FISHES
[Illustration]
SUGGESTIONS.—The behaviour of a live fish in clear water, preferably
in a glass vessel or an aquarium, should be studied. A skeleton may be
prepared by placing a fish in the reach of ants. Skeletons of animals
placed on ant beds are cleaned very thoroughly. The study of the
perch, that follows, will apply to almost any other common fish.
=Movements and External Features.=—What is the _general shape of the
body_ of a fish? How does the dorsal, or upper, region differ in form
from the ventral? Is there a narrow part or neck where the head joins
the trunk? Where is the body thickest? What is the ratio between the
length and the height? (Fig. 209.) Are the right and the left sides
alike? Is the symmetry of the fish bilateral or radial?
The _body of the fish may be divided_ into three regions—the head, the
trunk, and the tail. The trunk begins with the foremost scales; the tail
is said to begin at the vent, or anus. Which regions bear appendages? Is
the head movable independently of the trunk, or do they move together?
State the advantage or the disadvantage in this. Is the body depressed
(flattened vertically) or compressed (flattened laterally)? Do both
forms occur among fishes? (See figures on pages 123, 124.)
How is the _shape of the body advantageous for movement_? Can a fish
turn more readily from side to side, or up and down? Why? Is the head
wedge-shaped or conical? Are the jaws flattened laterally or vertically?
The fish swims in the water, the bird swims in the air. Account for the
differences in the shape of their bodies.
[Illustration:
FIG. 209.—WHITE PERCH (_Morone Americana_).
]
Is the _covering of the body_ like the covering of any animal yet
studied? The scales are attached in little pockets, or folds, in the
skin. Observe the shape and size of scales on different parts of the
body. What parts of the fish are without scales? Examine a single scale;
what is its shape? Do you see concentric lines of growth on a scale?
Sketch a few of the scales to show their arrangement. What is the use of
scales? Why are no scales needed on the head? How much of each scale is
hidden? Is there a film over the scale? Are the colours in the scale or
on it?
=The Fins.=—Are the movements of the fish active or sluggish? Can it
remain stationary without using its fins? Can it move backward? How are
the fins set in motion? What is the colour of the flesh, or muscles, of
a fish? Count the fins. How many are in pairs? (Fig. 209.) How many are
vertical? How many are on the side? How many are on the middle line? Are
the paired or the unpaired fins more effective in balancing the fish? In
turning it from side to side? In raising and lowering the fish? In
propelling it forward? How are some of the fins useful to the fish
besides for balancing and swimming?
The hard _spines_ supporting the fins are called the fin rays. The fin
on the dorsal line of the fish is called the _dorsal_ fin. Are its rays
larger or smaller than the rays of the other fins? The perch is
sometimes said to have two dorsal fins, since it is divided into two
parts. The fin forming the tail is called the tail fin, or _caudal_ fin.
Are its upper and its lower corners alike in all fishes? (Fig. 228.) On
the ventral side, just behind the vent, is the ventral fin, also called
the anal fin. The three fins mentioned are unpaired fins. Of the
four-paired fins, the pair higher on the sides (and usually nearer the
front) are the _pectoral_ fins. The pair nearer the ventral line are the
_pelvic_ fins. They are close together, and in many fish are joined
across the ventral line. The ventral fins are compared to the legs, and
the pectoral fins to the arms, of higher vertebrates. (Fig. 244.)
Compare fins of fish, pages 123, 124.
Make a =drawing= of the fish seen from the side, omitting the scales
unless your drawing is very large.
Are the =eyes= on the top or on the sides of the head, or on both? Can a
fish shut its eyes? Why, or why not? Is the eyeball bare, or covered by
a membrane? Is the covering of the eyeball continuous with the skin of
the head? Is there a fold or wrinkle in this membrane or the surrounding
skin? Has the eye a pupil? An iris? Is the eye of the fish immovable,
slightly movable, or freely movable? Can it look with both eyes at the
same object? Is the _range of vision_ more upward or downward? To the
front or the side? In what direction is vision impossible? Can a fish
close its eyes in sleep? Does the eyeball appear spherical or flattened
in front? The ball is really spherical, the lens is very convex, and
fish are nearsighted. Far sight would be useless in a dense medium like
water. In what direction from the eyes are the =nostrils= (Fig. 211.)
