BUILDERS OF THE PEARLY SHELLS--_Continued_
MUSSELS, SCALLOPS AND CHITONS
The familiar marine mussels of the family Mytilidæ will some day become
of great importance in this country as a food supply, as now they are
useful in resisting encroachment by the sea on certain parts of the
coast. They exist in vast numbers on both our coasts, and elsewhere in
the world, in two genera, Mytilus and Modiolus, which differ a little
in form, but not in habits. They have acquired the stationary habit,
and in place of a "foot" of serviceable size have developed a gland
that secretes an exceedingly tough, fibrous bunch of threads known as
a "byssus," by means of which the animal may not only attach itself
firmly to any sort of object, but may actually move about. The common
species of Modiolus, the "horse mussel," lives in great numbers north
of Cape Hatteras at and below the line of low water, and is much larger
than the edible mussel just described. A smaller species of Modiolus
is extremely numerous on the New England coast, and down to the
Carolinas, forming dense tangled beds on muddy patches as well as among
rocks, and serving to bind the mud and plants together and hold them
from disintegration by stormy waves, in spite of the thin and brittle
character of their shells. A southern species is bright yellow, with
dark rays; and the common modiola of the Pacific coast is dark, glossy
brown. Such mussels are eaten regularly in Europe, and come to us in a
pickled condition as a luxury. There is no reason why we should neglect
to add our own to our long list of sea foods.
The next useful mollusk to be considered is the scallop, one of
the many species of the family Pectinidæ, of which we eat only the
adductor muscle. The commercial species is _Pecten irradians_, the name
referring to the (nineteen) ridges that radiate from the flattened
hinges to the scalloped margin of the shell, which is prettily colored.
This species is common in sandy, shallow places from Cape Cod to
Florida, but the fishery is most productive about the eastern end of
Long Island and in Narragansett Bay. Farther north is a very much
larger species (_P. islandicus_) especially abundant on the Grand
Banks, off Newfoundland, where it forms an important food of the cod
and other fishes. It is well known to cooks, who use it in baking
their fish confections _en coquille_. A large number of other species
are distributed throughout the world, one (_P. jacobæus_), inhabiting
the Mediterranean having the name "pilgrim shell" in allusion to the
fact that in the days of medieval religious pilgrimages, those who had
visited the shrine of Saint James at Santiago de Compostela, Spain, to
pay homage on July 25, were accustomed to wear a scallop shell in their
hats in token of the fact--this mollusk being connected with traditions
of that saint.
Turning to the fresh-water mussels, or naids, as some books call
them, one is staggered to learn that more than 1,500 species have
been named, a large proportion of which belong to the United States,
which is peculiarly hospitable to them because of our many rivers and
lakes, together with the prevalence of limestone rocks, whose constant
dissolution in water supplies the store of calcareous matter that these
thick-shelled mollusks require. All belong to the family Unionidæ, in
which two divisions are noted--one (Anodon) in which the mussel has a
comparatively elongated thin shell with no "teeth" in the hinges; and
the other (Unio) in which the shell is thick, various in shape from
an oval to a triangle, and has prominent umbones, beneath which the
valves (which are always alike) are hinged together by interlocking
teeth embedded in a somewhat elastic gristle. The interior of all these
unios is richly nacreous, and consequently pearls are produced in the
same way as in the marine pearl-bearing shells; and some of the finest
known gems have been derived from them, in this country and abroad, as
well as innumerable specimens of moderate value. These mollusks like
clear streams or lakes with a sandy bottom, and are not to be looked
for in stagnant weedy waters. They keep an erect position, the nibs of
the shell half buried in the sand, and move slowly about, plowing a
path and dragging themselves along by means of the powerful foot, but
keeping the short siphons at the other (or longer) end of the shell
well above the mud.
We come next to our market clams. These are of two distinct
kinds--"hard" and "soft," or quahog and long clam, as they are
distinctively called. The quahog is a thick-shelled, roundish mollusk
with a distinctly heart-shaped outline when looked at endwise. It
dwells in fairly deep water, standing on its nibs half buried in the
sand, like a wedge, and moving slowly about. Young ones become the
"little necks" of our summer tables.
The soft clam belongs to a different race. Its elongated shell is thin
and chalky, is loosely hinged, and gapes widely at both ends, and
although it is used much as food, especially in chowders, it is by no
means as good as the hard clam. Its principal value, indeed, is as
bait in the cod fisheries, and for this purpose enormous quantities
are gathered. It lives in, rather than on, muddy beaches, sometimes
in crowds of thousands, its shell deeply buried, and its long siphons
reaching up to suck in water and food when the tide covers the flat.
