BRANCH CHORDATA (_Continued_): CLASS PISCES
(THE FISHES)
THE GOLDEN SUNFISH OR PUMPKIN SEED (_Apomotis_ sp.)
TECHNICAL NOTE.--The species of sunfish named, or some closely
related species, can be obtained in any brook or stream in the
United States. _Gibbosus_ lives in all streams north of Dubuque,
Chicago, Pittsburg, and along the eastern coast north of
Charleston. Closely allied species live in all the other parts of
the country except in the higher Rocky Mountains west of Bismarck,
Pueblo, and Santa Fe. One species is found in the streams of
California, but none occurs in Washington or Oregon. In the few
places where a sunfish cannot be had, any species of bass or perch
may be used. Sunfish live in ponds and sluggish streams in deep
holes under a log or at the foot of a stump. They take eagerly a
hook baited with a worm, or they may be caught in nets. When
sunfish cannot be kept fresh for study in class, specimens may be
preserved in alcohol or 4% formalin. But if possible to keep some
alive for a time in a jar or tub with plenty of fresh water, the
colors of the living fish, together with its manner of swimming
and mode of breathing, can be observed.
=External structure=[16] (fig. 112).--Examine the general
configuration and make-up of the body. Note the deep, laterally
flattened _trunk_ and paddle-like _tail_. The _head_ is closely fitted
to the trunk without any neck. Note that the body is thickly covered
with firm, hard _scales_, arranged like the shingles on a roof. Remove
one of these scales and examine it under a hand lens. What sort of an
edge has it? Such a scale is said to be _ctenoid_.
The body of the sunfish terminates behind in the _caudal fin_, a
series of cartilaginous rays connected by thin skin and attached to a
bony plate at the end of the backbone. Along the median dorsal line
will be noted another fin composed anteriorly of spines and
posteriorly of soft rays jointed and branched. This is the _dorsal
fin_. How many spines has it? Anterior to the caudal fin on the
ventral surface is a median unpaired _anal fin_. How many spines has
it? Anterior to the anal fin are the _ventral fins_, while on the
sides of the body back of the head in a line with the mouth are found
the _pectoral fins_. The ventral fins, attached to a rudimentary
pelvis, correspond to the hind legs of the other vertebrates. The
pectoral fins, attached to the shoulder girdle, correspond to the
arms. In front of the anal fin note a small pit-like opening, the
opening from the kidneys and reproductive organs, and just anterior to
this a large aperture, the _anus_. At the anterior end of the head
note the broad _mouth_, surrounded by a complicated system of bones.
Note the large _eyes_ surrounded by a series of small bones, the
_orbital chain_. Just anterior to the eyes are two pairs of openings,
one pair of each side opening into a closed sac. What are these
openings? Note the presence of various bones on the side of the head,
each covered with a thin layer of skin. These are _membrane bones_,
characteristic of fishes. Are there any external ears in the fish?
Examine the inside of the mouth. Is there a _tongue_? If so, of what
character? Are there _teeth_? If so, where are they situated?
Note along each side extending to the base of the tail a line of
modified scales, on each scale a little mucous tube, the whole series
constituting the _lateral line_. These scales are intimately
associated with a large nerve (the _vagus_), and probably serve an
important part, not yet clearly understood, in the life of the fish.
Lift up the flap in front of one of the pectoral fins. This is the
_opercular flap_ which covers the gills that lie beneath. Bend this
forward and find four _gill-arches_, each with its double fringe of
_gills_. Note the _gill-rakers_, short and blunt, on the first
gill-arch. Note also on the under side of the flaps turned back,
delicate red gill-like structures covered by a membrane. These are the
_false gills_ or _pseudo-branchiæ_, larger in most fishes than in the
sunfish. The gills in the fish subserve the same function as the gills
of the crayfish, that of purifying the blood by eliminating
carbonic-acid gas from it and taking up oxygen from the air mixed with
or dissolved in the water. Organs subserving the same purposes in
different kinds of animals as, for example, the gills in fish and in
crayfish, are called _analogous structures_. But there is an important
morphological difference between the fish's gills and the gills of the
crayfish. In the latter animal they are outgrowths of the basal segments
of the walking-legs; in the fish they are outgrowths from the alimentary
canal. The internal gills of the young toad (tadpole) arise in the same
way as those of a fish. Structures which are identical in their origin,
like the gills of tadpole and fish, are called _homologous structures_.
Make a drawing of the sunfish from a lateral aspect, showing the
external parts named.
=Internal Structure.=--TECHNICAL NOTE.--Insert one point of the
scissors a little to one side of the anus and cut dorsally on the
left side of the body to the backbone. Now cut anteriorly from the
anus along the ventral wall to where the jaws unite, and cut, also
anteriorly, along the dorsal wall until the left side of the body
can be removed. Bend the opercular flap backward over the eye and
pin the entire fish, uncut side down, to the bottom of the
dissecting-pan, covering it with water.
