BUILDERS OF THE PEARLY SHELLS
The mollusks, or "shellfish" (phylum Mollusca) are a homogeneous
group of soft-bodied, unsegmented, typically bilateral, elaborately
organized animals, mainly aquatic and marine, whose origin--probably as
a derivative from a wormlike stock--is lost in the mists of geologic
prehistory. In most cases the mollusks secrete from a larval gland
an external shell which serves as skeleton and defensive armor; are
bisexual and produce eggs, or if monoecious are never self-fertilizing.
They possess a heart, and blood circulation (usually colorless);
breathe in the water by means of gills, or, in the air, by a primitive
kind of lung; have a nervous system and senses in some cases of a high
order; the organs are normally paired, and protected by a general
covering integument called the "mantle"; and the creeping species move
by a muscular, elastic, ventral organ styled the "foot," while the
swimmers are provided with a variety of swimming organs. Mollusks vary
in size from all but microscopic minuteness to a bivalve weighing 500
pounds or a squid half as big as a right whale. They occur in all seas
at all depths, abound in fresh waters both swift and stagnant, and are
scattered over the earth wherever vegetation flourishes.
The phylum Mollusca is divided into five classes, as follows, and it
will be noticed that four of the names refer to the locomotive organ
or "foot" (Greek _pous_, "foot"):
I. _Pelecypoda_, the Mussels--mollusks inclosed in a bivalve shell
fastened by a muscular hinge, the adjacent part of the valves being
generally more or less toothed; the foot is as a rule roughly
comparable to the shape of an ax head.
II. _Amphineura_, the Chitons--flattened, bisymmetrical mollusks
whose shell consists of eight crosswise, overlapping plates.
III. _Gastropoda_, Snails, whelks, etc.--mollusks that crawl on the
flat undersurface of the body, or distensible foot.
IV. _Scaphopoda_, Tusk shells--mollusks that possess a long tubular
shell open at both ends; with their small and elongated foot they
are supposed to _dig_ into the mud in which they live.
V. _Cephalopoda_, Cuttlefishes, and Octopods--mollusks with
tentaclelike "arms" arranged about the mouth, and either an external
or internal shell. These are the highest in rank.
THE OYSTER AND ITS RELATIVES
The lowest in rank of these classes is the Pelecypoda, containing
the "bivalves"--mussels, clams, oysters, and the like, in which the
shell is in two parts or valves hinged together over the "back" of
the animal, and attached to it on each side by a powerful muscle,
the "adductor," by the contraction of which the shell may be tightly
shut. Within the shell the body is enveloped in a "mantle," or fleshy
membrane falling like a cloak on each side; and from it is secreted
the outer shell, which grows by additions to its ventral margin. These
additions are in a general way annual, so that the concentric lines of
growth on its exterior are an indication of the years of the mollusk's
life, which is slow in growth, and long-lived. The interior of the
shell is usually pearly, and marked with microscopic rugosities, which,
by breaking up the light, as if by innumerable prisms, gives the
iridescence so beautiful in the pearl oyster, the fresh-water unios and
many others. These pearly layers are called "nacre."
Bivalves were formerly classified in conchology as Acephala, because
they have no proper head, but at the posterior end are two openings of
tubes, provided with cilia. In one, the cilia induce a constant current
of water which after leaving the gills brings into the animal's stomach
floating microscopic food, both plants and animals, including eggs and
larvæ, where it is captured and assimilated while water is ejected
through the other (dorsal) pipe. This food includes bacteria, and if
the mollusk lives and feeds in water polluted by sewage, or otherwise
containing germs of disease, it becomes dangerous as human food; hence
oysters and clams exposed to such bad conditions ought never to be sent
to market because of the disease germs remaining in them.
In bivalves such as the oysters, horse mussels, piddocks, and others
that are sedentary, and often fixed in place, or that, like river
mussels, scallops, etc., move about freely, the mouth tubes are short;
but many bivalves, as the clams, pinnas, razor fish and so forth,
bury themselves in the sand of the bottom, by means of the strong
distensible foot protruding from the forward end of the shell. These
are provided with a double-barreled tube, called the "siphon," which
may be contracted within the protection of the closed shell, or may
be stretched out several inches; the animal may thus sink its body
deep in the sand while its siphon reaches to the surface and inhales
food-bearing water. The little squirts of water often seen jetting
out of the beach at low tide as one walks along it are from clams
so buried, and which, alarmed by the vibration of one's footsteps,
hastily eject the water and withdraw their siphons.
