MOLLUSCA: THE MOLLUSCS
THE FRESH-WATER MUSSEL (_Unio_ sp.)
=Structure= (fig. 103).--TECHNICAL NOTE.--The fresh-water or river
mussel lives commonly in the streams and lakes or ponds in the
United States. It frequents muddy or sandy bottoms. Specimens can
often be secured with a long-handled rake from the shore or picked
up in shallow streams with the hand. If possible to keep the animals
alive until ready for use, some of their habits may be observed.
Place them in a tub or trough with water and mud; when they have
settled themselves put some powdered carmine, starch, or similar
substance in the water near them, and note the water-currents.
Living mussels which have been placed in a dish with mud several
inches deep and covered with water will be seen to travel in a
definite direction. The end which is in front is the head end. Note
the process of thrusting out and retracting the fleshy _foot_ which
extends between the two _valves_ of the _shell_. Note that the two
valves are held together along the upper, or dorsal, surface by a
horny structure, the _hinge-ligament_. Note near the hinge-line a
prominence (_umbo_) in each valve from which extends a series of
concentric lines of growth. The umbo is the oldest part of the valve.
Note at the lower edge of the valves a soft membrane with a fringe
along its free border. This is the edge of the _mantle-lobes_, flaps
of the body-wall which cover the body and which aid in the functions
of respiration and nutrition.
TECHNICAL NOTE.--Specimens which are to be dissected should be
killed by dropping them for a few seconds into warm water, when
the muscles will relax enough so that a chip may be thrust between
the valves. If specimens are to be kept for some time before
dissecting they should be preserved in alcohol or 4% formalin. In
a dead specimen carefully remove the left valve. This is
accomplished by slipping in a thin knife-blade close to the inner
edge of the left valve and carefully cutting the two large
adductor muscles which bind the valves together. The dissection
should be made under water.
Before the removal of the valve, as just described, notice a portion
of the mantle adhering to the inner face of the valve, along a line of
attachment indicated by a crease. This is the _pallial line_. After
the left valve has been removed, the mantle being carefully separated
from it, note the large conical projections from the valves, the
_hinge teeth_, which fit into each other. Note the large muscle
impression just in front of the hinge-teeth; this is the point of
attachment of the _anterior adductor muscle_, while just behind and
adjoining it is the impression of the _anterior retractor muscle_.
Note posterior to the adductor and below the retractor a small
impression which affords attachment for the _protractor muscles_ of
the foot. At the other end of the valve, note the large impression of
the _posterior adductor muscle_ with the impression of the small
_posterior retractor muscle_ just above it.
TECHNICAL NOTE.--Lift back the left mantle-lobe, thus exposing the
body parts underneath.
Note the projecting muscular foot, the movements of which are governed
by the retractor and protractor muscles attached to the impressions
just mentioned. Note a pair of flattened plate-like structures
composed of thin, ribbed, membranous folds. These are the _gills_.
Note just beneath the anterior adductor muscle a small opening leading
into the soft _visceral mass_ of the body. This is the _mouth_. Note
near the mouth two pairs of plate-like structures much smaller than
the gills. These are the _labial palpi_, and it is by their action
that food-particles which have been brought in with the water are
conveyed to the mouth. Note at the posterior part of each mantle-lobe
a fringed portion which, together with a corresponding part on the
other side, forms the _inhalant siphon_. The cilia of the fringes
carry water and food-particles into the space enclosed by the
mantle-lobes; this space is the _mantle-cavity_. After the food has
been taken out and the water has passed through the finely striated
gills it is collected in a common cavity which extends above the two
sets of gills on each side. This space is called the _supra-branchial
cavity_. This cavity is continuous posteriorly with a space between
the right and left mantle-lobes, which is connected with the exterior
by an opening above the inhalant siphon called the _exhalant siphon_.
The function of the gills is partly to produce currents of water
carrying the food to the mouth, and partly respiratory. The mantle is
an important organ of respiration.
Make a drawing showing the organs described.
TECHNICAL NOTE.--Carefully cut away the mantle and gills from the
left side, and also the labial palpi, being careful not to disturb
the visceral mass.
Note two openings along the line where the gills and foot come together.