The above operation will have severed the large powerful _muscles_
forming the body-wall and extending along the sides. Note a membranous
sac completely filling a large dorsal cavity. This is the
_swim-bladder_, a float filled with air which tends to give the fish
the same weight as the water it displaces. It arises as a diverticulum
from the alimentary canal, but soon becomes permanently shut off from
it. Beneath the swim-bladder is a large cavity filled with various
organs, collectively known as the _viscera_. In vertebrate animals the
cavity which contains the viscera is generally called the _peritoneal
cavity_. It is lined by the _peritoneum_, a delicate membrane, part of
which is deflected as the _mesentery_ over the alimentary canal and
the other organs, thus suspending them all from the dorsal wall. Note
in the anterior end of the peritoneal cavity a large bi-lobed gland,
the _liver_, red in fresh, yellowish in alcoholic specimens. Its
function, like that of the liver of the toad, is to store up nutriment
for the blood and to secrete a digestive fluid called _bile_. Behind
the liver note a long, convoluted tube. What is this tube? Unfold this
tube, separating it from its enveloping membrane, the mesentery.
Thrust a probe down the throat and note that it passes into a
thick-walled sac, the _stomach_. The mouth and gill-slits open into
the front part of the alimentary canal called the _pharynx_, which
leads by a short tube, the _œsophagus_, into the stomach. Note the
large, thickened portion of the alimentary canal leading from the
stomach. This is the _pylorus_, and to its walls are attached a number
of finger-like projections, the _pyloric cæca_. The pyloric cæca
secrete a fluid which is poured into the alimentary canal and which
assists in the process of digestion somewhat as does the secretion
from the pancreas of the toad. From the pylorus, passing backwards in
one or two loops, is the _small intestine_. Trace this to its exit.
Lying within the mesentery near the posterior end of the body-cavity
note a small red glandular mass, the _spleen_.
At the anterior end of the body in front of the liver and between the
sets of gills note the small _pericardial cavity_ within which is
contained the _heart_. The pericardial cavity is separated from the
peritoneal cavity by a thick muscular wall against which the liver
abuts. The heart consists of four parts. The posterior part is a
thin-walled reservoir, the _sinus venosus_, into which blood enters
through the _jugular vein_ from the head and through the _cardinal vein_
from the kidney. From the sinus venosus it passes forward into a large
chamber, the _auricle_. Next it flows into the _ventricle_, where, by
the contraction of the walls, rhythmical pulsations force it into the
_conus arteriosus_, thence into the _ventral aorta_, and lastly into the
gills, where it is purified. After passing through the capillaries in
the fine gill-filaments it is again collected, now pure, by paired
arteries from each pair of gills, which arteries unite to form the
_dorsal aorta_ extending backward just below the backbone to the end of
the tail. From the dorsal aorta a pair of arteries, the _subclavian_,
are given off to the pectoral fins. At this point two other arteries
branch off ventrally, the first being the _cardiac artery_, which
distributes blood to the stomach and pyloric cæca. The second divides
into several long _mesenteric arteries_ supplying blood to all parts of
the intestine and spleen. In the caudal region blood is taken up through
the _caudal vein_ and carried forward to the kidneys. These strain out
the impurities arising from waste of tissues, after which the blood is
carried back to the sinus venosus through the cardinal vein. From the
intestine it is gathered into the large _portal vein_ as in the toad.
The portal vein carries blood to the liver, where nutriment may be
stored up, and from thence it flows back to the sinus venosus through a
very short thin-walled vessel, the _hepatic sinus_.
The _kidneys_, more or less united in one mass, lie in the posterior
part of the body-cavity along the dorsal wall. Note running from each
side of the kidney a _ureter_ which unites with its fellow and opens
into a small _urinary bladder_ which discharges through a small
opening immediately back of the anus.
The reproductive organs lie below the swim-bladder near the posterior
end of the body-cavity. If the fish are caught in the spring, the
greater part of the body-cavity of the female is found to be filled with
small eggs. When mature, these eggs are deposited by the mother fish in
the gravel of the stream-bed where they are fertilized by the
sperm-cells poured over them by the male and left floating in the water.
The nervous system of fishes is best studied in a specimen treated
with nitric acid. Carefully remove the roof of the skull, thereby
exposing the brain. Most anteriorly make out, as in the toad, the
paired _olfactory lobes_. These are attached by long stalks to the
_cerebrum_ or _forebrain_, which is followed by two large hollow
lobes, the _midbrain_ or _optic lobes_. Behind the midbrain is the
_cerebellum_. Following the cerebellum is the elongate _medulla
oblongata_, which tapers backward into the _spinal cord_. How far
backward does the spinal cord extend? On each side of the brain-case
about opposite the cerebellum are located the _auditory organs_, each
consisting of three _semicircular canals_ which lie in different
planes, and of the _vestibule_. These parts are filled with liquid,
and suspended in the liquid in the vestibule are small calcareous
bodies called _otoliths_ or _ear-stones_. Running out beneath from the
midbrain are the _optic nerves_, which cross, the left one connected
with the right eye, the right one with the left eye. From each side of
the medulla oblongata there is given off a large nerve, the _vagus_,
which sends branches to the lateral line organs on either side, and
extends backward to the stomach and viscera.
For further study of the nervous system see Parker's "Zootomy," pp.
122-130.
Make a drawing of the nervous system as worked out.
TECHNICAL NOTE.--To make a good skeleton immerse a fresh or
preserved specimen for some time in a hot soap solution. When the
muscles have commenced to soften remove the body from the
solution, pick the flesh away, and leave to dry.
