fine concentric incised lines, which, with the grooves beside
each rayed ridge, produce the cross-cut, or _decussate_ appearance
which suggested the name. The interior is dull white, like the surface
of an enamelled card, the muscular and pallial impressions polished,
and consequently very obvious.
The Virgin’s Carpet-shell (_T. virginea_) is smaller, the exterior
very smooth, the ornamentation taking the form of shallow concentric
lines. The colouration is a mottling of salmon-pink, with little specks
of white showing through, and irregularly shaped spots of dark brown
sprinkled sparingly over all. Viewed not too closely, it will be seen
that the whole surface is divided between about six broad rays of
lighter and darker tints. The interior is white and glossy, deepening
to pink or yellow towards the beak and hinge.
The Golden Carpet-shell (_Tapes aurea_) is similar, but some shade of
yellow takes the place of the pink in the last-mentioned species.
The Pullet Carpet-shell (_T. pullastra_) is broader from the hinge to
the edge of the shell, in proportion to its length at right angles
with that measurement. Its name, _pullastra_ (Latin, a pullet), has
evidently a relation to its colouring, which is similar to that of
_virginea_, though darker. If the two are compared it will be found
there is a further difference in the fact that whilst _virginea_ can
scarcely be said to have any radiate-grooves, _pullastra_ is covered
with them; but they are not appreciable to the sense of touch, and
scarcely so to ordinary eyesight, unless special attention is drawn to
them--they are so exceedingly finely cut. Inside, the shell is white,
that part lying between the impressions and the hinge being dull like
the whole interior of _decussata_; but the impressions and the outer
margin are polished.
The Tapes animals spin a byssus like the Mussel; they burrow in the
sand at low-water with their thick fleshy foot, or spin up to the roots
of _Laminaria_ and other seaweeds among the low-water rocks. Around
the shores of the European continent they are used as food, but do not
appear to be so utilised in Britain.
The Scallops are familiar to all in a general way, from the frequency
with which one species occurs on the fishmongers’ slabs. This is the
largest British species, and is generally distinguished as the Common
Scallop, Quin or Queen (_Pecten opercularis_), a deep-water species,
whose valves are frequently washed up on the beach. They occur in beds,
but are not fixed like the Oyster; on the contrary, by the sudden
closing of their valves and the consequent rapid expulsion of water,
the shell shoots hinge foremost through the water to some considerable
distance. The young ones can attach themselves by a byssus to the
rocks, as is done by the Mussels, Carpet Shells, and others. It is
a peculiarity of the Pectens that they have a pair of “ears” to the
shell, the edges of which afford a good foundation for the hinge
ligament, whilst in lieu of hinge-teeth to keep the valves firmly
together when closed by the muscles, the corrugations of the valves
extend right to the margins, and the ridges of the right valve fit into
the furrows of the left valve and _vice versâ_. It will be noticed
that these ears are not a good pair--one is always larger than the
other, and the smaller one is popularly supposed to have been broken;
that, however, is a mistake, the Pectens are built that way. The most
prominent ear is always the front one, and below that of the right
valve there is a notch where the byssal threads issue. In the Common
Pecten these ears are much more nearly equal than in the others, whilst
in _P. varius_ there is a great contrast in the size and shape of the
two, and in _P. tigrinus_ one is almost absent altogether.
[Illustration: COMMON SCALLOPS.]
The Common Pecten is sometimes dredged for, but as a rule it is
avoided by the fishermen, on account of the risk to their nets and the
small price realised for the mollusks after they have caught them. It
will be noted more conspicuously in this species, on account of its
size, that the right valve, which is the lower one when the creature is
lying on its bed, is far more convex than the left or upper one. It is
exceedingly variable in colour.
[Illustration: SCALLOP HUNG UP.]
The Variable Pecten (_P. varius_) carries out its name to the letter,
for out of a score one could scarcely find two that agreed in colour
and the disposition of the markings. Their usual tint will be found
among the red series of the chromatic scale; sometimes almost white
with dark red blotches, at other times dark red-brown with faintly
perceived cloudings of a still darker hue. The exterior is ornamented
by about twenty-eight bold ridges radiating from the sharp beak, and
each of these, as it approaches towards the other edge of the shell,
gives off irregular spiny processes. There is a rare variety of this
almost entirely white. _P. varius_ is not content with using its byssus
only in the days of its youth, but continues to do so, even when at
full age; it may sometimes be found thus hung up to a rock, as shown in
our illustration, or attached to the roots of _Laminaria_.