The uppermost is the opening of the ureter giving exit to the excretion
from the kidneys; the lower is the opening of the duct from the
reproductive organs and is called the _genital aperture_. The products
from both of these organs are carried out through the exhalant siphon.
Note that the mouth leads by a short tube (_œsophagus_ or _gullet_)
into a large cavity, the _stomach_, which is surrounded by a greenish
mass, the _digestive gland_.
TECHNICAL NOTE.--Carefully cut the delicate covering of the dorsal
portion of the visceral mass and expose a cavity.
The cavity thus exposed is the _pericardium_. Note within the
pericardium a long tube extending through it. This is a portion of the
alimentary canal, the _rectum_, which opens posteriorly through the
_anus_ into the _supra-branchial chamber_. Note a muscular sac about
the rectum midway of its course through the pericardium. This is the
unpaired _ventricle_ of the _heart_. Attached to each side of the
ventricle are thin-walled sacs, the _right_ and _left auricles_, which
are entered by fine blood-vessels, the _efferent branchial veins_,
from the right and left gills. The blood brought through these
blood-vessels from the gills flows into the auricles and from them
into the unpaired muscular ventricle, from which it is forced
anteriorly and posteriorly through two main arteries, the _anterior_
and _posterior aortas_, to all parts of the body. After bathing the
body-tissues the blood is collected into a median longitudinal vein
beneath the pericardium called the _vena cava_. From the vena cava the
blood passes through the kidneys and gills to be returned at last to
the heart. The mantle acts as an organ for the aeration of the blood,
and the blood it receives or at least part of it passes directly back
to the heart without passing through the kidneys and gills.
Note the delicate membranous dark-colored sac on the floor of the
pericardium, the _kidneys_ or _nephridia_. These are paired structures
which appear as two U-shaped tubes lying side by side. Each consists
of a lower portion with thick folded walls, the kidney proper, and an
upper thin-walled portion, the _ureter_. The kidneys open internally
through a pair of _reno-pericardial_ openings into the pericardium,
while the ureters communicate with the mantle-cavity by an opening on
the side of the body beneath the gills as already mentioned. The
kidneys are profusely supplied with fine blood-vessels and carry off
the waste matter from the blood.
Beneath the posterior adductor muscles note a small white
spider-shaped body, the more or less united _visceral ganglia_ of the
nervous system. Posteriorly these ganglia give off nerves to the
mantle and gills, while anteriorly there proceed two nerves, the
_cerebro-visceral connectives_, running forward, one on either side of
the foot close to the visceral mass, to the _cerebro-pleural ganglia_,
paired ganglia lying near the mouth. A delicate commissure running
over the gullet connects these ganglia.
TECHNICAL NOTE.--Cut away the skin and outer muscular layer from
the left side of the foot.
Note the large stomach-cavity, surrounded by the digestive gland. Trace
the convolutions of the alimentary canal through the foot to the anal
exit. Note in the anterior portion of the foot a fused pair of ganglia
similar to the visceral ganglia. These are the _pedal ganglia_, which
are connected by a pair of delicate commissures, the _cerebro-pedal
connectives_, with the cerebro-pleural ganglia. Note the glandular
tissue which fills the cavity of the foot and surrounds the loops of the
alimentary canal. This is the reproductive organ, which has its exit
beneath the gills on each side of the foot. The sexes of the mussel are
separate, but the reproductive organs are very similar.
=Life-history and habits.=--The eggs (ova) of the female pass first
into the supra-branchial chamber, whence, after being fertilized, they
drop into the outer pair of gill-chambers. These outer gills serve as
brood-pouches, and here it is that the embryonic stages are passed
through. The embryo when ready to issue has a soft body enclosed in
two triangular valves. At this stage it is called a _glochidium_. The
glochidium on being discharged through the exhalant siphon of the
parent falls to the bottom, where it remains for a time, when it
attaches itself to some fish by the lower hook-like projections of
the valves and leads a truly parasitic life for two months, after
which it undergoes a metamorphosis and falls to the bottom again,
there to begin an independent existence. Mussels often congregate in
favorite mud or sand banks. Their food consists primarily of small
organisms, both plants and animals, which are taken from the water
entering the mantle-cavity. Mussels move about slowly over the muddy
bottom of the stream by means of the muscular foot.
