Of all the grand divisions of the Animal Kingdom, the subkingdom
Mollusca is probably the least known to the ordinary observer, and if
one were asked to enumerate as many different kinds of “shell fish” as
he could, it is probable that not over six or eight different varieties
would be named. The majority of people think of a clam, oyster, mussel,
snail or Nautilus and their molluscan vocabulary ends with these names.
And yet this group of animals is second only to the insects in number of
different species, beauty of coloration and interest of habitat. They
may be found everywhere, in salt and fresh water, in our forests and
fields, our ponds, brooks and rivers; in the valleys and on the mountain
tops, and even in the waters of the frozen north, while in the warm
waters of the tropics they flourish in uncounted millions. In size they
range from the little sea-snails hidden in the eel grass along the
shore, with tiny shells scarcely an eighth of an inch in length, to the
giant squid, which measures forty feet or more from the tip of its tail
to the end of its long arms; and they range from the tide-washed beach
to the abyssal depths of the ocean. It is to these lowly creatures that
I would draw the reader’s attention.
In nearly all the species of the Mollusca the animal is protected by a
hard shell, made of carbonate of lime, which is covered with a horny
epidermis to protect the limy shell from being dissolved by the acids in
the water. This shell is generally capable of containing the entire
animal, thus affording, in most cases, adequate protection for the soft
body. Those animals not provided with a shell, as is the case with the
land slugs, are capable of covering themselves with a sort of mucus
which encysts and protects them from both extreme heat and cold.
The lowest branch of Mollusca is known as class Pelecypoda, which
comprises all of the different kinds of clams, mussels, quahaugs, etc.,
in which the body is protected by two hard, calcareous shells placed,
generally, opposite each other and connected on the upper margin by a
ligament, and the two valves work back and forth in teeth and sockets,
making a kind of hinge. A set of stout adductor muscles keep the two
shells or valves together and allow them to open and close at the will
of the animal. The majority of clams live in the mud in a horizontal
position, the anterior end being buried and the posterior end,
containing the siphons which draw in and expel the water, being out of
the mud, in the water. The clam progresses by pushing forward its
strong, muscular foot, getting a firm hold of the mud and then drawing
the shell after it. Some pelecypods, as the oyster, live attached to
some object on the bottom of the water, as a stone, piece of wood or
piling of an old wharf, and are not able to travel from place to place
as are the true clams, examples of the latter being fresh water mussels
and the marine quahaug or round clam.
Some bivalves also attach themselves by a byssus composed of a number of
silk-like threads, which anchor their shells to stones, sticks, and
other foreign objects. In one group (genus Pinna) found in the
Mediterranean Sea, this byssus is so fine and silky that the Italians
weave it with silk and make caps, gloves and other articles of wearing
apparel.
[Illustration: ]
First row:
Sunrise Shell (Tellina radiata)
Pearl Oyster (Margaritiphora radiata)
Second row:
Coccle (Cardium isocardia)
Spiny Oyster (Spondylus princeps)
Scallop (Pecten dislocatus)
Third row:
Mussel (Mytilus edulis)
Oyster (Ostrea lacerans)
Fourth row:
Fresh Water Clam (Unio luteolus)
Spiny Venus (Cytheria lupinaria)
Another wonderful and interesting arrangement for the comfort of the
animal is its breathing organs or branchiae. These are two or four in
number, and are made up of numerous small chambers, covered with little
whip-like organs or cilia, which keep up a constant motion, creating
currents of water, bring thousands of minute organisms to the clam to
serve as food. These little organisms, many of them microscopic, are
caught upon the surfaces of the gills, rolled into little masses, and
passed into the animal’s mouth. Besides being food-gatherers, the gills
serve to keep up a circulation by which fresh water is constantly
brought in to purify and aerate the blood and also to expel the waste
products. There is no head in this class, and the mouth is an oval slit
surrounded by four lips or palpi, and leads almost directly into the
stomach.
The currents of water spoken of above are controlled and directed in
several different ways. In attached forms, and those living above the
surface of the mud, like the oyster, mussel and scallop, the soft mantle
which lines the shell is divided, forming a slit nearly the whole
diameter of the shell, and the water is allowed to circulate freely
through the open edges of the shells. But in those animals which burrow
in the mud, as the common little neck clam, fresh water clam and
quahaug, this mantle is closed and prolonged posteriorly into one double
or two single siphons or tubes, one being fringed with little
finger-like cilia and drawing in the water by their motion, and the
other expelling the water after it has circulated through the animal.