When the tide is out, a tiny hole in the sand and a spurt of water show
the clammer where to dig, and his spade quickly unearths the clam.
The second class, Amphineura, contains the chitons and their relatives.
These chitons are flattened mollusks protected by an armature of eight
crosswise plates, overlapping like shingles, which creep about the
rocks close to shore, and when lifted curl up like sowbugs. The most
interesting thing about the chitons is the fact that they are provided
with excellent visual organs, "the whole dorsal surface of some forms
being studded with eyes, of which not less than 8,000 occasionally
exist on a single specimen." Many of them are complete, with cornea,
lens, and a pigment layer within the iris.
SNAILS AS TYPES OF GASTROPODS
The gastropods (Gastropoda), including the snails and slugs, limpets,
whelks, periwinkles, sea hares and the like, are Mollusca having the
mantle completely enveloping the body, and the shell, when present, in
a single piece, and usually in spiral form. There is a well-developed
ventral foot, on which the animal creeps, and in front of it a distinct
head bearing eyes and tentacles. These organs retain their normal
bilaterality, but the body is, as a rule, inequilateral. The cause
of this is the fact that on the animal's back is developed from the
first a shell, which, with its contents, amounts to a relatively
large weight, and it naturally falls over to one side. The mouth is
armed with a flat, distensible, ribbon-like organ, studded with rows
of chitinous teeth, that serves as a rasp and a boring instrument,
and which is called an odontophore, or, in snails, a radula. Most
gastropods are carnivorous.
The lowest in rank are the shell-less, or "naked" gastropods known as
"sea slugs," "sea hares," and so forth. One Mediterranean species of
Aplysia secretes a purple liquid utilized by the ancients as a dye, and
this is still sought for in Portugal, where storms sometimes cast vast
quantities of the mollusk on the beaches.
We come now to the great group of mollusks inhabiting fresh waters
and dry land--the snails, whose group name is "pulmonates," that is,
possessors of lungs, and breathing air. On the generally accepted
theory that all these are descended from marine ancestors, and have
gradually acquired the faculty of living on land, it would be natural
to look for a series of mollusks that were amphibious, and, as it were,
half-way fitted for a terrestrial existence, and such intermediates
exist in all parts of the world. The little black Melampus, which
covers the mud of tide flats on both the Atlantic and Pacific coasts
in tens of thousands, and seems just as happy when the tide is out as
when it is in, or when it is simply refreshed by the spray, is a good
example. A near relative, Carychium, is still more emancipated from the
sea.
First among these pulmonates are those common in ponds and still
streams the world over, of the family Limneidæ, called limneids or pond
snails. They are in various forms. Some are limpet-shaped (Ancylus),
some are flatly coiled (Planorbis), but most of them have shells drawn
out into a graceful spiral; in all cases the shell is not composed of
lime, but of the thin, fragile, horny substance "chitin." The best
known one is _Limnea stagnalis_, which sometimes reaches a length of
two inches, and inhabits almost every quiet piece of water in North
America, and in Europe and all Asia except India and China.
These water snails of our ponds and ditches are exclusively vegetable
feeders, and must come to the surface at frequent intervals to breathe,
letting out a bubble of vitiated air, and taking in a fresh supply.
Should the pond dry up in summer the limneids burrow down into the mud,
and remain in that heat trance called æstivation until the autumnal
rains refill the basin and let them come forth. The small kinds
called "physas," exceedingly common everywhere in this country and
Europe, differ from Limnea in having the shell partly enveloped in the
turned-up fringed edges of the mantle, and by being coiled from right
to left instead of clockwise. This reversal occasionally occurs in
individuals of all gastropods, which are then said to be "sinistral,"
as opposed to the normal "dextral" coiling; but in the physas it is the
rule.
Next come the wholly terrestrial pulmonates--snails and slugs,
distinguished from the pond snails, which have only one pair of
tentacles at the bases of which the eyes are embedded in the skin,
by having two pairs of "horns," one of which carries the eyes on
their tips--good eyes, which may be quickly withdrawn out of harm's
way by inversion of the tubular stalks. The thick, extensible foot is
surmounted by a body coiled within the shell; and this foot secretes a
viscid fluid that lubricates the creature's path, and often leaves a
silvery trail.
Snails are mainly vegetarians. The mouth lies just under the front
tentacles, and its upper lip is armed with a horny, crescentic "jaw."
