EMBRYOLOGY AND GROWTH OF FISHES
=Segmentation of the Egg.=--The egg of the fish develops only after
fertilization (amphimixis). This process is the union of its nuclear
substance with that of the sperm-cell from the male, each cell carrying
its equal share in the function of heredity. When this process takes
place the egg is ready to begin its segmentation. The eggs of all
fishes are single cells containing more or less food-yolk. The presence
of this food-yolk affects the manner of segmentation in general,
those eggs having the least amount of food-yolk developing most
typically. The simplest of all fish like vertebrates, the lancelet
(_Branchiostoma_) has very small eggs, and in their early development
it passes through stages that are typical for all many-celled animals.
The first stage in development is the simple splitting of the egg
into two halves. These two daughter cells next divide so that there
are four cells; each of these divides, and this division is repeated
until a great number of cells is produced. The phenomenon of repeated
division of the germ-cell is called cleavage, and this cleavage is the
first stage of development in the case of all many-celled animals.
Instead of forming a solid mass the cells arrange themselves in such a
way as to form a hollow ball, the wall being a layer one cell thick.
The included cavity is called the segmentation cavity, and the whole
structure is known as a blastula. This stage also is common to all the
many-celled animals. The next stage is the conversion of the blastula
into a double-walled cup, known as a gastrula by the pushing in of one
side. All the cells of the blastula are very small, but those on one
side are somewhat larger than those of the other, and here the wall
first flattens and then bends in until finally the larger cells come
into contact with the smaller and the segmentation cavity is entirely
obliterated. There is now an inner layer of cells and an outer layer,
the inner layer being known as the endoblast and the outer as the
ectoblast. The cavity of the cup thus formed is the archenteron and
gives rise primarily to the alimentary canal. This third well-marked
stage is called the gastrula stage; and it is thought to occur either
typically or in some modified form in the development of all metazoa,
or many-celled animals. In the lampreys, the Ganoids, and the Dipnoans
the eggs contain a much greater quantity of yolk than those of the
lancelet, but the segmentation resembles that of the lancelet in
that it is complete; that is, the whole mass of the egg divides into
cells. There is a great difference, however, in the size of the cells,
those at the upper pole being much smaller than those at the lower.
In _Petromyzon_ and the Dipnoans blastula and gastrula stages result,
which, though differing in some particulars from the corresponding
stages of the lancelet, may yet readily be compared with them. In
the hagfishes, sharks, rays, chimæras, and most bony fishes there
is a large quantity of yolk, and the protoplasm, instead of being
distributed evenly throughout the egg, is for the most part accumulated
upon one side, the nucleus being within this mass of protoplasm. When
the food substance or yolk is consumed and the little fish is able
to shift for itself, it leaves the egg-envelopes and is said to be
hatched. The figures on page 135 show some of the stages by which cells
are multiplied and ultimately grouped together to form the little fish.
=Post-embryonic Development.=--In all the fishes the development of the
embryo goes on within the egg long after the gastrula stage is passed,
and until the embryo becomes a complex body, composed of many differing
tissues and organs. Almost all the development may take place within
the egg, so that when the young animal hatches there is necessary
little more than a rapid growth and increase of size to make it a fully
developed mature animal. This is the case with most fishes: a little
fish just hatched has most of the tissues and organs of a full-grown
fish, and is simply a small fish. But in the case of some fishes the
young hatches from the egg before it has reached such an advanced state
of development, and the young looks very different from its parent. It
must yet undergo considerable change before it reaches the structural
condition of a fully developed and fully grown fish. Thus the
development of most fishes is almost wholly embryonic development--that
is, development within the egg or in the body of the mother--while the
development of some of them is to a considerable degree post-embryonic
or larval development. There is no important difference between
embryonic and post-embryonic development. The development is continuous
from egg-cell to mature animal and, whether inside or outside of an
egg, it goes on with a degree of regularity. While certain fishes are
subject to a sort of metamorphosis, the nature of this change is in no
way to be compared with the change in insects which undergo a complete
metamorphosis. In the insects all the organs of the body are broken
down and rebuilt in the process of change. In all fishes a structure
once formed maintains a more nearly continuous integrity although often
considerably altered in form.