Note that the main axis of the _skeleton_ is composed of _vertebræ_
placed end to end. How many vertebræ are there? What vertebræ bear
_ribs_? The ribless ones beyond the body-cavity are called _caudal
vertebræ_. Note the _interspinal bones_ which support the fins, with
large muscles on either side to control their action. Note that the
group of bones supporting the pectoral fin is attached to the back of
the brain-case and makes up the _shoulder girdle_. The ventral fins
are attached to a rudimentary _pelvic girdle_, attached in front to
the shoulder girdle, as the shoulder girdle is in turn attached to the
skull. It will be seen that the sunfish has no neck and we may say,
also, no back. Its skeleton consists only of a tail attached to the
skull. The brain-case is made up of a number of bones closely joined
together. From it is suspended the lower jaw, which comprises a number
of bones but loosely attached to each other. Overlying these is the
system of membrane bones already mentioned, including the opercle or
gill-cover.
For a detailed study of the fish-skeleton see Parker's "Zootomy," pp.
86-101, or Parker and Haswell's "Zoology," vol. II. pp. 183-195.
=Life-history and habits.=--The sunfish or "pumpkin-seed" lives in
quiet corners of the brooks and rivers, preferably under a log or at
the root of an old stump. It is a beautiful fish, shining "like a
coin fresh from the mint." Its body is mottled golden, orange and
blue, with metallic lustre, darker above, pale or yellowish below. Its
fins are of the same color. The tip of its opercle is prolonged like
an ear and jet black in color, with a dash of bright scarlet along its
lower edge. Nearly all the thirty species of sunfish found in the
United States have this black ear, but some have it long, some short,
and in some it is trimmed with yellow or blue instead of scarlet.
The sunfish lays its eggs in the spring in a rude nest it scoops in the
gravel, over which it stands guard with its bright fins spread, looking
as big and dangerous as possible. When thus employed it takes the hook
savagely, perhaps regarding the worm as a dangerous enemy. The young
fishes soon hatch, looking very much like their parents, although more
transparent and not so brightly colored. They grow rapidly, feeding on
insects and other small creatures, and reach their growth in two or
three years. They do not wander far and never willingly migrate.
Students should verify this account on the different species. A more
exact study of the nests of the different species and the fishes'
defence of them would be a valuable addition to our knowledge. The most
striking traits of the habits of this fish are its vivacity and courage;
it reveals its great muscular strength when captured. The sexes are
similar in appearance and both defend the nest alike.
OTHER FISHES.
[Illustration: FIG. 112.--Dissection of the sunfish, _Apomotis_ sp.]
Fishes constitute the largest class of vertebrate animals and are to
be found everywhere in ponds, streams, or ocean. About 15,000 species
of fish are known, of which 3,000 live in North America. The largest
of all fishes is the basking shark (_Cetorhinus_), which reaches a
length of thirty-six feet. The smallest is the dwarf goby
(_Mistichthys_), less than half an inch long, found in Luzon, one of
the Philippine Islands. Between these extremes is every variety in
size, form, and relative proportions. The body, for example, may be
greatly elongated and almost cylindrical as in the eels; or long and
flattened from side to side as in the ribbon-fishes; or the head may
be very large, wider and higher than the rest of the body as in the
anglers, or may have a great beak as in the sword-fish.
=Body form and structure.=--When we consider the fish as a whole, we
find first a body formed for progression in the water, the typical fish
being pointed at each end (the shorter point in front), and having the
sides flattened, the back and belly rather narrow, and the motive power
located in the fin on the tail. From this typical form diverge all
conceivable variations, adaptations to every sort of fish life.
Most fishes have the body covered with scales, although many have the
skin naked or covered with small scales so hidden in the skin as to be
hardly visible. The scales are small horny or bony plates which fit
into small pockets or folds of the skin, and are usually arranged
shingle-fashion, overlapping each other. They are of various shapes,
mostly classified as of three kinds, namely, squarish enamelled scales
called ganoid, roundish smooth-edged called cycloid, and roundish
tooth-edged called ctenoid.
The skeleton of the fish is relatively complex. Its bones are
comparatively soft, having little lime in them, indeed in many cases
they are mere cartilage. The vertebral column is made of twenty-four
vertebræ in the typical fishes, the number in the others being variously
increased, or sometimes diminished. These vertebræ are of two classes,
abdominal or body, and caudal or tail vertebræ. The former have a neural
arch which encloses the spinal cord and from which projects a spine.
Below, the processes spread apart, surrounding the kidneys and partly
enclosing the air-bladder. To these processes ribs are loosely attached.
The caudal vertebræ have no ribs and leave no room below for viscera.
Their lower arch (hæmal), similar to the dorsal (neural) arch, surrounds
a blood-vessel. The fins of a fish are composed of bony rods or rays
joined by membrane. Some of these rays may be unbranched and unjointed,
being then known as spines, and usually occupy the front part of the
fin. Other rays are made up of little joints and are usually branched
toward their tip. Such ones are called soft rays. Soft rays make up the
greatest part of most fins. The vertical fins are on the middle line of
the body. These are the dorsal above, anal below, and caudal forming the
end of the tail. The paired pectoral and ventral fins are ranged one on
each side corresponding to the arms and legs of higher animals. The
pectoral fin or arm is fastened to a series of bones called the shoulder
girdle. These bones do not correspond to those in the shoulder girdle of
the higher animals, and the various parts in the two structures are
differently named. The uppermost bone of the shoulder girdle is usually
attached to the skull. To the lowermost is attached the rudimentary
pelvis, which supports the hinder limb or ventral fin. Usually the
pelvis is farther back and loose in the flesh, but sometimes it is
placed far forward, being occasionally attached at the chin.