Within the mouth is the lingual ribbon, which may be brought up
against the cutting edge of the jaw. This tongue is studded with rows
of infinitesimal, flinty teeth, the radula of our big white-lipped
snail, a quarter of an inch long, furnishing room for 11,000 of these
denticles; and as all of them point backward the tongue easily seizes
and draws into the mouth whatever the jaw nips off. Substantially
the same sort of "tongue" is possessed by all the gastropods, but
the arrangement and shape of the microscopic denticles is different
in every species, and this is one of the "characters" used in
classification. With it the carnivorous rasp away their food; and by
bending it double and using it as a gimlet bandits like Nassa, the
oyster pest, drill through other shells and devour the occupant. You
may pick up on any seabeach scores of examples of the work of these
borers. In Europe some kinds of slugs and snails do great damage in
gardens, but we have little to complain of in this respect.
Largely dependent on moisture, the young snails that are hatched in
midsummer at once seek retreats, and may be looked for under leaves,
logs, and loose stones in the woods and pastures. Most American
snails are solitary, and will be found lurking in the moss beside
mountain brooklets--a favorite spot for the glassy vitrinas--hiding
in the crevices of rocky banks and old walls, crawling at the edge
of swampy pools, creeping in and out of the crannies of bark on aged
trees, or clinging to the underside of succulent leaves. Some forms,
very beautiful in their ornamentation when magnified, are so minute
that they might be encircled by the letter o in this type, yet you
will soon come to perceive them amid the grains of mud adhering to the
undersurface of a soaked chip or rotten log.
For fresh-water species various resorts are to be searched. Go to
the torrents with rocky bottoms for the paludinas and periwinkles
(Melania); to quiet brooks for physas and coil shells (Planorbis); for
limneas to the reeking swamps and weedy ponds. By pulling up the weeds
gently, you may get small species that otherwise easily escape your
dipper or net. In the Southern States and in the tropics certain forms
are to be picked off bushes and mangrove trees like fruit, especially
the round "apple snails" (Ampullaria) as big as your fist.
SEA SHELLS IN NATURE AND ART
Other familiar forms of gastropods are the limpets, keyhole and
half-deck; the abalones, so much used in the making of ornaments; and
the many small sorts of "periwinkles" studding the rocks and hiding
among the seaweeds of every coast. Then there are the pyramidal top
shells (Trochus), the bulging, wide-mouthed turbans (Turbo), and the
open-whorled wentletraps (Scalaria) which years ago were so rare that
collectors paid $100 or more for a good specimen. The two former
kinds are on sale in all seaside shops, with the natural rough brown
exterior ground away until they gleam outside in the prismatic glory
of the nacre layers that lie underneath. A group of heavy shells of
carnivorous tropical mollusks furnishes ornaments for the mantelshelf
also. These include the knobby volutes, often richly colored in
marbled patterns or in spiral rows of round spots; the olives, whose
ovate shells are sometimes dark purple, sometimes beautifully marked,
and always glossy, because enfolded during life inside flaps of the
mantle that completely protect them; the miters, that take their name
from their resemblance in shape to the headdress of a bishop, and
show splendid decorations in tints of red and orange; and the strong,
spiny murexes, a small Mediterranean species, which is the principal
source from which the ancients derived their Tyrian purple dye--a
coloring matter yielded by treatment of the blood of many species,
including one of the commonest little mollusks (Purpura) on our own
coast, which old-fashioned New Englanders yet utilize sometimes for
making an indelible ink for marking clothing. To this family belong the
"drills" that destroy thousands of dollars worth of oysters annually
in Long Island Sound by boring through them. Near relatives are the
whelks (Buccinum), extensively eaten in England; and two of the largest
and commonest shells on our eastern sand beaches, known to northern
fishermen as "winkles" and along the southern coast as "conchs."
These (Fulgur and Sycotyphus) are big, pear-shaped creatures with
chalky white shells that crawl about near shore, seizing and devouring
anything they can overcome, and working havoc on planted oyster beds;
they deposit their eggs in parchmentlike capsules shaped like gun wads
and connected into a long chain that are often thrown up on the beach,
where they are called sea necklaces.
Of great beauty in their rich variety of color and pattern are the
tropical cone shells, of which a large number of species are known,
some so rare as to bring great prices in the conchological market.
Their bite is poisonous. Equally numerous in species are the charmingly
decorated auger shells, some (Pleurotoma) spindle-shaped, others
(Terebra) that would serve as models for a church spire. Near them is
classified that white mollusk (Natica) whose globular shell is perhaps
the commonest relic of the sea seen on our northern beaches, and
sometimes is as large as a man's fist; to it belong the curious "sand
saucers" to be found in August, which contain its eggs. These naticas
are predatory, and burrowing their way through the loose sand come upon
and devour other shellfish, boring a circular, nicely countersunk hole
through their armor and feeding on its inmate; their depredations on
the northern oyster beds are a serious matter.