=General Laws of Development.=--The general law of development may
be briefly stated as follows: All many-celled animals begin life as
a single cell, the fertilized egg-cell; each animal goes through a
certain orderly series of developmental changes which, accompanied by
growth, leads the animal to change from single-cell to many-celled,
complex form characteristic of the species to which the animal belongs;
this development is from simple to complex structural condition; the
development is the same for all individuals of one species. While all
animals begin development similarly, the course of development in the
different groups soon diverges, the divergence being of the nature of
a branching, like that shown in the growth of a tree. In the free tips
of the smallest branches we have represented the various species of
animals in their fully developed condition, all standing clearly apart
from each other. But in tracing back the development of any kind of
animal we soon come to a point where it very much resembles or becomes
apparently identical with some other kind of animal, and going farther
back we find it resembling other animals in their young condition, and
so on until we come to that first stage of development, that trunk
stage where all animals are structurally alike. Any animal at any stage
in its existence differs absolutely from any other kind of animal, in
this respect: it can develop into only its own kind. There is something
inherent in each developing animal that gives it an identity of its
own. Although in its young stages it may be indistinguishable from some
other species of animal in its young stages, it is sure to come out,
when fully developed, an individual of the same kind as its parents
were or are. The young fish and the young salamander may be alike to
all appearance, but one embryo is sure to develop into a fish, and
the other into a salamander. This certainty of an embryo to become an
individual of a certain kind is called the law of heredity. Viewed in
the light of development, there must be as great a difference between
one egg and another as between one animal and another, for the greater
difference is included in the less.
=The Significance of Facts of Development.=--The significance of the
process of development in any species is yet far from completely
understood. It is believed that many of the various stages in the
development of an animal correspond to or repeat the structural
condition of the animal's ancestors. Naturalists believe that all
animals having a notochord at any stage in their existence are related
to each other through being descended from a common ancestor, the first
or oldest chordate or back-boned animal. In fact it is because all
these chordate animals--the lancelets, lampreys, fishes, batrachians,
the reptiles, the birds, and the mammals--have descended from a common
ancestor that they all develop a notochord, and those most highly
organized replace this by a complete back-bone. It is believed that
the descendants of the first back-boned animal have, in the course
of many generations, branched off little by little from the original
type until there came to exist very real and obvious differences among
the back-boned animals--differences which among the living back-boned
animals are familiar to all of us. The course of development of an
individual animal is believed to be a very rapid and evidently much
condensed and changed recapitulation of the history which the species
or kind of animal to which the developing individual belongs has passed
through in the course of its descent through a long series of gradually
changing ancestors. If this is true, then we can readily understand
why the fish and the salamander and the tortoise and bird and rabbit
are all alike in their earlier stages of development, and gradually
come to differ more and more as they pass through later and later
developmental stages.
=Development of the Bony Fishes.[9]= The mode of development of bony
fishes differs in many and apparently important regards from that of
their nearest kindred, the Ganoids. In their eggs a large amount of
yolk is present, and its relations to the embryo have become widely
specialized. As a rule, the egg of a Teleost is small, perfectly
spherical, and enclosed in delicate but greatly distended membranes.
The germ disc is especially small, appearing on the surface as an
almost transparent fleck. Among the fishes whose eggs float at the
surface during development, as of many pelagic Teleosts, e.g., the
sea-bass, _Centropristes striatus_, the yolk is lighter in specific
gravity than the germ; it is of fluid-like consistency, almost
transparent. In the yolk at the upper pole of the egg an oil globule
usually occurs; this serves to lighten the relative weight of the
entire egg, and from its position must aid in keeping this pole of the
egg uppermost.
[Illustration: FIG. 97.--Development of Sea-bass, _Centropristes
striatus_ (Linnæus). _a_, egg prior to germination; _b_, germ-disk
after first cleavage; _c_, germ-disk after third cleavage; _d_, embryo
just before hatching. (After H. V. Wilson.)]
In the early segmentation of the germ the first cleavage plane is
established, and the nuclear divisions have taken place for the second;
in the latter the third cleavage has been completed. As in other fishes
these cleavages are vertical, the third parallel to the first. A
segmentation cavity occurs as a central space between the blastomeres,
as it does in the sturgeon and garpike.
In stages of late segmentation the segmentation cavity is greatly
flattened, but extends to the marginal cells of the germ-disk; its
roof consists of two tiers of blastomeres, its floor of a thin film of
the unsegmented substance of the germ; the marginal blastomeres are
continuous with both roof and floor of the cavity, and are produced
into a thin film which passes downward, around the sides of the yolk.