The head contains the various bones of the cranium, usually closely
wedged together and not easily distinguished. The jaws are each made
of several pieces; the lower one is suspended from the skull by a
chain of three flat bones. The jaws may bear any one of a great
variety of forms of teeth or no teeth at all, and any of the bones of
the mouth-cavity and throat may have teeth as well. On the outside of
the head are numerous bones called membrane bones, because they are
made up of ossified membrane. The most important of these is the
opercle or gill-cover. Within are the tongue with the five gill-arches
attached to it below and to the floor of the skull above, the last
arch being usually modified to form the pharyngeal jaw.
The stomach may be a blind sac with entrance and exit close together,
or it may have the form of a tube or siphon. At its end are often
found the large glandular tubes called pyloric cæca which secrete a
digestive fluid; and to its right side is attached the red spleen. The
liver is large, having usually, but not always, a gall-bladder; it
pours its secretion into the upper intestine. In fishes which feed on
plants the intestine is long, but it is short in those which eat
flesh, because flesh is digested in the stomach, not in the
intestines. The kidney is usually a long slender forked gland showing
little variation. The egg-glands differ greatly in different sorts of
fishes, the size and number of eggs varying equally. The air-bladder
is a lung which has lost both lung structure and respiratory function,
being simply a sac filled with gas secreted from the blood, and lying
in the upper part of the abdominal cavity. It is subject to many
variations. In the gar pike, bow-fin and the lung-fishes of the
tropics, the air-bladder is a true lung used for breathing and
connected by a sort of glottis with the œsophagus. In others it is
rudimentary or even wholly wanting, while in still others its function
as an air-sac is especially pronounced, and in many it is joined
through the modified bones of the neck to the organ of hearing.
The blood of the fish is purified by circulation through its gills.
These are a series of slender filaments attached to bony arches. Among
them the blood flows in and out, coming in contact with the water
which the fish takes in through its mouth and which passes across the
gills to be expelled through the gill-openings. The blood is received
from the body into the first chamber of the heart, a muscular sac
called the auricle. From here it passes into the ventricle, a chamber
with thicker walls, the contraction of which sends it to the gills,
thence without return to the heart it passes over the body. The
circulation of blood in fishes is slow, and the blood, which receives
relatively little oxygen, is cold, being but little warmer than the
water in which the individual fish lives.
Inside the cranium or brain-case is the brain, small and composed of
ganglia which are smooth at the surface and contain little gray
matter. At the posterior end of the brain is the thickened end of the
spinal cord, called the medulla oblongata. Next overlapping this is
the cerebellum, always single. Before this lie the largest pair of
ganglia, the optic lobes or midbrain, round, smooth, and hollow. From
the under side of these, nerves run to the eyes with or without a
chiasma or crossing. In front of the optic lobes and smaller than them
is the cerebrum or forebrain, usually of two ganglia but sometimes (in
the sharks) united into one. In front of these are the small olfactory
lobes which send nerves to the nostrils.
The sense organs are well developed. The sense of touch has in some
fishes special organs for its better effectiveness. For instance
certain fin-rays in some fishes, or, as in the catfish, slender,
fleshy, whip-like processes on the head, are developed as feelers or
special tactile organs. Other fishes, the sucker and loach for
example, have specially sensitive lips and noses with which they
explore their surroundings. The sense of taste does not seem to be
well developed in this group. Taste-papillæ are often present in small
numbers on the tongue or on the palate. The sense of smell is good.
The olfactory organs, one on each side of the head, are hollow
sac-like depressions, closed at the rear. In most cases each sac has
two openings or nostrils. The sense of hearing is not very keen. The
ears are fluid-filled sacs buried in the skull, and without external
or (except in a few cases) internal opening. Fishes are far more
sensitive to sudden jars or sudden movements than to any sound. They
possess what is generally believed to be a special sense organ not
found in other animals. This is the lateral line which extends along
the sides of the body and which consists of a series of modified
scales (each one with a mucous channel) richly supplied with nerves.
The eyes are usually large and conspicuous. They differ mainly from
the eyes of other vertebrates in their myopic spherical crystalline
lens, made necessary by the density of the medium in which fishes
live. There are usually no eyelids, the skin of the body being
continuous but transparent over the eyes. Being near-sighted, fishes
do not discriminate readily among forms, their special senses fitting
them in general to distinguish motions of their enemies or prey rather
than to ascertain exactly the nature of particular things.
The colors of fishes are in general appearance protective. Thus most
individuals are white on the belly, mimicking the color of the sky to
the enemy which pursues them from below. Seen from above most of them
are greenish, like the water, or brownish gray and mottled, like the
bottom. Those that live on sand are sand-colored, those on lava black,
and those among rose-red sea-weeds bright red. In many cases,
especially among kinds that are protected by their activity, brilliant
colors and showy markings are developed. This is especially true among
fishes of the coral reefs, though species scarcely less brilliant are
found among the darters of our American brooks.