There are two pair of nostrils, but there is only one pair of nasal
cavities, with two nostrils opening into each. There are no nasal
passages to the mouth, as the test with a probe shows that the cavities
do not open into the mouth. What two functions has the nose in man? What
function has it in the fish?
[Illustration:
FIG. 210.—BLACKBOARD OUTLINE OF FISH.
]
There are _no external_ =ears=. The ear sacs are embedded in the bones
of the skull. Is hearing acute or dull? When you are fishing, is it more
necessary not to talk or to step lightly, so as not to jar the boat or
bank?
[Illustration:
FIG. 211.—HEAD OF CARP.
]
What is the use of the large openings found at the back of the head on
each side? (Fig. 211.) Under the skin at the sides of the head are thin
membrane bones formed from the skin; they aid the skin in protection.
Just under these membrane bones are the gill covers, of true bone. Which
consists of more parts, the membranous layer, or the true bony layer in
the gill cover? (Figs. 211 and 212.)
Is the =mouth= large or small? Are the _teeth_ blunt or pointed? Near
the outer edge, or far in the mouth? (Fig. 212.) Does the fish have
lips? Are the teeth in one continuous row in either jaw? In the upper
jaw there are also teeth on the premaxillary bones. These bones are in
front of the maxillary bones, which are without teeth. Teeth are also
found in the roof of the mouth, and the tongue bears horny appendages
similar to teeth. Are the teeth of the fish better suited for chewing or
for grasping? Why are teeth on the tongue useful? Watch a fish eating:
does it chew its food? Can a fish taste? Test by placing bits of brown
paper and food in a vessel or jar containing a live fish. Is the throat,
or gullet, of the fish large or small?
[Illustration:
FIG. 212.—SKELETON OF PERCH.
]
The =skeleton= of a fish is simpler than the skeleton of other backboned
animals. Study Fig. 212 or a prepared skeleton. At first glance, the
skeleton appears to have two vertebral columns. Why? What bones does the
fish have that correspond to bones in the human skeleton? Are the
projections (processes) from the vertebræ long or short? The _ribs_ are
attached to the vertebræ of the trunk, the last rib being above the
vent. The tail begins at the vent. Are there more tail vertebræ or trunk
vertebræ? Are there any neck (cervical) vertebræ (_i.e._ in front of
those that bear ribs)? The first few ribs (how many?) are attached to
the central body of the vertebræ. The remaining ribs are loosely
attached to processes on the vertebræ. The ribs of bony fishes are not
homologous with the ribs of the higher vertebrates. In most fishes there
are bones called intermuscular bones attached to the first ribs (how
many in the perch?) which are possibly homologous to true ribs; that is,
true ribs in the higher vertebrates may have been developed from such
beginnings.
[Illustration:
FIG. 213.
]
[Illustration:
FIG. 214.—SOFT-RAYED AND SPINY-RAYED FINS.
]
Which, if any, of the _fin skeletons_ (Fig. 214) are not attached to the
general skeleton? Which fin is composed chiefly of tapering, pointed
rays? Which fins consist of rays which subdivide and widen toward the
end? Which kind are stiff, and which are flexible? Which of the fin rays
are segmented, or in two portions? The outer segment is called the
radial, the inner the basal segment. Which segments are longer? There is
one basal segment that lacks a radial segment. Find it (Fig. 212).
[Illustration:
FIG. 215.—CARP, with right gill cover removed to show gills.
]
What is the advantage of the backbone plan of structure over the
armour-plate plan? You have seen the spool-like body of the vertebra in
canned salmon. Is it concave, flat, or convex at the ends?
[Illustration:
FIG. 216.—SKELETON AROUND THROAT OF FISH.
]
The =gills= are at the sides of the head (Fig. 215) under the opercula,
or gill covers. What is the colour of the gills? Do the blood vessels
appear to be very near the surface of the gills, or away from the
surface? What advantage in this? Are the gills smooth or wrinkled? (Fig.
215.) What advantage? The bony supports of the gills, called the gill
arches, are shown in Fig. 216 (_k_{1}_ to _k_{4}_). How many arches on
each side? The gill arches have projections on their front sides, called
gill rakers, to prevent food from being washed through the clefts
between the arches. The fringes on the rear of the gill arches are
called the gill filaments (_a_, Fig. 216). These filaments support the
thin and much-wrinkled borders of the gills, for the gills are
constructed on the plan of exposing the greatest possible surface to the
water. Compare the plan of the gills and that of the human lungs. The
gill opening on each side is guarded by seven rays (_kh_, Fig. 216)
along the hinder border of the gill cover. These rays grow from the
tongue bone. (_Zu_, Fig. 216. This is a rear view.)
