_MOLLUSCS_
THE FRESH-WATER MUSSEL
SUGGESTIONS.—The mussel is usually easy to procure from streams and
lakes by raking or dredging. In cities the hard-shelled clam, or
quahog, is for sale at the markets, and the following descriptions
apply to the anodon, unio, or quahog, with slight changes in regard to
the siphons. Mussels can be kept alive for a long time in a tub with
sand in the bottom. Pairs of shells should be at hand for study.
=External Features.=—The shell is an elongated oval, broader and blunter
at one end (Fig. 188). Why does the animal close its shell? Does it open
the shell? Why? Does it thrust the foot forward and pull up to it, or
thrust the foot back and push? (Mussels and clams have no bones.) Does
it go with the blunt end or the more tapering end of the shell forward?
(Fig. 188.) Can a mussel swim? Why, or why not?
[Illustration:
FIG. 188.—ANODON, or fresh-water mussel.
]
Lay the shells, fitted together, in your hand with _the hinge side away
from you and the blunt end to the left_ (Fig. 188). Is the right or the
left shell uppermost? Which is the top, or dorsal, side? Which is the
front, or anterior, end? Is the straight edge at the top or at the
bottom? Our word “valve” is derived from a word meaning shell, because
the Romans used shells for valves in pumps. Is the mussel a univalve or
a bivalve? Which kind is the oyster? The snail?
Does the mussel have _bilateral symmetry_? Can you find a _horny
covering_, or epidermis, over the limy shell of a fresh specimen? Why is
it necessary? Does water dissolve lime? Horn? Find a bare spot. Does any
of the shell appear to be missing there?
[Illustration:
FIG. 189.—DIAGRAM OF SHELL open and closed, showing muscle, _m_, and
ligament, _b_.
]
The bare projection on each shell is called the _umbo_. Is the umbo near
the ventral or the dorsal line? The posterior or anterior end? Is the
surface of the umbones worn? Do the umbones rub against the sand as the
mussel ploughs its way along? How are the shells held together? Where is
the _ligament_ attached? (Fig. 189.) Is it opposite the umbones or more
to the front or to the rear? (Fig. 189.) Is the ligament of the same
material as the shell? Is the ligament in a compressed condition when
the shell is open or when it is closed? (Fig. 189.) When is the muscle
relaxed?
[Illustration:
FIG. 190.—MUSSEL crawling in sand.
]
Notice the _lines_ on the outside of the shell (Figs. 188 and 190). What
point do they surround? They are _lines of growth_. Was each line once
the margin of the shell? If the shell should increase in size, what
would the present margin become? (Fig. 191.) Does growth take place on
the margin only? Did the shell grow thicker as it grew larger? Where is
it thinnest?
=Draw= the outside of the shell from the side. Draw a dorsal view. Near
the drawings write the names of the margins of the shell (p. 98) and of
other parts learned, using lines to indicate the location of the parts.
Study the surface of the shell inside and out. The inside is called
_mother-of-pearl_. Is it of lime? Is the deeper layer of the shell of
lime? (When weak hydrochloric acid or strong vinegar is dropped on limy
substances, a gas, carbon dioxide, bubbles up.) Compare the thickness of
the _epidermal layer_, the middle _chalky layer_, and the inner, _pearly
layer_.
[Illustration:
FIG. 191.—DIAGRAM.
Change of points of attachment of muscles as mussel enlarges.
(Morgan.)
]
=Anatomy of the Mussel.=—What parts protrude at any time beyond the edge
of the shell? (Fig. 190.) The shell is secreted by two folds of the
outer layer of the soft body of the mussel. These large, flaplike folds
hang down on each side, and are called the mantle. The two great flaps
of the _mantle_ hang down lower than the rest of the body and line the
shell which it secretes (Fig. 192). The epidermis of the mantle secretes
the shell just as the epidermis of the crayfish secretes its crust. Can
you find the pallial line, or the line to which the mantle extended on
each shell when the animal was alive? A free portion of the mantle
extended like a fringe below the pallial line.
[Illustration:
FIG. 192.—CROSS SECTION OF MUSSEL. (Diagram, after Parker.)
]
The shells were held together by two large _adductor muscles_. The
anterior adductor (Fig. 193) is near the front end, above the foot. The
posterior adductor is toward the rear end, but not so near the end as
the anterior. Can you find both _muscle scars_ in the shells? Are they
nearer the ventral or the dorsal surface? The points of attachment
travelled downward and farther apart as the animal grew (see Fig. 191).