Later the segmentation cavity is still further flattened; its roof
is now a dome-shaped mass of blastomeres; the marginal cells have
multiplied, and their nuclei are seen in the layer of the germ, below
the plane of the segmentation cavity. These are seen in the surface
view of the marginal cells of this stage; they are separated by
cell boundaries only at the sides; below they are continuous in the
superficial down-reaching layer of the germ. The marginal cells shortly
lose all traces of having been separate; their nuclei, by continued
division, spread into the layer of germ flooring the segmentation
cavity, and into the delicate film of germ which now surrounds the
entire yolk. Thus is formed the _periblast_ of the Teleost development,
which from this point onward is to separate the embryo from the yolk;
it is clearly the specialized inner part of the germ, which, becoming
fluid-like, loses its cell-walls, although retaining and multiplying
its nuclei. Later the periblast comes into intimate relations with the
growing embryo; it lies directly against it, and appears to receive
cell increments from it at various regions; on the other hand, the
nuclei of the periblast, from their intimate relations with the yolk,
are supposed to subserve some function in its assimilation.
Aside from the question of periblast, the growth of the blastoderm
appears not unlike that of the sturgeon. From the blastula stage to
that of the early gastrula, the changes have been but slight; the
blastoderm has greatly flattened out as its margins grow downward,
leaving the segmentation cavity apparent. The rim of the blastoderm
has become thickened as the 'germ-ring'; and immediately in front of
the dorsal lip of the blastopore its thickening marks the appearance
of the embryo. The germ-ring continues to grow downward, and shows
more prominently the outline of the embryo; this now terminates at
the head region; while on either side of this point spreads out
tail-ward on either side the indefinite layer of outgrowing mesoderm.
In the next stage the closure of the blastopore is rapidly becoming
completed; in front of it stretches the widened and elongated form of
the embryo. The yolk-plug is next replaced by periblast, the dorsal
lip by the tail-mass, or more accurately the dorsal section of the
germ-rim; the coelenteron under the dorsal lip has here disappeared,
on account of the close approximation of the embryo to the periblast;
its last remnant, the Kupffer's vesicle, is shortly to disappear. The
germ-layers become confluent, but, unlike the sturgeon, the flattening
of the dorsal germ-ring does not permit the formation of a neurenteric
canal.
[Illustration: FIG. 98. Sea-bass, _Centropristes striatus_, natural
size. (From life, by R. W. Shufeldt.)--Page 137.]
The process of the development of the germ-layers in Teleosts appears
as an abbreviated one, although in many of its details it is but
imperfectly known. In the development of the medullary groove, as
an example, the following peculiarities exist: the medullary region
is but an insunken mass of cells without a trace of the groove-like
surface indentation. It is only later, when becoming separate from the
ectoderm, that it acquires its rounded character; its cellular elements
then group themselves symmetrically with reference to a sagittal plane,
where later, by their dissociation, the canal of the spinal cord is
formed. The growth of the entoderm is another instance of specialized
development. In an early stage the entoderm exists in the axial
region, its thickness tapering away abruptly on either side; its lower
surface is closely apposed to the periblast; its dorsal thickening
will shortly become separate as the notochord. In a following stage of
development the entoderm is seen to arch upward in the median line as a
preliminary stage in the formation of the cavity of the gut. Later, by
the approximation of the entoderm-cells in the median ventral line, the
condition is reached where the completed gut-cavity exists.
The formation of the mesoderm in Teleosts is not definitely understood.
It is usually said to arise as a process of 'delamination,' i.e.,
detaching itself in a mass from the entoderm. Its origin is, however,
looked upon generally as of a specialized and secondary character.
The mode of formation of the gill-slit of the Teleost does not differ
from that in other groups; an evagination of the entoderm coming in
contact with an invaginated tract of ectoderm fuses, and at this point
an opening is later established.
The late embryo of the Teleost, though of rounded form, is the more
deeply implanted in the yolk-sac than that of the sturgeon; it is
transparent, allowing notochord, primitive segments, heart, and
sense-organs to be readily distinguished; at about this stage both anus
and mouth are making their appearance.
[Illustration: FIG. 99.--Young Sword-fish, _Xiphias gladius_ (Linnæus).
(After Lütken.)]
=The Larval Development of Fishes.[10]=--"When the young fish has freed
itself from its egg-membranes it gives but little suggestion of its
adult form. It enters upon a larval existence, which continues until
maturity. The period of change of form varies widely in the different
groups of fishes, from a few weeks' to longer than a year's duration;
and the extent of the changes that the larva undergoes are often
surprisingly broad, investing every organ and tissue of the body, the
immature fish passing through a series of form stages which differ one
from the other in a way strongly contrasting with the mode of growth
of amniotes; since the chick, reptile, or mammal emerges from its
embryonic membranes in nearly its adult form.