[Illustration:
FIG. 217.—CIRCULATION IN GILLS.
]
[Illustration:
FIG. 218.—NOSTRILS, MOUTH, AND GILL OPENINGS OF STING-RAY.
]
Watch a live fish and determine how the water is forced between the
gills. Is the mouth opened and closed in the act of breathing? Are the
openings behind the gill covers opened and closed? How many times per
minute does fresh water reach the gills? Do the mouth and the gill
covers open at the same time? Why must the water in contact with the
gills be changed constantly? Why does a fish usually rest with its head
up stream? How may a fish be kept alive for a time after it is removed
from the water? Why does drying of the gills prevent breathing? If the
mouth of a fish were propped open, and the fish returned to the water,
would it suffocate? Why, or why not?
[Illustration:
FIG. 219.—GILL OPENINGS OF EEL.
]
=Food Tube.=—The gullet is short and wide. The stomach is elongated
(Fig. 220). There is a slight constriction, or narrowing, where it
joins the intestine. Is the intestine straight, or does it lie in few
or in many loops? (Fig. 220.) The liver has a gall bladder and empties
into the intestine through a bile duct. Is the liver large or small?
Simple or lobed? The spleen (_mi_, Fig. 220) lies in a loop of the
intestine. The last part of the intestine is straight and is called
the rectum. Is it of the same size as the other portions of the
intestine? The fish does not possess a pancreas, the most important
digestive gland of higher vertebrates.
[Illustration:
FIG. 220.—ANATOMY OF CARP. (See also coloured figure 4.)
_bf_, barbels on head (for feeling); _h_, ventricle of heart; _as_,
aortic bulb for regulating flow to gills; _vk_, venous sinus; _ao_,
dorsal aorta; _ma_, stomach; _l_, liver; _gb_, gall cyst; _mi_,
spleen; _d_, small intestine; _md_, large intestine; _a_, vent; _s_,
_s_, swim bladder; _ni_, _ni_, kidney; _hl_, ureter; _hb_, bladder;
_ro_, eggs (roe); _mhe_, opening of ducts from kidney and ovary.
=Questions=: Are the kidneys dorsal or ventral? The swim bladder? Why?
Why is the swim bladder double? Does blood enter gills above or
below?
]
The _ovary_ lies between the intestine and the air bladder. In Fig.
220 it is shown enlarged and filled with egg masses called roe. It
opens by a pore behind the vent. The silver lining of the body cavity
is called the peritoneum.
Is the _air bladder_ in the perch simple or partly divided? In the
carp? (Fig. 220.) Is it above or below the centre of the body? Why?
The air bladder makes the body of the fish about as light as water
that it may rise and sink with little effort. When a fish dies, the
gases of decomposition distend the bladder and the abdomen, and the
fish turns over. Why?
Where are the _kidneys_? (Fig. 220.) Their ends unite close under the
spinal column. The ureters, or tubes, leading from them, unite, and
after passing a small urinary bladder, lead to a tiny urinary pore
just behind the opening from the ovary. (Coloured figure 4.)
=The Circulation.=—The fish, unlike other vertebrates, has its
breathing organs and its heart in its head. The gills have already
been described. The heart of an air-breathing vertebrate is near its
lungs. Why? The _heart_ of a fish is near its gills for the same
reason. The heart has one auricle and one ventricle. (Coloured figure
1.)
[Illustration:
FIG. 221.—PLAN OF CIRCULATION.
_Ab_, arteries to gills; _Ba_, aortic bulb; _V_, ventricle.
]
Blood returning to the heart comes through several veins into a
_sinus_, or antechamber, whence it passes down through a valve into
the _auricle_; from the auricle it goes forward into the _ventricle_.
The ventricle sends it into an _artery_, not directly, but through a
_bulb_ (_as_, Fig. 220), which serves to maintain a steady flow,
without pulse beats, into the large artery (_aorta_) leading to the
gills. The arteries leading from the gills join to form a _dorsal
aorta_ (_Ao_, Fig. 221), which passes backward, inclosed by the lower
processes of the spinal column. After going through the _capillaries_
of the various organs, the blood returns to the heart through veins.