A live Scallop of any species, in a glass vessel of sea-water, is
a beautiful object. It will soon open its valves and exhibit the
richly-frilled edges of its brightly-coloured mantle; this organ, in
fact, has a double margin, the inner of the two finely fringed, and at
its base a row of eye-like beads.
When prying curiously about the rocks at low-water, under the scrub of
weeds and corallines, we are sure to discover little flat pearly shells
which we shall almost as surely decide to be young oysters. They are a
kind of oyster, though not edible oysters of the genus _Ostrea_, but
Saddle Oysters, of the genus _Anomia_ (_A. ephippium_). Although they
appear to be firmly cemented to the rock by the lower (right) valve,
this is not really so. The thin blade of a penknife gently pushed
beneath will move it off with the merest touch, for instead of being
fixed by its whole under surface, it is merely attached by a muscular
plug that passes through a comparatively large oval hole in the under
shell, near the hinge, and sticks like a sucker to the rock. As it
grows older it will probably alter its form, to adapt itself to things
it comes in contact with, as its diameter increases. Small specimens,
not so large as a threepenny-bit, usually have a colony of much younger
individuals located on their upper shell.
Odd specimens of the Common Mussel (_Mytilus edulis_) will be found
among the roots of weeds on the low-water rocks, but to obtain them
in quantity one must go to a mud-bank, as at the mouth of a river; or
they may be found clinging in masses to the wooden piles of piers and
breakwaters by means of their byssus-threads. The first thing a mussel
does on being placed in an aquarium is to attach itself to the side by
this means. Possibly he will wander a little, by means of his foot,
to make sure of the right spot upon which to cast anchor, but having
settled that point and found that he has made the right choice, there
he will remain.
A mussel is the best of all bivalves for aquarium life. It is true
he is not very lively, and does not flit through the water like the
young Scallops. He is anchored, and there he stays, simply opening his
shell a little way and putting out the frilled edges of his mantle,
with their openings for the inward and outward currents--the inward
bringing both oxygen and food, the outward carrying off carbonic acid
gas and other waste.
The large, thick, coarse-looking mussel shells we occasionally find
on the sands, measuring five or six inches in length, belong to a
different genus, and are called Horse Mussels (_Modiola modiolus_).
The valves in question may not have come far, for the species occurs
in sand and mud as near as low-water. But it will not be found moored
to rocks and weeds by its byssus; instead, it burrows and weaves its
enormous byssus into a nest with sand and gravel mixed up with the
threads. They are said to be coarse and unpleasant tasting, so that
they are not used as food, and hence the name, Horse-mussel; the
prefix, horse or dog, before a popular name for animals or plants,
denoting its worthlessness as food, the sole criterion of worth to the
popular imagination being found in the answer to the query, “Is it good
to eat?”
[Illustration: COMB-SHELL.]
A very handsome shell, as well as a common one, is the Comb-shell
(_Pectunculus glycimeris_), whose thick round valves may be found
rolling on the beach, where they have been washed up from the
zoophyte ground in deep water. It is very variable in its markings,
and yet there is so strong a family likeness running through all
its variations, that there is not the slightest difficulty in its
identification. To the touch the exterior is quite smooth, though not
glossy, but examined with a lens it will be found to be covered with
very fine and regular lines running from the beak to the opposite
edges. So fine and close-set are these, that a line an inch long,
drawn across them at right angles, will cross about ninety of them.
The valves are more or less covered with a colouring of rusty red,
relieved by numerous long sharp wedges of white. These are on parts or
the whole of the shell, sometimes so plentiful that there is no room
for solid masses of the red colour, and it only shows in zigzag lines.
It is difficult to get two shells that at all agree in the distribution
of white and red, and even the two valves of the same shell will
differ widely in this respect. The interior, also, exhibits characters
sufficiently striking to prevent its mis-identification. There is a
broad flange below the hinge, whereon are cut about twenty teeth, in
two series. Immediately below the ligament is a smooth space, clear of
teeth, but these are arranged in a row of about ten on each side of
this space. As the shell grows the flange lengthens, and more teeth are
added to the ends of the rows farthest from the beak; but those nearest
the smooth central space are being rubbed down or absorbed at the same
rate, so that the net increase is about _nil_. The free edge of the
valves, internally, has a series of raised marks, like the tips of the
teeth of a comb, and it is from these the creature gets its name. The
pallial impression is much deeper than those of the muscles at each end
of it, and it is uninterrupted by any sinus. This shell is about two
inches in length.