[Illustration: FIG. 100.--Sword-fish, _Xiphias gladius_ (Linnæus).
(After Day.)]
The fish may, in general, be said to begin its existence as a larva as
soon as it emerges from its egg-membranes. In some instances, however,
it is difficult to decide at what point the larval stage is actually
initiated: thus in sharks the excessive amount of yolk material which
has been provided for the growth of the larva renders unnecessary the
emerging from the egg at an early stage; and the larval period is
accordingly to be traced back to stages that are still enclosed in the
egg-membranes. In all cases the larval life may be said to begin when
the following conditions have been fulfilled: the outward form of the
larva must be well defined, separating it from the mass of yolk, its
motions must be active, it must possess a continuous vertical fin-fold
passing dorsally from the head region to the body terminal, and thence
ventrally as far as the yolk region; and the following structures,
characteristic in outward appearance, must also be established: the
sense-organs--eye, ear, and nose--mouth and anus, and one or more
gill-clefts.
[Illustration: FIG. 101.--Larva of the Sail-fish, _Istiophorus_, very
young. (After Lütken.)]
[Illustration: FIG. 102.--Larva of Brook Lamprey, _Lampetra wilderi_,
before transformation, being as large as the adult, toothless, and more
distinctly segmented.]
[Illustration: FIG. 103.--Common Eel. _Anguilla chrisypa_ Rafinesque.
Family _Anguillidæ_.]
Among the different groups of fishes the larval changes are brought
about in widely different ways. These larval peculiarities appear at
first of far-reaching significance, but may ultimately be attributed,
the writer believes, to changed environmental conditions, wherein one
process may be lengthened, another shortened. So, too, the changes
from one stage to another may occur with surprising abruptness. As a
rule, it may be said the larval stage is of longest duration in the
Cyclostomes, and thence diminished in length in sharks, lung-fishes,
Ganoids, and Teleosts; in the last-named group a very much curtailed
(i.e., precocious) larval life may often occur.
[Illustration: FIG. 104.--Larva of Common Eel, _Anguilla chrisypa_
(Rafinesque), called _Leptocephalus grassii_. (After Eigenmann.)]
The metamorphoses of the newly hatched Teleost must finally be
reviewed; they are certainly the most varied and striking of all larval
fishes, and, singularly enough, appear to be crowded into the briefest
space of time; the young fish, hatched often as early as on the fourth
day, is then of the most immature character; it is transparent,
delicate, easily injured, inactive; within a month, however, it may
have assumed almost every detail of its mature form. A form hatching
three millimeters in length may acquire the adult form before it
becomes much longer than a centimeter.
[Illustration: FIG. 105.--Larva of Sturgeon, _Acipenser sturio_
(Linnæus). (After Kupffer, per Dean.)]
[Illustration: FIG. 106.--Larva (called _Tholichthys_) of _Chætodon
sedentarius_ (Poey). Cuba. (After Lütken,)]
[Illustration: FIG. 107.--Butterfly-fish, _Chætodon capistratus_
Linnæus. Jamaica.]
=Peculiar Larval Forms.=--The young fish usually differs from the
adult mainly in size and proportions. The head is larger in the young,
the fins are lower, the appendages less developed, and the body
more slender in the young than in the adult. But to most of these
distinctions there are numerous exceptions, and in some fish there
is a change so marked as to be fairly called a metamorphosis. In
such cases the young fish in its first condition is properly called
a larva. The larva of the lamprey (_Petromyzon_) is nearly blind and
toothless, with slender head, and was long supposed to belong to a
different genus (_Ammocoetes_) from the adult. The larva of sharks and
rays, and also of Dipnoans and Crossopterygians, are provided with
bushy external gills, which disappear in the process of development.
In most soft-rayed fishes the embryonic fringe which precedes the
development of the vertical fins persists for a considerable time.
In many young fishes, especially the _Chætodontidæ_ and their allies
(butterfly-fishes), the young fish has the head armed with broad
plates formed by the backward extension of certain membrane-bones.