Among fresh-water fishes bright colors, crimson, scarlet, blue, creamy
white, are developed in the breeding season, the then vigorous males
being the most highly colored. Many of the feeble minnows even become
very brilliant in the nuptial season of May and June. Color in fishes is
formed by minute oil-sacs on the scales, and it often changes quickly
with changes in the nervous condition of the individuals.
=Development and life-history.=--The breeding habits of fishes are
extremely varied. Most fishes do not pair, but in some cases pairing
takes place as among higher animals. Ordinarily fishes lay their eggs
on the bottom in shallow water, either in brooks, lakes, or in the
sea. The eggs of fishes are commonly called spawn, and egg-laying is
referred to as spawning. The spawn of some fishes is esteemed a
special food delicacy. Spring is the usual time of spawning, though
some fishes spawn in summer and some even in winter; generally they
move from their usual haunts for the purpose. The eggs of the
different species vary much in size, ranging from an inch and a half
in diameter (barn-door skate) down to the tiniest dots, like those of
the herring. The number of eggs laid also varies greatly. The trout
lays from 500 to 1,000, the salmon about 10,000, the herring 30,000 to
40,000, and some species of river fish 500,000, while certain
flounders, sturgeons, and others each lay several millions of eggs.
The adults rarely pay any attention to the eggs, which are hatched
directly by the heat of the sun or by heat absorbed from the water.
The length of incubation varies much. When the young fish leaves the
egg-shell it carries, in the case of most species, a part of the yolk
still hanging to its body. Its eyes are very large, and its fins are
represented by thin strips of membrane. It usually undergoes no great
changes in development from the first, resembling the adult except in
size. But some of the ocean fishes show a metamorphosis almost as
striking as that of insects or toads or frogs.
Some fishes build nests. Sticklebacks build elaborate nests in the
brooks and defend them with spirit. Sunfishes do the same, but the
nests are clumsier and not so well cared for.
The salmon is the type of fishes which run up from the sea to lay
their eggs in fresh water. The king salmon of the Columbia River, for
example, leaves the sea in the high waters of March and ascends
without feeding for over a thousand miles, depositing its spawn in
some small brook in the fall. After making this long journey to lay
the eggs, the salmon become much exhausted, battered and worn, and are
often attacked by parasitic fungi. They soon die, probably none of
them ever surviving to lay eggs a second time.
=Classification.=--A fish is an aquatic vertebrate, fitted to breathe
the air contained in water, and never developing fingers and toes.
Accepting this broad general definition we find at once that there are
very great differences among fishes. Some differ more from others than
the ordinary forms differ from rabbits or birds. So although we have
entitled this chapter as if all fishes belonged to the class Pisces,
we cannot arrange them satisfactorily in less than three classes.
=The lancelets (Leptocardii).=--The lowest class of fish-like animals
is that of the lancelets, the Leptocardii. These little creatures,
translucent, buried in the sand, of the size and form of a small
toothpick, are fishes reduced to their lowest terms. They have the
form, life, and ways of a fish, but no differentiated skull, brain,
heart, or eyes. Moreover they have no limbs, no jaws, no teeth, no
scales. The few parts they do have are arranged as in a fish, and they
show something in common with the fish embryo. Lacking a distinct
head, the lancelets are put by some zoologists in a group called the
Acrania, as opposed to the Craniata, which includes all the other
vertebrates. Lancelets have been found in the North Atlantic and
Mediterranean, on the west coast of North America, on the east coast
of South America and on the coasts of Japan, Australia, New Zealand,
the East Indies and Malayan Islands. The best-known members of the
group belong to the genus _Amphioxus_. There are but one to two other
genera in the class.
[Illustration: FIG. 113.--A lamprey, _Petromyzon marinus_. (After
Goode.)]
=The lampreys and hag-fishes (Cyclostomata).=--The next class of
fish-like animals is that of the lampreys (fig. 113) and hag-fishes,
the Cyclostomata. The lampreys and hags are easily distinguished from
the true fishes by their sucking mouth without jaws, their single
median nostril, their eel-like shape and lack of lateral appendages or
paired fins. The hag-fishes (_Myxine_), which are marine, attach
themselves by means of a sucker-like mouth to living fishes (the cod
particularly), gradually scraping and eating their way into the
abdominal cavity of the fish. These hags or "borers" "approach most
nearly to the condition of an internal parasite of any vertebrate."
The lampreys, or lamprey-eels as they are often called because of
their superficial resemblance to true eels, are both marine and
fresh-water in their habitat, and most of them attach themselves to
live fishes and suck their blood. They also feed on crustacea,
insects, and worms. The brook-lamprey, _Lampetra wilderi_, is never
parasitic. It reaches its full size in larval life and transforms
simply for spawning. The sea- and lake-lampreys ascend small
fresh-water streams when ready to lay their eggs, few living to
return. Sometimes small piles of stones are made for nests. The young
undergo a considerable metamorphosis in their development. The largest
sea-lampreys reach a length of three feet. The common brook-lampreys
are from eight to twelve inches long only.
=The true fishes (Pisces).=--All the other fish-like animals are
grouped in the class Pisces. They are characterized, when compared
with the lower fish-like forms just referred to, by the presence of
jaws, shoulder girdle, and pelvic girdle. The class includes both the
cartilaginous and bony fishes, and is divided into three sub-classes,
namely, the Elasmobranchii, including the sharks, rays, skates,
torpedoes, etc., the Holocephali, including the chimæras (a few
strange-bodied forms), and the Teleostomi, including all the other
fishes, as the trout, catfishes, darters, bass, herring, cod,
mackerel, sturgeons, etc., etc.