We must not omit to mention a group of shells that are fairly common
upon many shores, and are usually found among the bucketful the
children have collected.
[Illustration: RAYED TROUGH-SHELL.]
First of these is the bold Rayed Trough-shell (_Mactra stultorum_),
more plentiful in the north than the south of Britain. The various
species of Mactra are inhabitants of sand in deep water, but their
shells are freely cast up on the shore. These are smooth, except that
the annual periods of rest from shell-making is plainly marked in deep
concentric grooves. Like that of the Spiny Cockle, the foot of Mactra
can be extended and used like a finger, and also as a leaping pole.
They are destroyed in great numbers by star-fish, and many empty valves
may be found with the clean round boring that shows the animal fell a
victim to the whelk. _M. stultorum_ is usually coloured some shade of
brown, with a number of white bands radiating from the beak. The hinge
arrangements in this genus are worthy of note, as indeed they are in
all the genera, and must be carefully studied by anybody who wishes to
have anything more than the merest superficial knowledge of conchology.
In the Trough-shells the ligament of the hinge is short and thick,
and contained in a spoon-like cavity in each valve. Immediately in
front of it there are two shelly teeth, joined above in the form of a
Ʌ, and from each side of the beak there runs off a strong ridge-like
tooth, the surface of which is “milled” like the edge of a shilling
or a sovereign. The Elliptic Trough-shell (_M. elliptica_) is not so
triangular as _M. stultorum_, and is without the white rays. The Cut
Trough-shell (_M. truncata_) might be appropriately styled the hatchet
shell, for its truncated ends give it a very close likeness to the head
of a hatchet.
Related to the Trough-shells are the Otter-shells (_Lutraria_), of
which we have two species. They burrow in the mud and sands, of
estuaries especially, and are found from low-water to about twelve
fathoms. Having found a complete, though empty, shell, you will be
surprised to discover that it will not close properly, and you not
unnaturally suppose that you have got hold of a malformed specimen,
whose shell has got a twist somehow. That, however, would be a mistake,
as you would find when other specimens came in your way, and you found
they all had the same objection to closing at the ends. From one end,
when the creature is alive, protrude its united syphons, large and
thick; and from the other end is thrust out the useful “foot,” with
which its burrowing is effected. Where you happen to find the usually
broken valves of the Otter-shells, it is worth while to dig in the
muddiest spots thereabout at extreme low-water, and you will probably
be rewarded with perfect specimens, and have the greater satisfaction
of seeing the living creature within.
Then there are the Tellen-shells (_Tellina_), a bright and
delicate-looking group, with shells that appear as though they had
been subjected to considerable pressure. They are finely grooved with
concentric lines, and decorated with broad bands of pink. One of the
most plentiful of these is the Thick Tellen (_Tellina crassa_), in
which the pink bands radiate from the beak across the shell. Thick is
a comparative term, and is so used here, for the shell, as compared
with a _Mactra_, for instance, would be considered rather thin; but in
contrast with other Tellens, it is solid and substantial. The interior
is delicately tinted with pink or orange. The pallial sinus is large
and rounded. The Fragile Tellen (_T. tenuis_) has thin shells that
are very easily broken. Its surface is very smooth, of an orange tint
marked with bands of pink and white. There are half-a-dozen other
British species. The Tellens burrow slightly in sandy mud, frequently
in shallow water. They may be dug for on a suitable beach between
tide-marks, though their range extends to about fifty fathoms.
Somewhat similar to the Tellens in their delicacy and style of ornament
are the Sunset-shells (_Psammobia_), so called on account of the
crimson patch around the beak, from which rays of a similar hue run off
to the margin. If the shell is so placed before you that the beak is
downwards, these rays suggest the far-reaching rays from the sun that
streak all the western sky, when Sol dips below the horizon for the
night. There are four British species. The two ends of the shell are
nearly equally rounded, but in an allied genus--
The Wedge-shells (_Donax_), the hinder end is much more acute than
the front, so that their popular name is very suitable. They have a
suggestion of sunset rays, too, but not so strong or so symmetrical as
in _Psammobia_. The most familiar species is the Common Wedge-shell
(_Donax anatinus_), which may easily be distinguished from the others
by the milling of the interior edge of the valves. The Polished Wedge
(_D. politus_) may be equally well separated by its superior gloss, and
by a white band which runs backward from the beak.