[Illustration:
FIG. 222.—BRAIN OF PERCH, from above.
_n_, end of nerve of smell; _au_, eye; _v_, _z_, _m_, fore, mid, and
hind brain; _h_, spinal bulb; _r_, spinal cord.
]
The _colour of the blood_ is given by red corpuscles. These are
nucleated, oval, and larger than the blood corpuscles of other
vertebrates. The blood of the fish is slightly above the temperature
of the water it inhabits.
Notice the general shape of the brain (Fig. 222). Are its subdivisions
distinct or indistinct? Are the lobes in pairs? The middle portion of
the brain is the widest, and consists of the two _optic lobes_. From
these lobes the optic nerves pass beneath the brain to the eyes (_Sn_,
Fig. 223). In front of the optic lobes lie the two cerebral lobes, or
the _cerebrum_. The small _olfactory lobes_ are seen (Fig. 224) in
front of the cerebrum. The olfactory nerves may be traced to the
nostrils. Behind the optic lobes (mid brain) is the cerebellum (hind
brain) and behind it is the _medulla oblongata_ or beginning of the
spinal cord.
[Illustration:
FIG. 223.—BRAIN OF PERCH, side view.
]
[Illustration:
FIG. 224.—BRAIN OF PERCH, from above.
]
If you take the eyeball for comparison, is the whole brain as large as
one eyeball? (Fig. 222.) If you judge from the size of the parts of
the brain, which is more important with the fish, thinking or
perception? Which is the most important sense?
The scales along a certain line on each side of the fish, called the
lateral line, are perforated over a series of lateral line sense
organs, supposed to be the chief organs of _touch_ (see Fig. 209).
[Illustration:
FIG. 225.—THE STICKLEBACK. Instead of depositing the eggs on the
bottom, it makes a nest of water plants—the only fish that does
so—and bravely defends it.
]
=Questions.=—Which of the fins of the fish have a use which corresponds
to the keel of a boat? The rudder? A paddle for sculling? An oar? State
several reasons why the head of the fish must be very large, although
the brain is very small. Does all the blood go to the gills just after
leaving the heart?
[Illustration:
FIG. 226.—ARTIFICIAL FECUNDATION. The egg cells and sperm-cells are
pressed out into a pan of water.
]
Make a list of the different species of fish found in the waters of your
neighbourhood; in the markets of your town.
[Illustration:
FIG. 227.—NEWLY HATCHED TROUT, with yolk-sac adhering, eyes large, and
fins mere folds of the skin. (Enlarged.)
]
=Reproduction.=—The female fish deposits the unfertilized eggs, or ova,
in a secluded spot on the bottom. Afterward the male fish deposits the
sperms in the same place (see Fig. 225). The eggs, thus unprotected, and
newly hatched fish as well, are used for food by fish of the same and
other species. To compensate for this great destruction, most fish lay
(spawn) many thousands of eggs, very few of which reach maturity. Higher
vertebrates (_e.g._ birds) have, by their superior intelligence, risen
above this wasteful method of reproduction. Some kinds of marine fish,
notably cod, herring, and salmon, go many miles up fresh rivers to
spawn. It is possible that this is because they were originally
fresh-water species; yet they die if placed in fresh water except during
the spawning season. They go because of _instinct_, which is simply an
inherited habit. Rivers may be safer than the ocean for their young.
They are worn and exhausted by the journey, and never survive to lay
eggs the second time.
[Illustration:
FIG. 228.—A SHARK (_Acanthias vulgaris_).
]
The _air bladder is developed from the food tube_ in the embryo fish,
and is homologous with lungs in the higher vertebrates. Are their
functions the same?
Fish that _feed on flesh have a short intestine_. Those that eat plants
have a long intestine. Which kind of food is more quickly digested?
There are _mucous glands in the skin_ of a fish which supply a secretion
to facilitate movement through the water; hence a freshly caught fish,
before the secretion has dried, feels very slippery.
The air bladder, although homologous to lungs, is not a breathing organ
in common fishes. It is filled by the formation of gases from the blood,
and can be made smaller by the contraction of muscles along the sides of
the body; this causes the fish to sink. In the gar and other ganoids,
the air bladder contains blood vessels, is connected with the gullet,
and is used in breathing. Organs _serving the same purpose_ in different
animals are said to be _analogous_. To what in man are the gills of the
fish analogous? Organs having _a like position and origin_ are said to
be _homologous_. The air bladders of a fish are homologous with the
lungs of man; but since they have not the same use they are not
analogous.