The old name for this class, Lamellibranchiata, referred to the gills,
two of which, on each side, hang like curtains inside the mantle and
between it and the saclike body containing the viscera; when the shell
is open they are laved by the water, and extract from it, by some
quality hardly understood, the oxygen necessary to regenerate the blood
that flows through them; and, in addition, respiration is carried on
through the skin.
The nervous system is very primitive, and the sense organs consist of
an otocyst (a minute sac in which a hard particle floats in a liquid)
in the foot, by which, it is believed, a sense of direction is had,
and which also serves the purpose of an ear; an organ that tests the
water; and in some, as the scallop, rudiments of eyes are situated on
the margin of the mantle. Most pelecypods are of two sexes, but some,
such as our American oysters, are hermaphrodite. Eggs in vast number,
and a cloud of spermatozoa, are thrown out in midsummer, and a little
of the latter succeeds in reaching and so fertilizing fortunate eggs,
but almost all merely serve as food for the host of mollusks, worms,
sea anemones and what not that subsist on such provender. The few
fertilized larvæ drift about and happily escaping multiplied perils,
presently settle to the bottom to attach themselves to some fixed
object, or otherwise get a chance to grow big enough to defy ordinary
enemies. Some interesting variations in this rather commonplace larval
history occur, however, in certain families.
It will be possible to name only a few of the most useful or otherwise
conspicuous bivalves, beginning with the oyster, concerning which
an immense amount of detailed information is accessible to the
reader in the reports of the United States Government (Tenth Census,
and documents issued by the Fisheries authorities) and in those of
States, like Connecticut, New York, and Maryland, where oyster culture
is an extensive industry, said to be worth in the aggregate about
$20,000,000. The oyster of the eastern American coast is to be found
in the Gulf of St. Lawrence, but not in considerable numbers between
there and western Maine, whence it is present southward to the Gulf of
Mexico, except on the shifting sands of the outer beaches. It seeks
protected waters and a rocky or weedy bottom furnishing objects to
which it may, when young, attach itself, and later will not be torn
adrift by storms, for where an oyster establishes itself in infancy it
means to stay all its life. Hence the sheltered waters of Buzzards and
Narragansett Bays, Long Island Sound, and the lagoons and inlets that
lie behind the outer line of sandy beaches from Long Island to Florida
are the sources of our supply--especially Chesapeake Bay.
A full-grown oyster will produce about 9,000,000 eggs, each being about
one five-hundredth of an inch in diameter. When the little oyster
(spat) is about one-eighth inch wide shells begin to form on its
sides, and it settles to the bottom with its left side down, usually
where other oysters are; and hence extensive colonies, or "reefs," of
these mollusks form, and "rise on their dead selves" to a level where
they may be reached by the oysterman's rake. Many years ago, however,
it was discovered that large, marketable oysters were becoming very
scarce. Oystermen therefore sought favorable places, and raking the
natural beds transplanted their catch, little and big, to new ground,
where they were left to mature. This crude method was next improved
on by sowing thickly over the new ground, just before spawning time
in midsummer, a great quantity of empty oyster and other shells.
These were favorable to the catching of "spat," and would result in a
new bed that in about four years would furnish salable oysters; and
annual plantings produced, after a time, an annual crop. These are the
essential facts of oyster culture everywhere, although methods differ
somewhat in other parts of the world--in France, for example, fascines
of twigs are spread over tidal flats to catch the spat, instead of
shells.
Our eastern American oysters are undoubtedly the largest and finest for
the table of the many species that exist all round the globe. Those of
the Pacific coast of the United States are excellent, but small; and
the same is true of the European species; nor is the use of oysters
abroad so general and extensive as in the United States.
The pearl-bearing oysters are somewhat distant relatives of the edible
oyster (Ostræa), the thorny oysters (Spondylus), the hammer shell, the
windowglass shell (Placuna) and others. The pearl oyster of commerce is
named _Meleagrina margaritifera_ and is found in scattered localities
within the tropics on both continents. The chief fisheries are in the
Persian Gulf, around Ceylon, in Australia, among the Sulu Islands
and on the west coast of Panama. The Pearl Islands, south of Panama,
yielded to the early Spanish adventurers riches in gems that rivaled
those their competitors obtained from gold mines; but now they are a
field of small importance. In fact, the pearl fishery is carried on
now far less in hope of a profitable collection of gems than for the
profit in the shells, which have a nacreous interior of remarkable
beauty--the mother-of-pearl--and the great advantage of offering this
in almost flat surfaces, sometimes eight or nine inches broad, making
it useful in the arts as well as in the more practical line of buttons,
knife handles, etc. Sometimes the whole surface of a fine shell has
been carved, cameowise, with cunning art and an exquisite effect.