Higher than the larger scars are small scars, or impressions, where the
protractor and retractor muscles that extend and draw in the foot were
attached.
[Illustration:
FIG. 193.—ANATOMY OF MUSSEL. (Beddard.)
]
The muscular _foot_ extends downward in the middle, halfway between the
shells (Fig. 193). On each side of the foot and behind it hang down the
two pairs of =gills=, the outer pair and the inner pair (Fig. 192). They
may be compared to four V-shaped troughs with their sides full of holes.
The water enters the troughs through the holes and overflows above. Is
there a marked difference in the size of the two pairs of gills? A kind
of chamber for the gills is made by the joining of the mantle flaps
below, along the ventral line. The mantle edges are separated at two
places, leaving openings called _exhalent_ and _inhalent siphons_.
[Illustration:
FIG. 194.—MUSSEL.
_A_, left shell and mantle flap removed.
_B_, section through body.
=Question=: Guided by other figures, identify the parts to which lines
are drawn.
]
Fresh water with its oxygen, propelled by _cilia_ at the opening and on
the gills, enters through the lower or inhalent siphon, passes between
the gills, and goes to an upper passage, leaving the gill chamber by a
slit which separates the gills from the foot. For this passage, see
arrow (Fig. 194). The movement of the water is opposite to the way the
arrow points. After going upward and backward, the water emerges by the
exhalent siphon. The gills originally consisted of a great number of
filaments. These are now united, but not completely so, and the gills
still have a perforated or lattice structure. Thus they present a large
surface for absorbing oxygen from the water.
The =mouth= is in front of the foot, between it and the anterior
adductor muscle (Fig. 194). On each side of the mouth are the _labial
palps_, which are lateral lips (Fig. 195). They have cilia which convey
the food to the mouth after the inhalent siphon has sent food beyond the
gill chamber and near to the mouth. Thus both food and oxygen enter at
the inhalent siphon. The foot is in the position of a lower lip, and if
regarded as a greatly extended lower lip, the animal may be said to have
what is to us the absurd habit of using its lower lip as a foot. The
foot is sometimes said to be hatchet-shaped (Fig. 195). Do you see any
resemblance? Does the foot penetrate deep or shallow into the sand?
(Fig. 190.) Why, or why not?
[Illustration:
FIG. 195.—MUSSEL. From below. Level cut across both shells.
_Se_, palp; _P_, foot; _O_, mouth; _G_, liver; _Gg_, _Vg_, _Pg_,
ganglia.
]
The =food tube= of the mussel is comparatively simple. Behind the
mouth it enlarges into a swelling called the _stomach_ (Fig. 193). The
bile ducts of the neighbouring liver empty into the stomach. The
_intestine_ makes several turns in the substance of the upper part of
the foot and then passing upward, it runs approximately straight to
the vent (or anus), which is in the wall of the exhalent siphon. The
intestine not only runs through the _pericardial cavity_ (celome)
surrounding the heart, but through the ventricle of the heart itself
(Fig. 196).
[Illustration:
FIG. 196.—HEART OF MUSSEL, with intestine passing through it.
]
The =kidneys= consist of tubes which open into the pericardial chamber
above and into the gill chamber below (_Neph._, Fig. 193). The tubes
are surrounded by numerous blood vessels (Fig. 198) and carry off the
waste matter from the blood.
[Illustration:
FIG. 197.
]
The =nervous system= consists of _three pairs of ganglia_ and nerves
(Fig. 197). The ganglia are distinguishable because of their orange
colour. The pedal ganglia on the front of the foot are easily seen
also; the visceral ganglia on the posterior adductor muscle may be
seen without removing the mussel from the shell (Fig. 193). The
reproductive organs open into the rear portion of the gill cavity
(Fig. 193). The sperms, having been set free in the water, are drawn
into the ova by the same current that brings the food. The eggs are
hatched in the gills. After a while the young mussels go out through
the siphon.
=Summary.=—In the gills (Fig. 198) the blood gains what? Loses what?
From the digestive tube the blood absorbs nourishment. In the kidneys
the blood is partly purified by the loss of nitrogenous waste.
[Illustration:
FIG. 198.—DIAGRAM OF MUSSEL CUT ACROSS, showing mantle, _ma_; gills,
_kie_; foot, _f_; heart, _h_; intestine, _ed_.
]
The cilia of the fringes on the inhalent, or lower, siphon, vibrate
continually and drive water and food particles into the mouth cavity.