OTHER MOLLUSCS.
The branch Mollusca includes the fresh-water mussels, the clams,
oysters, snails, and slugs, the cuttlefishes, and all that host of
animals we call "shells" or shell-fish, which we know familiarly only
by the shell which they make, live in, and leave at death to tell the
tale of their existence. Not all the molluscs, however, form shells,
that is, external shells which serve as houses. The familiar slugs do
not, nor do a number of ocean forms called nudibranchs, which are
somewhat like the land-slugs, only much prettier and more attractive.
All the cuttlefishes and octopi are also without the hard calcareous
shell. But most of the molluscs are shell-bearing animals. The shell
may be bivalved, as in the mussel and clam, or univalved, that is,
composed of a single piece which may be spirally twisted, as with the
snail, or otherwise curiously shaped. The variety in the form, colors,
and markings of the shells indicates the great diversity among
molluscs. Molluscs live on land, in fresh water and in the ocean. No
depths of the ocean abysses are too great for the octopi, no coast but
has its many shells, hardly a pond or stream is without its mussels
and pond-snails, and in all regions the land-snails and slugs abound.
=Body form and structure.=--The molluscs are not to be mistaken for
any other of the lower animals; they have a structure peculiarly their
own. In them the body is not articulated or segmented as with the
worms and arthropods, nor radiate as in the echinoderms, nor
plant-like as with the sponges and polyps. (Where the typical
molluscan body is well developed it is composed of four principal
parts: a head, with the mouth, feelers, eyes, and other organs of
special sense; a trunk containing the internal organs; a foot which is
a thick muscular mass not at all foot- or leg-like in shape, but which
is the organ of locomotion by means of which the mollusc crawls; and a
mantle which is a fold of the skin enclosing most of the body and
which produces the shell. Such a typical molluscan body is possessed
by most of the snails. But in most of the other molluscs one or more
of these four body-regions are so fused with some other region as to
be indistinguishable. In the mussels and clams the head is not at all
set off from the rest of the body, the cuttlefishes and octopi have no
foot, the slugs have no shell. In the case of some of the molluscs
without external shell there are inside the body the rudiments or
vestiges of a shell.
With regard to the internal organs we note the constant presence of
three pairs of ganglia, viz., the brain, lying above the pharynx,
which sends nerves to the feelers, eyes, and auditory organs; the
pedal ganglion, which sends nerves to the foot, and the visceral
ganglion, which sends nerves to the viscera. This is a condition of
the nervous system characteristic of all molluscs. The heart is a
well-developed pulsating sac in the upper part of the body composed of
either two or three chambers, and there is a well-defined closed
system of arteries and veins, specially complete in the cuttlefishes
and octopi. This highly developed condition of the circulatory system
also distinguishes the molluscs from the other invertebrates.
=Development.=--Reproduction among the molluscs is always sexual.
Multiplication by budding or by the parthenogenetic production of eggs
is not known to occur. The eggs are usually laid in a mass held together
by a gelatinous substance. In most species the young mollusc on hatching
from the egg does not resemble its parent, but is a free-swimming larva
called a _veliger_. It is provided with cilia for organs of locomotion.
It must undergo a radical change in order to reach the adult stage. Thus
metamorphosis occurs in this branch as well as among the Arthropods and
Echinoderms. In the development of some molluscs, however, there is
little or no metamorphosis, the young being hatched in a condition much
resembling, except in size, the parent.
Some of the special characteristics of structure, life-history, and
habits of the molluscs will be noted in our consideration of the
various kinds.
=Classification.=--The branch Mollusca is divided into five classes,
three of which include the more familiar kinds. These three classes
are the Pelecypoda, including the mussels, cockles, clams, scallops,
oysters, etc., molluscs with a shell composed of two pieces, one on
each side of the body and hinged together; the Gastropoda, including
the snails, slugs, periwinkles, whelks, and a host of other univalved
shell-fish, that is, molluscs which have a shell composed of a single
piece; and the Cephalopoda, including the squids, cuttlefishes,
octopi, and the pearly nautilus.