=The sharks, skates, etc. (Elasmobranchii).=--The sharks and skates
are characterized by the possession of a skeleton composed of
cartilage and not bone, as in the bony fishes; they have no operculum;
their teeth are distinct, often large and highly specialized, and
their eggs are few and very large. There are two principal groups
among Elasmobranchii, viz., the sharks, which usually have an elongate
body, and always have the gill-openings on the sides, and the rays or
skates, which have a broad flattened body with the gill-openings
always on the under side. All the members of both groups are marine.
The sharks are active, fierce, usually large fishes, which live in the
surface-waters of the ocean and make war on other marine animals, all
of the species except half a dozen being fish-eaters. The shark's
mouth is on the under side of the usually conical head, and the
animal often turns over on its back in order to seize its prey. The
largest American sharks, and the largest of all fishes, are the great
basking-sharks (_Cetorhinus_), which reach a length of nearly forty
feet. They get their name from their habit of gathering in numbers and
floating motionless on the surface. They feed chiefly on fishes.
The hammer-headed sharks (_Sphyrna_) are odd sharks which have the
head mallet or kidney shaped, twice as wide as long, the eyes being
situated on the ends of the lateral expansions of the head. The
man-eating or great white sharks (_Carcharodon_) are nearly as large
as the basking-sharks, and are extremely voracious. They will follow
ships for long distances for the refuse thrown overboard. They do not
hesitate to attack man. Among the more familiar smaller sharks are the
dog-fishes and sand-sharks of our Atlantic coast.
The rays and skates are also carnivorous, but are with few exceptions
sluggish, lying at the bottom of shallow shore-waters. They feed on
crabs, molluscs, and bottom-fishes. The small common skates,
"tobacco-boxes" (_Raja erinacea_) (fig. 114), about twenty inches long,
and the larger "barn-door skates" (_R. lævis_), are numerous along the
Atlantic coast from Virginia northward. Especially interesting members
of this group, because of the peculiar character of the injuries
produced by them, are the sting-rays and torpedoes or electric-rays. The
sting-rays (_Dasyatis_) have spines near the base of the tail which
cause very painful wounds. The torpedoes (_Narcine_) have two large
electrical organs, one on each side of the body just behind the head,
with which they can give a strong electric shock. "The discharge from a
large individual is sufficient to temporarily disable a man, and were
these animals at all numerous they would prove dangerous to bathers."
Very different from the typical rays in external appearance are the
saw-fishes (_Pristis pectinatis_) which belong to this group. The body
is elongate and shark-like, and has a long saw-like snout. This saw,
which in large individuals may reach a length of six feet and a breadth
of twelve inches, makes its owner formidable among the small sardines
and herring-like fishes on which it feeds. The saw-fishes live in
tropical rivers, descending to the sea.
[Illustration: FIG. 114.--The common skate, _Raja erinacea_. (From
Kingsley.)]
=The bony fishes (Teleostomi).=--The bony or true fishes are
distinguished from the lampreys and sharks and rays by having in
general the skeleton bony, not cartilaginous, the skull provided with
membrane bones, and the eggs small and many. In this group are
included all the fishes of our fresh-water lakes, ponds, and streams
as well as most of the marine forms. Fish life, being spent under
water, is not familiar to most of us, and beginning students are
rarely helped enough in getting acquainted with the different kinds
and the interesting habits of fishes. But they offer a field of study
which is really of unusual interest and profit. We can refer in the
following paragraphs to but few of the numerous common and readily
found kinds, and to these but briefly.
Closely related to the sunfish, studied as example of the bony fishes,
are the various kinds of bass, as the "crappie" (_Pomoxis annularis_),
the calico bass (_P. separoides_), the rock-bass (_Ambloplites
rupestris_) and the large-mouthed and small-mouthed black bass
(_Micropterus salmoides_ and _M. dolomieu_ respectively). All the
members of this sunfish and bass family are carnivorous fishes
especially characteristic of the Mississippi valley.
Another family of many species especially common in the clear, swift,
and strong Eastern rivers is that of the darters and perches. The
darters are little slender-bodied fishes which lie motionless on the
bottom, moving like a flash when disturbed and slipping under stones
out of sight of their enemies. Some are most brilliantly colored,
surpassing in this respect all other fresh-water fishes.
Unlike the sunfishes and darters are the catfishes, composing a great
family, the Siluridæ. The catfish (_Ameiurus_) gets its name from the
long feelers about its mouth; from these feelers also come its other
names of horned pout, or bull-head. It has no scales, but its spines are
sharp and often barbed or jagged and capable of making a severe wound.
Remotely allied to the catfish are the suckers, minnows, and chubs,
with smooth scales, soft fins and soft bodies and the flesh full of
small bones. These little fish are very numerous in species, some
kinds swarming in all fresh water in America, Europe, and Asia. They
usually swim in the open water, the prey of every carnivorous fish,
making up by their fecundity and their insignificance for their lack
of defensive armature. In some species the male is adorned in the
spring with bright pigment, red, black, blue, or milk-white. In some
cases, too, it has bony warts or horns on its head or body. Such forms
are known to the boys as horned dace.