Food particles that are brought near the labial palps are conveyed by
them to the mouth. As the water passes along the perforated gills, its
oxygen is absorbed; the mantle also absorbs oxygen from the water as it
passes. The water, as stated before, goes next through a passage between
the foot and the palp into the cavity above the gills and on out through
the exhalent siphon. By stirring the water, or placing a drop of ink
near the siphons of a mussel kept in a tub, the direction of its flow
may be seen. The pulsations of the heart are plainly visible in a living
mollusc.
=Habits of the Mussel.=—Is it abundant in clear or in muddy water;
swift, still, or slightly moving water? Describe its track or furrow.
What is its rate of travel? Can you distinguish the spots where the foot
was attached to the ground? How long is one “step” compared to the
length of the shell? The animal usually has the valves opened that it
may breathe and eat. The hinge ligament acts like the case spring of a
watch, and holds the valves open unless the adductor muscles draw them
together (Fig. 189).
[Illustration:
FIG. 199.—OYSTER.
_C_, mouth; _a_, vent; _g_, _g′_, ganglia; _mt_, mantle; _b_, gill.
]
When the mussel first hatches from the egg, it has a triangular shell.
It soon attaches itself to some fish and thus travels about. After two
months it drops to the bottom again.
[Illustration:
FIG. 200.—TROCHUS.
]
=Other Mollusca.=—The _oyster’s_ shells are not an exact pair, the shell
which lies upon the bottom being hollowed out to contain the body, and
the upper shell being flat. Can you tell by examining an oyster shell
which was the lower valve? Does it show signs of having been attached to
the bottom? The young oyster, like the young mussel, is free-swimming.
Like the arthropoda, most molluscs undergo a metamorphosis to reach the
adult stage (Fig. 199).
[Illustration:
FIG. 201.—CYPRÆA. (Univalve, with a long opening to shell.)
]
Examine the shells of clams, snails, scallops, and cockles. Make
drawings of their shells. The slug is very similar to the snail except
that it has no shell. If the shell of the snail shown in Fig. 202 were
removed, there would be left a very good representation of a slug.
=Economic Importance of Mollusca.=—Several species of clams are eaten.
One of them is the _hardshell clam_ (quahog) found on the Atlantic coast
from Cape Cod to Texas. Its shell is white. It often burrows slightly
beneath the surface. The _softshell clam_ is better liked as food. It
lives along the shores of all northern seas. It burrows a foot beneath
the surface and extends its siphons through the burrow to the surface
when the tide is in, and draws into its shell the water containing
animalcules and oxygen.
_Oysters_ to the value of many millions of dollars are gathered and sold
every year. The most valuable oyster fisheries of North America are in
Chesapeake Bay. The young oysters, or “spat,” after they attach
themselves to the bottom in shallow water, are transplanted. New oyster
beds are formed in this way. The beds are sometimes strewn with pieces
of rock, broken pottery, etc., to encourage the oysters to attach
themselves. The dark spot in the fleshy body of the oyster is the
digestive gland, or liver. The cut ends of the tough adductor muscles
are noticeable in raw oysters. The starfish is very destructive in
oyster beds.
[Illustration:
FIG. 202.—A SNAIL.
_l_, mouth; _vf_, _hf_, feelers; _e_, opening of egg duct; _f_, foot;
_ma_, mantle; _lu_, opening to lung; _a_, vent.
]
_Pearls_ are deposited by bivalves around some irritating particle that
gets between the shell and the mantle. The pearl oyster furnishes most
of the pearls; sometimes pearls of great value are obtained from
fresh-water mussels. Name articles that are made partly or wholly of
mother-of-pearl.
=Study of a Live Snail or Slug.=—Is its body dry or moist? Do land
snails and slugs have lungs or gills? Why? How many pairs of tentacles
have they? What is their relative length and position? The eyes are
dark spots at bases of tentacles of snail and at the tips of the rear
tentacles of slug. Touch the tentacles. What happens? Do the tentacles
simply stretch, or do they turn inside out as they are extended? Is
the respiratory opening on the right or the left side of the body? On
the mantle fold or on the body? (Figs. 202–3–4.) How often does the
aperture open and close?
[Illustration:
FIG. 203.—A SLUG.
]
Place the snail in a moist tumbler. Does the whole under surface seem
to be used in creeping? Does the creeping surface change shape as the
snail creeps? Do any folds or wrinkles seem to move either toward the
front or the rear of its body? Is enough mucus left to mark the path
travelled? The fold moves to the front, adheres, and smooths out as
the slug or snail is pulled forward.