One of the most attractive families of bivalve shells is the Veneridae,
or venus shells, in which the shelly skeleton is ornamented by many
bright colors, the patterns occurring in spots, dashes, zigzag lines and
rays. Some varieties, as the spiny venus (Cytheria lupinaria) have the
posterior end of the shell provided with long, sharp, curved spines, and
the shell is also frilled in a beautiful manner. The common quahaug
(round or hard-shelled clam), which is esteemed an article of diet on
the Atlantic coast, and also to some extent in the interior, is a
prominent member of this family. The Veneridae comprise some five
hundred species, found throughout the world, and ranging from the shore
between tides to several hundred fathoms in depth.
The family Cardiidae, the heart-shells or cockles, comprise some of the
largest and most attractive of mollusks. The name Cardium, signifying a
heart, is given them because of the close resemblance to that organ when
a shell is viewed from the anterior end. These animals live in sandy or
muddy bays, and generally congregate by thousands. In England, the
edible cockle (Cardium edule) is considered quite a delicacy and
thousands are used for this purpose. In our own country they are not
generally eaten, except by the poor in Florida and in some places along
the Gulf of Mexico, but the waters of Florida furnish some very handsome
species, among them the Cardium isocardia figured on our plate, and the
large Cardium magnum, which grows to a length of five inches and whose
shell is ornamented by beautiful color-patterns of brown and yellow. The
foot of the Cardium is very peculiar, being shaped like a sickle, which
enables the animal to pull itself along at a lively gait. A California
cockle (Liocardium elatum) grows to a diameter of seven inches and would
furnish a meal for several people.
In the family Tridacuidae size seems to have reached its limit.
Tridacena gigas, found in the Indian Ocean, grows to a length of nearly
six feet and weighs upwards of eight hundred pounds. Tryon records that
a pair of these shells, weighing five hundred pounds, and two feet in
diameter, are used as benetiers in the church of St. Sulpice, Paris. In
some parts of the Indian Ocean, where pearl and sponge-fishing are
carried on, this clam (known as the giant clam), is a source of great
danger to the divers, many losing their lives by being caught between
the great valves of the shell, by either hands or feet. Many times a
diver has amputated his fingers, hand or foot, and thus saved his life
at the expense of one or more of these members.
The Tellinas (family Tellinidae) number among its five hundred or more
species some very beautiful and interesting animals. They live for the
most part buried in sand or sandy mud and are found throughout the
entire world. Our common Tellina radiata, familiarly called sunshell, is
found in Florida and the West Indies, and a typical valve looks not
unlike the horizon at sunrise, the brilliant rays of color spreading in
different directions from a common center. At Newport, Rhode Island, the
writer has gathered many thousand specimens of a beautiful little Tellen
(Tellina tenera), whose shell measures scarcely half an inch in diameter
and is tinted a lovely pink or pinkish white. The siphons of this family
are very long and are separated, the upper one being half or
three-quarters as long as the lower one, and the foot is rather long and
pointed, admirably adapted for burrowing. The long siphons enable the
animal to bury itself to quite a depth beneath the surface of the sand.
Closely related to the Tellinidae is the Psammobiidae, a characteristic
form of which (Psammobia rubroradiata) is thus spoken of by Prof. Josiah
Keep, in his interesting little book, “West Coast Shells:” “But I wanted
to see more of him, so I took a large jar, filled it half full of beach
sand, added as much sea-water as it would hold, and plunged my prize
into the same. He rested quietly for a few minutes, and then began to
open his shell and cautiously put out his two siphons. Soon afterward,
from between the edges of his shells, came his big, white, spade-shaped
foot. He drove it down into the sand, curved it a little to one side,
gave a vigorous pull, and lo! his shell followed, though just why I
could not clearly understand. Though the jar was large he reached the
bottom before his shell was wholly covered with sand, and had to content
himself with a half-above-ground tenement.”
“Next morning his siphons were stretched out some six inches in length.
* * * I never thought before that there was any particular beauty to the
siphons of a clam, but for this red-lined one my opinions quickly
changed. Imagine two tubes made of the finest pink and white silk,
stretched over delicate hoops arranged at regular intervals; then think
of them as endowed with life, and waving with a graceful motion through
the water, and you will have a faint idea of their exquisite texture and
elegant appearance.”