Then there are the familiar Razor-shells (_Solen_) that must be dug
out of the sand at low-water; and quick work you will find it, if
you succeed in catching any specimens. Very good examples may often
be picked up on a wide sandy beach, but minus the animal. They are
sought for food, and the professional catchers are very expert in their
movements--they need to be, or the business would not pay a dividend.
Everybody knows the razor-handle-like shells of _Solen siliqua_,
if they have no acquaintance with the animal. They are flattened
cylinders, widely open at each end for the extrusion of the foot and
the syphons. The hinge is near the front extremity of the shell, the
ligament in a full-grown specimen measuring an inch and a half. There
are two teeth in each valve, though some have three in the left; but
it is difficult to pick up empty shells in which the teeth are intact.
The Razors spend all their lives buried vertically in the sand. When
the sands are covered by water they rise to the mouth of their burrow
and protrude the syphons, but those that are situated so far in shore
as to be uncovered at low-water, then plunge in to a depth of a foot or
two. They never leave their burrows, except on compulsion, in the shape
of the salt and spade of their enemy, the fisherman. The species, with
a very straight margin to its shell, is the Pod Razor (_S. siliqua_)
which attains a length of eight inches; that with a distinctly curved
outline is the Sabre Razor (_S. ensis_).
[Illustration:
RED-NOSED BORER. PIDDOCK.
]
A brief glance at some borers and excavators must suffice to close
this long chapter. The small, upper figure in accompanying plate is
the Red-nosed Borer (_Saxicava rugosa_), a species that largely helps
the sea in its ceaseless attacks upon the coast line. It is the office
of the Borer to excavate cells in the face of the rock, and as it is
never solitary in its work, but attacks a rock in “gangs,” as a human
excavator would put it, the result is the complete honey-combing of
the surface. They may often be found free in crevices of the rocks
and about the roots of seaweeds--that Alsatia for a very varied
population. The shell has a distorted look about it, and the valves
will not fit properly, the ends gaping to allow the foot and the
syphons free play. It is very variable, however, and consequently has
been a splendid subject for the variety-mongers and species-splitters,
who have manufactured quite a long list of species and genera out
of it. It changes a good deal at different periods of its life,
and thus affords opportunities for careful descriptions made from
isolated specimens utterly disagreeing with each other; therefore,
the individuals described must belong to different species! In its
early state the shell is symmetrical, and has two minute teeth in each
valve; but before it has reached maturity it has lost its claim to be
considered graceful or symmetrical, and has either worn its milk-teeth
out or abandoned them as useless. The shell is covered with ridges and
wrinkles, and it is by their help that it carves out its chamber in the
rock. Sometimes on turning aside a curtain of weeds from a rock-face
you will see a large number of crimson points, which, however,
instantly disappear if they have been ever so lightly touched by the
weeds. These are the ends of the borers’ syphons, protruded from their
ventilation holes; they are united almost to their extremities, and
present the appearance shown in our figure.
The Piddock, or Finger Pholas (_Pholas dactylus_), is a much larger
species with some difference of structure. Its pure white shell, though
thin and fragile, is covered in front with rasp-like ridges, and by
means of it the chambers and tunnels of the rock are bored. Holding
to the rock with the clear crystalline foot, the Pholas gives its
shell a swing half way round in one direction, then a swing back, and
so by alternate half-revolutions, the rasping of the shell gradually
excavates a chamber sufficiently large to shelter it, communication
with the outer world being maintained by the large syphons. So far
there is no very great difference between the Pholas and the Saxicava;
but the Pholas is peculiar in that it possesses neither ligament
nor hinge, and in addition to the orthodox two valves, it has some
additional ones. The hinge-plates are reflected back over the beaks,
and a powerful muscle is attached thereto to keep the ordinary valves
together. Above these, and to protect this muscle, are two short
accessory valves, and a third, which is long, and extends back over
the dorsal edges of the big valves. In other species of Pholas these
arrangements give scope for variation.
And now it is time we gave some thought to the one-valved and valveless
mollusks of the shore.