=Clams, scallops, and oysters (Pelecypoda).=--TECHNICAL
NOTE.--Shells of scallops, oysters, and sea-mussels should be had
for examination; also specimens of _Teredo_ or _Pholas_ in alcohol
or formalin, and pieces of pile bored by _Teredo_. Make drawings
of various bivalve shells, and of _Teredo_.
[Illustration: FIG. 103.--Dissection of fresh-water mussel, _Unio_ sp.]
The fresh-water mussel which we have studied is an example of the
bivalve molluscs. The members of this class show a range in size
from the little fresh-water _Cyclas_ about 1 cm. long to the giant
clam of the Indian and Pacific islands "which is sometimes 60 cm. (2
feet) in length and 500 pounds in weight." They show also some variety
in the form and appearance of the shell, but not anything like the
degree of variety shown by the shells of the Gastropods.
The edible clams are of several different species. The hard-shell clam
(_Venus mercenaria_), or "quohog" as it is often called, is found
along the Atlantic coast from Texas to Cape Cod. It is "common on
sandy shores, living chiefly on the sandy and muddy plots, just beyond
low-water mark.... It also inhabits estuaries, where it most abounds.
It burrows a short distance below the surface, but is frequently found
crawling at the surface with the shell partly exposed." The shells of
this edible clam are white. The soft-shell clam (_Mya arenaria_), "the
clam _par excellence_, which figures so largely in the celebrated New
England clam-bake, is found in all the northern seas of the world....
All along the coasts of the eastern States, every sandy shore, every
mud flat, is full of them, and from every village and hamlet the
clam-digger goes forth at low tide to dig these esculent bivalves. The
clams live in deep burrows in the firm mud or sand, the shells
sometimes being a foot or fifteen inches beneath the surface. When the
flats are covered with water his clamship extends his long siphons up
through the burrow to the surface of the sand, and through one of
these tubes the water and its myriads of animalcules is drawn down
into the shell, furnishing the gills with oxygen and the mouth with
food, and then the water charged with carbonic acid and fæcal refuse
is forced out of the other siphon. When the tide ebbs the siphons are
closed and partly withdrawn." Ocean clams and mussels have furnished
food for man for ages, and along coasts are found here and there
great mounds made of heaps of clam-shells which have become covered
over with soil and vegetation. Such mounds are the old feasting-places
of the early coast inhabitants, and the archæologist often finds in
these "kitchen-middens," as they are called, various relics of the
early natives of the continent.
[Illustration: FIG. 104.--A group of marine Pacific Coast molluscs; in
upper left-hand corner, _Purpura saxicola_; next to the right,
_Littorina scutulata_; farthest to right, limpets, _Acmara spectrum_;
left-hand lower corner, _Mytilus californianus_; in right-hand lower
corner the black shells just above the large clam-shell, _Chlorostomum
funebrale_. (From living specimens in a tide pool in the Bay of
Monterey, California.)]
Even more widely known that the clams are the oysters (_Ostrea
virginiana_), also members of this class of molluscs. The oyster is
carefully cultivated by man in many countries. It has its two shells
or two shell-halves dissimilar, one valve being hollowed out to
receive the body, while the other is nearly flat. The oyster is
attached to the sea-bottom by the outside of the hollowed-out valve.
When first hatched the young oyster swims freely by means of its
cilia; after a few days it attaches itself to some solid object and
grows truly oyster-like. Much care has to be taken in cultivating
oysters to furnish proper conditions for growth and development. The
young oysters when first attached are called "spat"; when a little
older this "spat," now called "seed," may be transplanted to new beds,
which are stocked in this way. In fact some beds have constantly to be
thus restocked, the young oysters produced on them not finding good
places to attach themselves, and so swimming away. Sometimes pieces of
slate, pottery, etc., are strewed about the oyster-beds to serve as
"collectors," that is, as places for the attachment of the young
oysters. The extent of the acreage of the American oyster-beds is
larger than that of any other country. "The Baltimore oyster-beds on
the Chesapeake River and its tributaries cover 3,000 acres, and
produce an annual crop of 25,000,000 bushels."
[Illustration: FIG. 105.--_Dactylus_ sp., a mollusc, excavating granite.
(Photograph by C. H. Snow; permission of Amer. Soc. Civil Engineers.)]