Most interesting to the angler are the fishes of the salmon and trout
(fig. 115) family, because they are gamy, beautiful, excellent as food
and above all perhaps because they live in the swiftest and clearest
waters in the most charming forests. The salmon live in the ocean most
of their life, but ascend the rivers from the sea to deposit their
eggs. The king salmon (_Oncorhynchus tschawytscha_) of the Columbia
goes up the great river more than a thousand miles, taking the whole
summer for it, and never feeding while in fresh water. Besides the
different kinds of salmon, the black-spotted or true trout, the charr
or red-spotted trout of various species, the whitefish (_Coregonus_),
the grayling (_Thymallus signifer_) and the famous ayu of Japan belong
to this family.
[Illustration: FIG. 115.--The rainbow-trout, _Salmo iridens_. (From
specimen.)]
In the sea are multitudes of fish forms arranged in many families. The
myriad species of eels agree in having no ventral fins and in having
the long flexible body of the snake. Most of them live in the sea, but
the single genus (_Anguilla_) or true eel which ascends the rivers is
exceedingly abundant and widely distributed. Most eels are extremely
voracious, but some of them have mouths that would barely admit a
pin-head. The codfish (_Gadus callarias_) is a creature of little
beauty but of great usefulness, swarming in all arctic and subarctic
seas. The herring (_Clupea harengus_), soft and weak in body, are more
numerous in individuals than any other fishes. The flounders (fig.
116) of many kinds lie flat on the sea-bottom. They have the head so
twisted that the two eyes occur both together on the uppermost side.
The members of the great mackerel tribe swim in the open sea, often in
great schools. Largest and swiftest of these is the sword-fish
(_Xiphias gladius_), in which the whole upper jaw is grown together to
form a long bony sword, a weapon of offence that can pierce the wooden
bottom of a boat.
[Illustration: FIG. 116.--The winter flounder, _Pseudopleuronectes
americanus_. (After Goode.)]
Many of the ocean fishes are of strange form and appearance. The
sea-horses (_Hippocampus_ sp.) (fig. 117) are odd fishes covered with
a bony shell and with the head having the physiognomy of that of a
horse. They are little fishes rarely a foot long, and cling by their
curved tails to floating seaweed. The pipefish (_Syngnathus fuscum_)
is a sea-horse straightened out. The porcupine-fishes and swellfishes
(_Tetraodontidæ_) have the power of filling the stomach with air which
they gulp from the surface. They then escape from their pursuers by
floating as a round spiny ball on the surface. The flying-fishes
(_Exocœtus_) leap out of the water and sail for long distances through
the air, like grasshoppers. They cannot flap their long pectoral fins
and do not truly fly; nevertheless they move swiftly through the air
and thus escape their pursuers. In its structure a flying-fish differs
little from a pike or other ordinary fish.
[Illustration: FIG. 117.--A sea-horse, _Hippocampus heptagonus_.
(After Goode.)]
For an account of the fishes of North America see Jordan's "Manual of
Vertebrates," eighth edition, pp. 5-173, and Jordan and Evermann's
"Fishes of North and Middle America," where the 3,127 species known
from our continent are described in detail with illustrative figures.
=Habits and adaptations.=--The chief part of a fish's life is devoted
to eating, and as most fishes feed on other fishes, all are equally
considerably occupied in providing for their own escape.
In general the provisions for seizing prey are confined to sharp teeth
and the strong muscles which propel the caudal fin. But in some cases
special contrivances appear. In one large group known collectively as
the "anglers" the first spine of the dorsal fin hangs over the mouth.
It has at its tip a fleshy appendage which serves as a bait. Little
fishes nibble at this, the mouth opens, and they are gone. In the deep
seas, many fishes are provided with phosphorescent spots or lanterns
which light up the dark waters, and enable them to see their prey. In
storms these lantern-fishes sometimes lose their bearings and are
thrown upward to the surface.
In general the more predatory in its habits any fish is the sharper
its teeth, and the broader its mouth. Among brook-fishes the pickerel
has the largest mouth and the sharpest teeth. It has been called a
"mere machine for the assimilation of other organisms." The trout has
a large mouth and sharp teeth. It is a swift, voracious, and predatory
fish, feeding even on its own kind. The sunfish is less greedy and its
mouth and teeth are smaller, though it too eats other fish.
As means of escape, most fishes depend on their speed in swimming. But
some hide among rocks and weeds, disguising themselves by a change in
color to match their surroundings. Others, like the flounders and
skates, lie flat on the bottom. Still others retreat to the shallows
or the depths or the rock-pools or to any place safer than the open
sea. Some are protected by spines which they erect when attacked. Some
erect these spines only after they have been swallowed, tearing the
stomach of their enemy and killing it, but too late to save
themselves. Again in some species the spines are armed with poison
which benumbs the enemy. Sometimes an electric battery about the head
or on the sides gives the biting fish a severe shock and drives him
away. Such batteries are found in the electric rays or torpedo, in the
electric eel of Paraguay, the electric catfish of the Nile, the
electric stargazer and other fishes.