How does the tail of a shark or a gar differ from the tail of common
fishes? (Fig. 228.) Do you know of fish destitute of scales? Do you know
of fish with whiplike feelers on the head? (Figs.) Why are most fishes
white on the under side?
=Comparative Review.=—(Copy table on one page or two facing pages of
notebook.)
═══════════╤═══════════╤═══════════╤═══════════╤════════════╤══════════
│IS THERE A │ METHOD OF │ DIGESTIVE │REPRODUCTION│ SENSES
│ HEAD? A │ FEEDING │ORGANS AND │ │
│ NECK? │ │ DIGESTION │ │
───────────┼───────────┼───────────┼───────────┼────────────┼──────────
Amœba │ │ │ │ │
───────────┼───────────┼───────────┼───────────┼────────────┼──────────
Sponge │ │ │ │ │
───────────┼───────────┼───────────┼───────────┼────────────┼──────────
Hydra │ │ │ │ │
───────────┼───────────┼───────────┼───────────┼────────────┼──────────
Starfish │ │ │ │ │
───────────┼───────────┼───────────┼───────────┼────────────┼──────────
Earthworm │ │ │ │ │
───────────┼───────────┼───────────┼───────────┼────────────┼──────────
Wasp │ │ │ │ │
───────────┼───────────┼───────────┼───────────┼────────────┼──────────
Mussel │ │ │ │ │
───────────┼───────────┼───────────┼───────────┼────────────┼──────────
Fish │ │ │ │ │
═══════════╧═══════════╧═══════════╧═══════════╧════════════╧══════════
[Illustration:
FIG. 229.—DRAWING THE SEINE.
]
[Illustration:
FIG. 230.—SUNFISH.
]
[Illustration:
FIG. 231.—TUNNY.
]
[Illustration:
FIG. 232.—SWORDFISH.
]
[Illustration:
FIG. 233.—SWELLFISH.
]
[Illustration:
FIG. 234.—TURBOT.
]
[Illustration:
FIG. 235.—CARP.
]
[Illustration:
FIG. 236.—HERRING.
]
[Illustration:
FIG. 237.—SPECKLED TROUT.
]
[Illustration:
FIG. 238.—PERCH.
]
[Illustration:
FIG. 239.—SALMON.
]
=Seven Food Fish. Three Curious Fish.=
SPECIAL REPORTS. (Encyclopedia, texts, dictionary.)
[Illustration:
FIG. 240.—SEA HORSE (_Hippocampus_), with incubating pouch, _Brt_.
]
[Illustration:
FIG. 241.—BAND FISH.
]
[Illustration:
FIG. 242.—TORPEDO. Electrical organs at right and left of brain.
]
[Illustration:
FIG. 243.—LANTERN FISH (_Linophryne lucifer_). (After Collett.)
]
[Illustration:
FIG. 244.—LUNG FISH of Australia (_Ceratodus miolepis_).
]
[Illustration:
FIG. 245.—TRUNK FISH.
]
[Illustration:
FIG. 246.—SEAWEED FISH. × ⅕ (_Phyllopteryx eques_).
]
=Remarkable Fish.= SPECIAL REPORTS. (Encyclopedia, texts, dictionary.)
RECOGNITION GROUP CHARACTERS
The commoner members of the several branches may be recognized by the
following characters:—
1. The =Protozoans= are the only one-celled animals.
2. The =Sponges= are the only animals having pores all over the body for
the inflow of water.
3. The =Polyps= are the only many-celled animals having a single opening
into the body, serving for both mouth and vent. They are radiate in
structure, and usually possess tentacles.
4. The =Echinoderms= are marine animals of more or less radiate
appearance, having a food tube in the body separate from the body wall.
The following groups are plainly bilateral: that is, dorsal and ventral
surfaces, front and hind ends are different.
5. The =Vermes= have usually a segmented body but lack jointed legs.
6. The =Arthropods=[3] have an external skeleton and jointed legs.
Footnote 3:
Insects and crustaceans.
7. The =Molluscs= have soft bodies, no legs, no skeleton, but usually a
limy shell.
8. The =Vertebrates= have an internal skeleton of bones, and usually two
pairs of legs.
[Illustration:
FIG. 247.—A SNAIL. (Which branch? Why?)
]