[Illustration: FIG. 106.--_Pholas_ sp., a mollusc, burrowing in
sandstone. (Photograph by C. H. Snow; permission of Amer. Soc. Civil
Engineers.)]
The "pearl-oyster" is not a true oyster, that is, not a member of the
family to which the edible oysters belong, but it is a member of the
same class, that is, it is a bivalve mollusc. Pearls are obtained from
a number of different "pearl-oysters," but the finest pearls and
mother-of-pearl come from the tropical species _Meleagrina
margaritifera_. This pearl-oyster "has an extensive distribution,
being found in Madagascar, the Persian Gulf, Ceylon, Australia,
Philippine Islands, South Sea Islands, Panama, West Indies, etc."
Mother-of-pearl is simply the inner lining of the shell, which is
composed of numerous thin layers of carbonate of lime so arranged that
the edges of the successive layers produce many fine striæ very close
together. The beautiful iridescence of this inner shell-lining is
caused by the complicated diffraction and reflection (interference
effects) of the light by the fine striæ and the translucent superposed
thin plates of shell material. Pearls are simply isolated deposits of
shell material usually around some particle of foreign substance which
has found lodging in the mantle-cavity. Sometimes small objects are
purposely introduced into the shell in order to stimulate the
formation of pearls. The pearl-fishers go out in boats and dive to the
bottom, filling baskets with pearl-oysters. These are piled up in a
bin and left to die and decompose. "When the flesh is pretty
thoroughly disintegrated, it is washed away with water, great care
being taken that none of the pearls loose in the flesh are lost. When
the washing is concluded the shells themselves are examined for pearls
which may be attached to the interior of the valves." The principal
pearl-fishery is that on the coast of Ceylon; pearl-fishing has been
carried on here for over 2000 years.
[Illustration: FIG. 107.--_Martesia xylophaga_, a Pholad, in Panama
mahogany. (Photograph by C. H. Snow; permission of Amer. Soc. Civil
Engineers.)]
The ship-worm (_Teredo_) is an interesting member of this class of
bivalve molluscs, because of its unusual habits, and strangely
modified body form. The teredo is long and worm-like in general
appearance, with a small bivalve shell at one end and two elongated
siphons at the other. The young teredo is a free-swimming ciliated
embryo like the young of the other bivalve molluscs, but it soon
settles on a piece of submerged wood, usually the pile of a wharf, or
the bottom of a ship, and burrows into this wood. As it grows it
enlarges and deepens its tube-like burrow, and lines it with a
calcareous deposit. The burrow may be a foot long or longer, and when
thousands of teredos attack a pile or the bottom of a ship, the wood
soon becomes riddled with holes. These boring molluscs do great
damage to wharves and ships. In Holland where they were first
discovered they caused such injuries to the piles and other submerged
wood which supported the dikes and sea-walls that they seriously
threatened the safety of the country.
[Illustration: FIG. 108.--The giant yellow slug of California,
_Ariolimax californica_. This slug reaches a length when outstretched
of 12 inches. (From living specimen.)]
=Snails, slugs, nudibranchs and "sea-shells"
(Gastropoda).=--TECHNICAL NOTE.--Pond-snails can be readily found
clinging to submerged stems, leaves, or pieces of wood in almost any
pond. Collect some and carry alive, in a jar of water, to the
schoolroom. Observe the habits of these live snails in the school
aquarium. Note the movements, the coming to the surface to breathe,
the eating (by scraping the surface of the leaves with the "radula"
or tongue; provide fresh bits of cabbage or lettuce-leaves), the use
of the feelers. Make drawings illustrating these habits. Examine the
shell; note that it is univalved, that is, composed of one piece. Do
the whorls of all the shells turn the same way? Make a drawing of
the shell, naming such parts as the apex, spire (all the whorls
taken together), the aperture, the columella (the axis of the
spire), the lip (outer edge of the aperture), the lines of growth
(parallel to the tip), the suture (the spiral groove on the
outside). Examine the snail; note the character of the foot; note
the protrusible tentacles or feelers, the eyes (dark spots at bases
of the tentacles), the mouth, the respiratory opening (on right side
of body in the edge of the mantle which protrudes beneath the lip
when the snail's body is extended), the radula or ribbon-like tongue
with fine teeth. Compare with the body of the mussel.