To those readers who live in the West, away from the ocean, the Unio, or
freshwater mussel, is more or less familiar. What child in Chicago has
not played on the sands of Lake Michigan and scooped up the little
grains with the broken half of a clam shell? Or who, wading in the muddy
water of Lake Calumet, has not wondered what the curious little hollow,
fringed objects were which protruded from the surface of the mud? These
latter were the siphons of the clam and if you were to dig under them a
little way you would find the beautiful green-rayed shell of a river
mussel. These are no less interesting than the marine shells already
described and in beauty of ornamentation they frequently excel many of
their salt-water relatives. Such excrescences as knobs, spines and
rib-like undulations are common, while the colors of the interior range
from pure silvery white through orange, pink and salmon to dark purple,
and the rich, pearly iridescence rivals that of any of the marine
shells. In many parts of the West mussels are collected by men in search
of pearls, which are generally of an inferior quality, and thousands of
shells are used annually in the manufacture of pearl buttons.
One of the most familiar objects to the seaside visitor is the huge
banks of sea-mussels (Mytilus) which line the shore at low water. The
shells are generally dark-colored, our common mussel (Mytilus edulis)
being frequently jet black, and are more or less wedge-shaped in form.
They attach themselves to mud banks and shore vegetation by a strong
byssus made up of stout, more or less silky threads. The mussels are of
great value economically, thousands of bushels of the edible mussel
(Mytilus edulis) being consumed annually in Europe. They are also used
as bait, and millions of the mussels are thus used every year. Although
considered a delicacy in parts of Great Britain and Europe, it has not
yet been adopted as an article of diet in this country, the clam and
quahaug taking its place.
The family Aviculidae, comprising the wing-shells or pearl oysters, is
of great interest, both scientifically and economically. At the present
time there are a little over one hundred species living, but the family
has been known from early geological times and over a thousand species
have been found in the rocks. The pearl-oyster (Melleagrina
margaritifera) is the most important member of this family, furnishing
as it does the beautiful pearls of commerce. These animals are found at
Madagascar, Ceylon and other parts of the Indian Ocean, several hundred
tons being imported into Europe annually. These pearls are formed by
some irritating substance, as a grain of sand or some parasite, getting
in between the shell and the animal, or lodging in some soft part, which
causes the animal to cover it with pearly matter to prevent irritation.
The shells also furnish a considerable part of the “mother-o’-pearl”
which is so largely used for ornamental purposes. The Margaritifera
radiata, figured on our plate, is a member of this family.
The scallop is an object well known to the tourist visiting New England
summer resorts, who has reveled in “fried scallops.” The family to which
this belongs (Pectinidae) is composed of rounded shells, many with
frills or ribs and nearly all ornamented with beautiful colors. Unlike
the animals which we have been considering, these mollusks have no
siphons and the shell is open all the way around save at the hinge, and
the edge of the mantle is provided with little, round, black eyes. It is
an interesting sight to observe a beach at low water, the receding tide
having left on the shore or in little pools of water hundreds of these
mollusks, attached by a byssus to bits of sea weed. As one is gazing
wonderingly over this vast field of yellow sand and green weed, an
object will suddenly move through a pool of water with astonishing
rapidity, accompanying the movement by a quick snapping sound. This is
the scallop, which is imprisoned in the pool and which desires to get
out. The movement is effected by rapidly closing and opening the two
valves of the shell, thereby causing a clicking sound. The noise of
several hundred of these shells opening and closing and the sight of as
many scallops with strings of sea weed attached to them, shooting
through the water, looking not unlike a comet with a long tail, is quite
bewildering. In Europe, the scallop is considered quite a delicacy and
several tons are gathered annually. One species (Pecten jacobaeus) has
been dignified as a badge of several orders of knighthood and it was
also worn by pilgrims to the Holy Land a good many years ago. It was
called “St. James’ Shell.”
The most common shell to the layman is the oyster (Ostrea virginica),
the cultivation of which occupies the attention of a large number of men
and the investment of considerable capital. The oyster is free and
active when young, but becomes attached to some submerged object early
in life. Oyster culturists take advantage of this habit by erecting
poles in the water to which the young oysters attach themselves. The
shells of the different species of oyster are not generally of much
beauty, but a related family, the Spondylidae, or spiny oysters, are
among the most beautiful of bivalves. In this family the shell is
ornamented by many long spines and frills, and the colors are different
shades of red, yellow and pink. The most beautiful species are found in
the Gulf of California.
The space at our command is far too limited to adequately discuss the
many curious and interesting animals which make up the class Pelecypoda.
Much might be said of the Solen or razor-shell, with its curious foot
which is so great a help in digging burrows; of the Pholads, which
perforate and make burrows in clay, wood and even in the hardest rock;
and of the strange Teredo or “shipworm,” with a long, worm-like body
which bores into ships, wharves and any wooden object within reach. But
enough has been written and pictured to show the reader that the
unpretentious clam, mussel or oyster and their relatives have many
interesting habits, are encased in beautiful shells, and that some
species are of great economic importance to man.
Frank Collins Baker.