Some fishes are protected by their poor and bitter flesh. Some have
bony coats of mail and sometimes the coat of mail is covered with
thorns, as in the porcupine-fish. This fish and various of its
relatives have the habit of filling the stomach with air when
disturbed, then floating belly upward, the thorny back only within
reach of its enemies.
[Illustration: FIG. 118.--The remora, or cling fish, _Remoropsis
brachyptera_. Note sucker on top of head. (After Goode.)]
Many species (cling fishes) attach themselves to the rocks by a fleshy
sucking-disk. Some (_Remora_) (fig. 118) cling to larger fishes by a
strange sucking-disk on the head, a transformed dorsal fin, being thus
shielded from the attacks of fish smaller than their protectors. Some
small fishes seek the shelter of the floating jellyfishes, lurking
among their poisoned tentacles. Others creep into the masses of
floating gulf-weed. Some creep into the shell of clams and snails. In
the open channel of a sponge, the mouth of a tunicate and in similar
cavities of various animals, little fishes may be found. A few fishes
(hag-fishes) are parasitic on others, boring their way into the body
and devouring the muscles with their rasp-like teeth.
Some fishes are provided with peculiar modifications of the gills
which enable them to breathe for a time out of water. Such fish have
the pectoral fins modified for a rather poor kind of locomotion on
land, thus enabling them to move from pond to pond or from stream to
stream. In cold climates the fishes must either migrate to warmer
latitudes in winter, as some do, or withstand variously the cold,
often freezing weather. Some fish can be frozen solid, and yet thaw
out and resume active living. Some lie at the bottoms of deep pools
through the colder periods, while many others, such as the minnows,
chubs, and other kinds common in small streams, bury themselves in the
mud, and lie dormant or asleep through the whole winter. On the other
hand in countries where the long intense rainless summers dry up the
pools, some fishes have the habit of burying themselves in the mud,
which, with slime from the body, forms about them a sort of tight
cement ball in which they lie dormant until the rains come. "Thus a
lung-fish (called _Protopterus_), found in Asia and Africa, so
completely slimes a ball of mud around it that it may live for more
than one season, perhaps many; it has been dug up and sent to England,
still enclosed in its round mud-case, and when it was placed in warm
water it awoke as well as ever."
=Food-fishes and fish-hatcheries.=--Most fishes are suitable for food,
though not all. Some are too small to be worth catching or too bony to
be worth eating. Some of the larger ones, especially the sharks, are
tough and rank. A few are bitter and in the tropics a number of
species feed on poisonous coelenterates about the coral reefs,
becoming themselves poisonous in turn. But a fish is rarely poisonous
or unwholesome unless it takes poisonous food. Where fishes of a kind
specially used for food gather in great numbers at certain seasons of
the year, fishing is carried on extensively and with an elaborate
equipment. Such fisheries, some of which have been long known, are
scattered all over the world. Along the shores of the Mediterranean
Sea, and on the coasts of Norway, France, the British Isles and Japan
are numerous great fishing-places. But "nowhere are there found such
large fisheries as those along the northern Atlantic coasts of our own
continent, extending from Massachusetts to Labrador. Especially on the
banks of Newfoundland are codfish, herring, and mackerel caught."
Among our fresh-water fisheries the great salmon fisheries of the
Penobscot and Columbia rivers and of the Karluk and other rivers of
Alaska are the best known. The whitefish of our Great Lakes is also
one of the important food-fishes of the world.
In many places fishes are raised in so-called hatcheries, not usually
for immediate consumption but for the purpose of stocking ponds and
streams either in the neighborhood of the hatchery or in distant
waters which the special species cultivated has not been able
naturally to reach. The eggs of some fishes are large and
non-adherent, two features which greatly favor artificial impregnation
and hatching. In the hatcheries the eggs are put first into warm
water, where development begins; they are then removed into cool
water, which arrests development without injury, making shipment
possible. The eggs of salmon and trout in particular can be sent long
distances to suitable streams or ponds. The eggs of the shad have been
thus carried from the East to the streams of California and trout have
been distributed to many streams in our country which by themselves
they could never have reached.
The salmon is a conspicuous example of those fishes which can be
artificially propagated. The eggs of the salmon are large, firm, and
separate from each other. If the female fish be caught when the eggs
are ripe and her body be pressed over a pan of water the eggs will
flow out into the water. By a similar process the milt or male
sperm-cells can be procured and poured over the eggs to fertilize
them. The young after hatching are kept for a few days or weeks in
artificial pools, till the yolk-sacs are absorbed and they can take
care of themselves. They are then turned into the stream, where they
drift tail foremost with the current and pass downward to the sea. All
trout may be treated in similar fashion, but there are many
food-fishes which cannot be handled in this way. In some the eggs are
small or soft, or viscid and adhering in bunches. In others the
life-habits make artificial fertilization impossible. Such species are
artificially reared only by catching the young and taking them from
one stream to another. To this type belong the black bass, the
sunfish, the catfish and other familiar forms.
FOOTNOTE:
[16] The author wishes to call the attention of teacher and student to
the plan (referred to in the Preface, page v) adopted in writing the
directions for the dissections. The sequence of the references to the
various organs depends on the actual course of the dissection, and not
upon the association of organs in systems. And the directions are so
much condensed that they are hardly more than a means of orienting the
student, leaving him to work out independently, or by the aid of more
detailed accounts (sometimes specifically referred to), the details of
the dissection.