Slugs may be found during the day concealed under boards or
elsewhere; they are nocturnal in habit. If specimens can be
obtained, compare with the pond-snails, noting the absence of a
shell, and the fleshy mantle on the dorsal surface near the head;
note the presence of two pairs of tentacles (the eyes being at the
tips of the second or hinder pair), and the respiratory pore. Note
the streak of mucus left by the slugs in crawling about.
Some sea-shells can be got from private collections of "curios" to
illustrate the variety of form of the univalve shells.
Perhaps one-half of all the known species of molluscs are snails and
slugs (fig. 108). Snails are either aquatic or terrestrial in habit,
but in either case they (the true pulmonate snails) breathe not by
means of gills, as do most of the other molluscs, but by means of a
so-called "lung." This lung is a sac with an external opening on the
right side of the body and with its inner surface richly furnished
with fine blood-vessels. The exchange of gases between the blood and
the outer air takes place through the thin walls of the blood-vessels.
Most snails which live in the water, as the pond-snails and the
river-snails, have to come occasionally to the surface to breathe.
These fresh-water and land-molluscs which possess a lung-sac instead
of gills constitute the order Pulmonata. The pulmonate pond- and
land-snails and slugs are vegetable feeders and where they occur in
large numbers do much injury to vegetation. While the common
pond-snails have but one pair of feelers, at the base of which are
found the eyes, most of the land-snails and slugs have two pairs of
"horns," the eyes being on the tips of the second pair. The lung-sac,
besides serving as a breathing organ, also enables the snail to rise
or sink according as the animal varies the size of the sac and
consequently the amount of air in it. All the Pulmonata are
hermaphroditic, each individual producing both sperm- and egg-cells.
The eggs of the pond-snail "are laid in gelatinous transparent
capsules, half an inch to an inch in length, flattened and linear or
oblong in outline. After a few snails have been kept a short time in a
small vessel of water with their appropriate food, these egg-capsules
may be looked for on the bottom and sides of the vessel or closely
adherent to the stems or leaves of plants placed in the water. They
are so transparent as to be easily overlooked." Young snails may be
reared from these eggs.
There are other snails common in ponds, also called, like the
pulmonate forms, pond-snails, which have gills and no lung-sac. These
pond-snails belong to a different order of molluscs, and live on the
bottom of the pond, crawling about in the soft mud and feeding on
animal instead of vegetable food.
The shells of the various kinds of snails vary much. In many of the
land-snails the spiral is not spire-shaped or conical, but is flat. In
some the whorls of the spiral run from left to right (dextral) when
the shell is looked at with apex held toward one, while in others the
whorls run from right to left (sinistral).
[Illustration: FIG. 109.--Three Pacific Coast nudibranchs; _Doris
tuberculata_ (in lower left-hand corner), _Echinodoris_ sp. (upper
one), and _Triopha modesta_ (at right). (From living specimens in a
tide-pool on the Bay of Monterey, California.)]
Of the hosts of marine Gastropods we can notice only a few kinds. The
nudibranchs (fig. 109) are a group of beautiful forms in which the
shell is wholly wanting and the mantle is usually absent. The gills
are thus exposed and are usually in the shape of delicate freely
projecting tufts arranged in rows along the back. The body is often
strikingly and variedly colored. These soft, naked "sea-slugs" live
near the shore, creeping about among the rocks and seaweeds. About a
thousand species of nudibranchs are known.
Among the shell-forming marine Gastropods there is great variety in
the size and shape and coloring of the shells. Many are beautifully
colored and patterned; others are oddly and fantastically shaped. The
cowries, or porcelain shells, familiar in collections of ocean
curiosities, have a large body whorl and a very short flat spire, and
the brightly colored shell looks as if enamelled. Some of the coast
tribes of Africa once used, and perhaps still use to some extent,
cowries as money. The limpets (fig. 104) are among the most abundant
of the seashore molluscs, their low, broadly conical shells being
plentifully scattered over the rocks between tide-lines. The
"oyster-drills" are Gastropods with odd spiny shells which do much
harm in oyster-beds by settling down on the oysters, boring holes
through the shells and eating the soft parts within. The
helmet-shells, from which shell cameos are cut, are composed of layers
of shell material of different colors. Among the specially beautiful
shells are the cone-shells, the olive-shells, the ivory-shells, etc.