[Illustration:
FIG. 204.—CIRCULATION AND RESPIRATION IN SNAIL.
_a_, mouth; _b_, _b_, foot; _c_, vent; _d_, _d_, lung; _h_, heart.
Blood vessels are black. (Perrier.)
]
=Cephalopods.=—The highest and best developed molluscs are the
=cephalopods=, or “head-footed” molluscs. Surrounding the mouth are
eight or ten appendages which serve both as feet and as arms. These
appendages have two rows of sucking disks by which the animal attaches
itself to the sea bottom, or seizes fish or other prey with a firm
grip. The commonest examples are the _squid_, with a long body and ten
arms, and the _octopus_, or devilfish, with a short body and eight
arms. Cephalopods have strong biting mouth parts and complex eyes
somewhat resembling the eyes of backboned, or vertebrate, animals. The
large and staring eyes add to the uncanny, terrifying appearance.
[Illustration:
FIG. 205.—A SQUID.
]
The sepia or “ink” discharged through the siphon of the squid makes a
dark cloud in the water and favours its escape from enemies almost as
much as does its swiftness (Fig. 205). The squid sometimes approaches
a fish with motion so slow as to be imperceptible, and then suddenly
seizes it, and quickly kills it by biting it on the back behind the
head.
[Illustration:
FIG. 206.—PEARLY NAUTILUS. (Shell sawed through to show chambers used
when it was smaller, and siphuncle, _S_, connecting them. Tentacles,
_T_.)
]
The octopus is more sluggish than the squid. Large species called
devilfish sometimes have a spread of arms of twenty-five feet. The
_pearly nautilus_ (Fig. 206) and the _female of the paper argonaut_
(Fig. 207) are examples of cephalopods that have shells. The
_cuttlefish_ is closely related to the squid.
[Illustration:
FIG. 207.—PAPER ARGONAUT (female). × ⅓ (_i.e._ the animal is three
times as long and broad as figure).
]
[Illustration:
FIG. 208.—PAPER ARGONAUT (male). × ½.
]
=General Questions.=—The living parts of the mussel are very soft, the
name mollusca being derived from the Latin word _mollis_, soft. Why is
it that the softest animals, the molluscs, have the hardest coverings?
To which class of molluscs is the name acephala (headless) appropriate?
Lamellibranchiata (platelike gills)?
Why is a smooth shell suited to a clam and a rough shell suited to an
oyster? Why are the turns of a snail’s shell so small near the centre?
Why does the mussel have no use for head, eyes, or projecting feelers?
In what position of the valves of a mussel is the hinge ligament in a
stretched condition? How does the shape of the mussel’s gills insure
that the water current and the blood current are brought in close
contact?
The three main classes of molluscs are: the pelecypoda (hatchet-footed);
gastropoda (stomach-footed); and cephalopoda (head-footed). Give an
example of each class.
Comparison of Mollusks
═════════════════╤═════════════════╤═════════════════╤═════════════════
│ MUSSEL │ SNAIL │ SQUID
─────────────────┼─────────────────┼─────────────────┼─────────────────
Shell │ │ │
─────────────────┼─────────────────┼─────────────────┼─────────────────
Head │ │ │
─────────────────┼─────────────────┼─────────────────┼─────────────────
Body │ │ │
─────────────────┼─────────────────┼─────────────────┼─────────────────
Foot │ │ │
─────────────────┼─────────────────┼─────────────────┼─────────────────
Gills │ │ │
─────────────────┼─────────────────┼─────────────────┼─────────────────
Eyes │ │ │
═════════════════╧═════════════════╧═════════════════╧═════════════════
=Comparative Review.=—(To occupy an entire page in notebook.)
════════════════════╤═══════════╤═════════╤═════════╤═════════╤════════
│GRASSHOPPER│ SPIDER │CRAYFISH │CENTIPEDE│ MUSSEL
────────────────────┼───────────┼─────────┼─────────┼─────────┼────────
Bilateral or radiate│ │ │ │ │
────────────────────┼───────────┼─────────┼─────────┼─────────┼────────
Appendages for │ │ │ │ │
locomotion │ │ │ │ │
────────────────────┼───────────┼─────────┼─────────┼─────────┼────────
Names of divisions │ │ │ │ │
of body │ │ │ │ │
────────────────────┼───────────┼─────────┼─────────┼─────────┼────────
Organs and method of│ │ │ │ │
breathing │ │ │ │ │
────────────────────┼───────────┼─────────┼─────────┼─────────┼────────
Locomotion │ │ │ │ │
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