In other forms the bones of the head are in the young provided with
long spines or with serrations, which vanish totally with age. Such a
change is noticeable in the swordfish. In this species the production
of the bones of the snout and upper jaw into a long bony sword, or
weapon of offense, takes place only with age. The young fish have jaws
more normally formed, and armed with ordinary teeth. In the headfish
(_Mola mola_) large changes take place in the course of growth, and the
young have been taken for a different type of fishes. Among certain
soft-rayed fishes and eels the young is often developed in a peculiar
way, being very soft, translucent, or band-like, and formed of large
or loosely aggregated cells. These peculiar organisms, long known as
leptocephali, have been shown to be the normal young of fishes when
mature very different. In the ladyfish (_Albula_) Dr. Gilbert has
shown, by a full series of specimens, that in their further growth
these pellucid fishes shrink in size, acquiring greater compactness
of body, until finally reaching about half their maximum length as
larvæ. After this, acquiring essentially the form of the adult fish,
they begin a process of regular growth. This leptocephalous condition
is thought by Günther to be due to arrest of growth in abnormal
individuals, but this is not the case in _Albula_, and it is probably
fully normal in the conger and other eels. In the surf-fishes the
larvæ have their vertical fins greatly elevated, much higher than in
the adult, while the body is much more closely compressed. In the
deal-fish (_Trachypterus_) the form of the body and fins changes
greatly with age, the body becoming more elongate and the fins lower.
The differences between different stages of the same fish seem greater
than the differences between distinct species. In fact with this and
with other forms which change with age, almost the only test of species
is found in the count of the fin-rays. So far as known the numbers of
these structures do not change. In the moonfishes (_Carangidæ_) the
changes with age are often very considerable. We copy Lütken's figure
of the changes in the genus _Selene_ (fig. 113). Similar changes take
place in _Alectis_, _Vomer_, and other genera.
[Illustration: _Fig. 108._--_Mola mola_ (Linnæus). Very early larval
stage of the Headfish, called _Centaurus boöps_. (After Richardson.)]
[Illustration: FIG. 109.--_Mola mola_ (Linnæus). Early larval stage,
called _Molacanthus nummularis_. (After Ryder.)]
[Illustration: FIG. 110.--_Mola mola_ (Linnæus). Advanced larval stage.
(After Ryder.)]
=The Development of Flounders.=--In the great group of flounders and
soles (_Heterosomata_) the body is greatly compressed and the species
swim on one side or lie flat on the bottom, with one side uppermost.
This upper side is colored like the bottom, sand-color, gray, or brown,
while the lower side is mostly white. Both eyes are brought around
to the upper side by a twisting of the cranium and a modification or
division of the frontal bones. When the young flounder is hatched it
is translucent and symmetrical, swimming vertically in the water, with
one eye on either side of the head. After a little the young fish rests
the ventral edge on the bottom. It then leans to one side, and as its
position gradually becomes horizontal the eye on the lower side moves
across with its frontal and other bones to the other side. In most
species it passes directly under the first interneurals of the dorsal
fin. These changes are best observed in the genus _Platophrys_.
=Hybridism.=--Hybridism is very rare among fishes in a state of nature.
Two or three peculiar forms among the snappers (_Lutianus_) in Cuba
seem fairly attributable to hybridism, the single specimen of each
showing a remarkable mixture of characters belonging to two other
common species. Hybrids may be readily made in artificial impregnation
among those fishes with which this process is practicable. Hybrids of
the different salmon or trout usually share nearly equally the traits
of the parent species.
=The Age of Fishes.=--The age of fishes is seldom measured by a
definite period of years. Most of them grow as long as they live, and
apparently live until they fall victims to some stronger species.
It is reputed that carp and pike have lived for a century, but the
evidence needs verification. Some fishes, as the salmon of the Pacific
(_Oncorhynchus_), have a definite period of growth (usually four
years) before spawning. After this act all the individuals die so
far as known. In Japan and China the Ice-fish (_Salanx_), a very
long, slender, transparent fish allied to the trout, may possibly be
annual in habit, all the individuals perhaps dying in the fall to
be reproduced from eggs in the spring. But this alleged habit needs
verification.
[Illustration: FIG. 111.--Headfish (adult), _Mola mola_ (Linnæus).
Virginia.]
=Tenacity of Life.=--Fishes differ greatly in tenacity of life. In
general, fishes of the deep seas die at once if brought near the
surface. This is due to the reduction of external pressure. The
internal pressure forces the stomach out through the mouth and may
burst the air-bladder and the large blood-vessels. Marine fishes
usually die very soon after being drawn out from the sea.
[Illustration: FIG. 112.--_Albula vulpes_ (Linnæus). Transformation
of the Ladyfish, from the translucent, loosely compacted larva to the
smaller, firm-bodied young. Gulf of California. (After Gilbert.)]
[Illustration: FIG. 113.--Development of the Horsehead-fish, _Selene
vomer_ (Linnæus). Family _Carangidæ_. (After Lütken.)]