Well known and always admired are the cowries, smooth, brightly colored
shells, shaped like an olive with a gash down the length of one side.
This long and narrow aperture is usually toothed, and it is only in the
young that any indication of a typical spiral growth is discernible.
The money cowrie of Africa is small and cream-white.
Lastly a word must be said about the largest of known gastropods, the
big "conchs" or wing shells (Strombus), the helmet shells (Cassis),
and the tuns (Dolium). They are West Indian. The species most commonly
seen in the United States, forming a border for flower beds in seaside
villages, is _Strombus gigas_, with a delicate orange-red or pink
interior, from which are cut most of the shell cameos offered to art
lovers. This shell, like the great spiral triton of the South Seas, is
also converted into a horn much used in foggy weather by the spongers
and small coasters of Floridian and West Indian waters. The helmet
shell, a heavy, rounder and smoother mollusk than the Strombus, is also
extensively used in cameo cutting, especially the African black helmet,
in which a white outer layer covers an almost black underlayer on the
broad lip. Dolium has a large, globose but thin shell, ornamented with
revolving ribs.
The class Scaphopoda is composed of a single family (Dentalidæ) known
as tusk shells, because the little shells, one to two inches long, are
shaped like an elephant's tusk, open at both ends. The structure of the
occupant is so singular, the animal lacking head, heart, gills, and
some other ordinary features, that naturalists believe it is a hopeless
degenerate. One of the species of the Pacific coast is famous as the
shell strung as ornaments and serving practically as money among the
northwestern Indians until very recent times, under the name "hiqua."
NAUTILUS, DEVILFISH, AND SQUID
We have now arrived at the last and highest division of the
Mollusca--the Cephalopoda, the class of the nautilus, ammonite, and
other fossil forms, and of the squid, cuttles, and octopuses of our
modern seas. The cephalopods are very different in shape, activity, and
in their higher organization and intelligence, from other mollusks,
but their general anatomy is the same. The special characteristic,
as indicated by the name, is the fact that the head is surrounded by
tentaclelike extensions of the "foot," which is here fused in part
with the head, and divided into the long "foot arms," which are the
instruments by which these predatory creatures obtain their prey. The
underpart of the foot forms a tube called the funnel (or siphon).
Through the funnel the animal expels water from the mantle cavity, and
thus propels itself through the water. When the siphon is in its normal
position the animal swims backward; but it can be turned back over
the edge of the mantle, giving a forward movement. In cephalopods the
sexes are separate, the male being often much smaller than the female.
The eggs are usually laid in gelatinous capsules, commonly known in
New England as "sea grapes," and the development is direct, that is,
without any free-swimming larval stage.
The class is divided into two subclasses: 1. _Tetrabranchiata_,
cephalopods with four plumelike gills inside the mantle; and 2.
_Dibranchiata_, with only two such gills. In the first subclass belong
all those very ancient cephalopods called in a general way ammonites,
goniatites, orthoceratites, etc., that are found in such great numbers
and astonishing variety in the Paleozoic rocks, from the Ordovician
age onward, although but few groups survived beyond the Carboniferous
period, and only two families can be traced as high as the Tertiary
deposits, one of which--that of the nautilus--survives to the present
day as the final remnant of one of the conspicuous and interesting
populations of the primitive ocean.
The pearly or chambered nautilus is one of several species inhabiting
the East Indies and the coral region of the South Pacific seas,
creeping along the bottom in deep water, most numerously at the
depth of about 1,000 feet. Hence the animal is not often taken
alive, although the smoothly coiled and handsome shells are cast
on the beaches in great numbers; and little is known of its habits
or embryology. It is a soft lumpish sort of creature, with a great
number of short arms and tentacles around the mouth, none armed with
suckers. It begins life as a mere globule covered by a minute hood of
shell; but presently, growing too large for this hood, it enlarges it
by additions to the rim, and then forms behind its body a partition
(septum) across the shell, cutting off the part in which it was born.
As growth advances, this enlarging and partitioning continues until the
nautilus has attained its full size. Then, as before, it occupies only
the outermost chamber, behind which the whole interior of the shell is
divided by the septa into chambers, abandoned and empty, but filled
with a gas that buoys it up in the water. Oriental artists are fond of
grinding away the dull exterior of the shell and exposing the gleaming
nacre underneath; and of carving in this mother-of-pearl picturesque
designs, examples of which are often to be seen in curiosity shops.
This is not only the last remnant of the great group of ancient
nautiloids, but one of the smallest, for some of the Paleozoic coiled
forms were as big as a washtub, and the straight ones were often six
feet long.