=Squids, cuttlefishes, and octopi (Cephalopoda).=--TECHNICAL
NOTE.--Small squids preserved in alcohol or formalin can be had of
all dealers in biological supplies (see p. 453), and specimens
should be examined.
The squids (fig. 110), cuttlefishes, octopi or "devil-fishes," and the
three living species of _Nautilus_ constituting the class Cephalopoda
are very different from the other molluscs in appearance, and are in
fact different in important structural characters. They can move
swiftly, have strangely modified organs of prehension, strong biting
mouth-parts, and eyes of very complex organization. They are the most
highly organized molluscan forms, and their predaceous habits and the
great size to which some of them attain have given them distinction
among the fierce and dangerous creatures of the sea. They are all
strictly marine in habitat, and are all carnivorous. Most of them have
no shell, or where the shell is present it is internal in all but a very
few forms. The tentacle-like arms or feet surrounding the mouth which
occur in all the Cephalopods are provided with sucking organs or
suckers, in some cases with a horny toothed rim. These long, powerful,
grasping, tentacular feet, with the suckers and five hooks, are very
effective means of securing prey, and the pair of strong, sharp, cutting
mandibles or beaks are equally effective in tearing to pieces. The eyes
of the Cephalopods are almost as highly developed as those of the
vertebrates. They are unusually large and staring, and add much to the
terrifying appearance of the "devil-fishes." Cephalopods have the power
of quickly changing color, because of the presence in the skin of many
pigment-cells which can expand so as nearly to touch each other, thus
producing a uniform tint over the whole body, or which can contract so
as to destroy this uniformity of color. There are several sets of these
color-carrying cells or chromatophores, each set of a color different
from the others. The purpose of this change of color is protective, the
animal being thereby able to make its color so harmonize with that of
its immediate surroundings as to become indistinguishable.
There are two principal groups of Cephalopods, viz., the Decapods and
the Octopods. The Decapods, as their name indicates, have ten feet or
arms surrounding the mouth, and in them the body is usually elongate,
containing a horny "pen" or calcareous "bone." This group includes the
cuttlefishes or sepias, from which are obtained sepia ink and the
cuttlefish bone used to feed canary birds. The ink is a secretion
which the cuttlefish discharges when attacked to create a cloud in the
water and thus escape unperceived. The squids (_Loligo_) commonly used
as bait by fishermen belong to the Decapoda. The two extra feet or
arms which the Decapods have in addition to the eight possessed by the
Octopods, differ from the others in being longer and slenderer and
having suckers only on the distal extremities which are expanded into
"clubs" (fig. 110).
[Illustration: FIG. 110.--The giant squid, _Ommatostrephes californica_.
(From specimen with body (exclusive of tentacles) four feet long, thrown
by waves on shore of the Bay of Monterey, California.)]
The Octopods have a short, sac-like, sub-spherical body and neither
external nor internal shell. To this group belong the famous
devil-fishes (_Octopus_), whose strange and terrifying appearance
combined with their frequently great size has furnished the basis for
many a weird tale of the sea. _Octopi_ have been killed having
tentacles more than 30 feet in length. The largest members of the
class, however, are probably the giant squids (belonging to the
Decapoda) specimens of which have been captured with a body-length of
twenty feet, and arms thirty-five feet long.
The beautiful paper sailor or argonaut (_Argonauta argo_), which
secretes a thin shell (not homologous with the shell of the other
molluscs) to protect her eggs, is a member of the Octopod group. In
fine weather the argonauts sail in fleets on the surface of the ocean.
The pearly nautilus (_Nautilus pompilius_) is a Cephalopod with four
gills instead of two, as with the Decapoda and Octopoda, and is the
only existing member of what was in the earlier times of the earth's
history a large group of animals. The nautili live in rather shallow
water usually creeping over the bottom feeding on small marine
animals. They make a many-chambered spiral shell with its inner
surface lined with beautiful pearly nacre.