Some fresh-water fishes are very fragile, dying soon in the air, often
with injured air-bladder or blood-vessels. They will die even sooner
in foul water. Other fishes are extremely tenacious of life. The
mud-minnow (_Umbra_) is sometimes ploughed up in the half-dried mud of
Wisconsin prairies. The related Alaskan blackfish (_Dallia_) has been
fed frozen to dogs, escaping alive from their stomachs after being
thawed out. Many of the catfishes (_Siluridæ_) will live after lying
half-dried in the dust for hours. The Dipnoan, _Lepidosiren_, lives in
a ball of half-dried mud during the arid season, and certain fishes,
mostly Asiatic, belonging to the group _Labyrinthici_, with accessory
breathing organ can long maintain themselves out of water. Among these
is the China-fish (_Ophiocephalus_), often kept alive in the Chinese
settlements in California and Hawaii. Some fishes can readily endure
prolonged hunger, while others succumb as readily as a bird or a mammal.
[Illustration: FIG. 114.--Ice-fish, _Salanx hyalocranius_ Abbott.
Family _Salangidæ_. Tientsin, China.]
[Illustration: FIG. 115.--Alaska Blackfish, _Dallia pectoralis_ (Bean).
St. Michaels, Alaska.]
=The Effects of Temperature on Fish.=--The limits of distribution of
many fishes are marked by changes in temperature. Few marine fishes can
endure any sudden or great change in this regard, although fresh-water
fishes adapt themselves to the seasons. I have seen the cutlass-fish
(_Trichiurus_) benumbed with cold off the coast of Florida while the
temperature was still above the frost-line. Those fishes which are
tenacious of life and little sensitive to changes in climate and food
are most successfully acclimatized or domesticated. The Chinese carp
(_Cyprinus carpio_) and the Japanese goldfish (_Carassius auratus_)
have been naturalized in almost all temperate and tropical river
basins. Within the limits of clear, cold waters most of the salmon
and trout are readily transplanted. But some similar fishes (as the
grayling) are very sensitive to the least change in conditions. Most of
the catfish (_Siluridæ_) will thrive in almost any fresh waters except
those which are very cold.
[Illustration: FIG. 116.--Snake-headed China-fish, _Ophiocephalus
barca_. India. (After Day.)]
=Transportation of Fishes.=--The eggs of species of salmon, placed in
ice to retard their development, have been successfully transplanted
to great distances. The quinnat-salmon has been thus transferred
from California to Australia. It has been found possible to stock
rivers and lakes with desirable species, or to restock those in which
the fish-supply has been partly destroyed, through the means of
artificially impregnated eggs.
The method still followed is said to be the discovery of J. L. Jacobi
of Westphalia (about 1760). This process permits the saving of nearly
all the eggs produced by the individuals taken. In a condition of
nature very many of these eggs would be left unfertilized, or be
destroyed by other animals. Fishes are readily kept in captivity
in properly constructed aquaria. Unless injured in capture or
transportation, there are few species outside the deep seas which
cannot adapt themselves to life in a well-constructed aquarium.
=Reproduction of Lost Parts.=--Fishes have little power to reproduce
lost parts. Only the tips of fleshy structures are thus restored after
injury. Sometimes a fish in which the tail has been bitten off will
survive the injury. The wound will heal, leaving the animal with a
truncate body, fin-rays sometimes arising from the scars.
[Illustration: FIG. 117.--Monstrous Goldfish (bred in Japan),
_Carassius auratus_ (Linnæus). (After Günther.)]
=Monstrosities among Fishes.=--Monstrosities are rare among fishes
in a state of nature. Two-headed young are frequently seen at
salmon-hatcheries, and other abnormally divided or united young
are not infrequent. Among domesticated species monstrosities are
not infrequent, and sometimes, as in the goldfish, these have been
perpetuated to become distinct breeds or races. Goldfishes with
telescopic eyes and fantastic fins, and with the green coloration
changed to orange, are reared in Japan, and are often seen in other
countries. The carp has also been largely modified, the changes taking
place chiefly in the scales. Some are naked (leather-carp), others
(mirror-carp) have a few large scales arranged in series.
FOOTNOTES:
[9] This account of the normal development of the Teleost fishes is
condensed from Dr. Dean's "Fishes Living and Fossil," in which work the
details of growth in the Teleost are contrasted with those of other
types of fishes.
[10] This paragraph is condensed from Dean's "Fishes Living and Fossil."