The Dibranchiata, on the other hand, are comparatively modern, as their
ancestry dates back only to the Trias, and our seas still harbor a long
list of living representatives. This subclass has two divisions: 1.
Octopoda--octopods, the eightarmed argonaut and other octopuses; and
2. Decapoda--decapods, the ten-armed cuttlefishes, or calamaries, and
the squids.
[Illustration: THE CHAMBERED NAUTILUS--SECTION
_H_, Head. _T_, Tentacles. _E_, Eye. _M_, Muscles. _S_, Shell. _A_,
Air Chambers]
The octopods have a saclike body with eight arms of about equal size,
in some kinds thick and short, in others long and snaky. Every arm has
along its underside a double row of round, muscular suckers without
horny rims; and whatever is seized by one or more of these arms is
drawn into the mouth at their base, where it is bitten by a beaklike
jaw of sharp horn, and further devoured by means of a toothed tongue
similar to the radula of gastropods. Nearly all are tropical, but some
species exist in deep water considerably to the northward. Certain
species are used as food in many parts of the world, and are considered
a delicacy in Italy and other Mediterranean countries. The fishermen
of Japan and the Philippines capture them by the simple process of
lowering big earthen urns and leaving them on the bottom overnight;
when they are hauled up in the morning many will contain entrapped
devilfish, as sailors call them, which at once go to market.
A very singular octopod is the little argonaut, or "paper sailor." Its
body is not larger than a walnut--that is the body of the female, for
the male is only a tenth of that bigness. Its home is mainly in the
tropics and in deep water, but in the summer spawning season it rises
to the surface, and is occasionally met with far northward on the Gulf
Stream, drifting, apparently, in a snug little boat. The two dorsal
arms are expanded into broad, roundish membranes at their ends, and old
stories said that they were used as sails--a supposition of much use to
poets; but the "boat," shaped somewhat like the shell of the nautilus,
is not a shell proper, but a membranous pouch secreted by the mantle in
spawning time, and not vitally attached to the body, but held in place
beneath it by the two broadened arms, and serving as a receptacle for
eggs and a cradle for the embryos hatching from them.
Turning now to the Decapoda, we treat of things much nearer home and
familiar on both sides of the continent, for these are the cuttlefish
and squids, none of which have an external shell, but possess an
interior brace to their muscles either of lime or of chitin. The
cuttlefish proper, or calamaries, are those of the family Sepiadæ,
which have an oval, flattened body bordered by a fin; and two of the
ten arms are, in the female, in the form of long, slender tentacles.
In addition to being edible and easy to get, as they stay near shore,
their calcareous back brace is the "cuttlebone" fed to cage birds; and
they furnish the substance from which the drawing ink called "sepia"
is made--principally in Rome. This is a brownish black liquid that the
animal jets out through its siphon when it thinks itself in danger in
order to make an inky cloud in the water behind which, as a sort of
smoke screen, it may run and hide. Other cephalopods use this means of
escape.
The squids, however, are all elongated in shape, and have finlike
expansions of the mantle only on the tail. Two of their arms are long
and slender, and are broadened at the tips, and studded with suckers.
These suckers in some squids are strengthened by a horny rim, or by
recurved hooks, or by both. The eyes are large, perfectly formed,
and as serviceable as those of the fishes on which they prey. These,
and some other animals, including small ones of their own kind, they
capture by darting backward, swinging quickly to one side and seizing
the victim in their sucker-bearing arms. They themselves are devoured
by whales, seals, and many kinds of fishes; and enormous quantities
of squids of various species are annually collected by fishermen for
use as bait in the Newfoundland fisheries. In place of the calcareous
cuttlebone of the sepia the squids have their bodies stiffened by an
internal strip of chitinous substance called the "pen."
Squids are of all sizes from an inch to twelve feet in length; then
there is a surprising jump to the giants (Architeuthis) of the North
Atlantic, which, when the tentacles are stretched out in front, may
measure seventy-five feet from tip to tail. These are little different
in structure or habits from their smaller brethren that exist in so
many species near all coasts and throughout the midseas right around
the globe; but their huge size makes them fit antagonists of the sperm
whale, which hunts them, and whose hide often bears a record, left by
their powerful suckers, to show how hardly some big squid struggled
for life. These monsters are the greatest invertebrates known in
present or past time; and it is probable that the long wriggling arms
of one and another, glimpsed at the surface, may account for some of
the sea serpent stories brought home by apparently perfectly honest
sailors, especially those which in many cases recount that the supposed
"serpent" was in conflict with a whale. Carcasses of these gigantic
squids are occasionally cast on the shores of Labrador and Greenland.