THE EXTERIOR OF THE FISH
=Form of Body.=--With a glance at the fish as a living organism and
some knowledge of those structures which are to be readily seen without
dissection, we are prepared to examine its anatomy in detail, and to
note some of the variations which may be seen in different parts of the
great group.
In general fishes are boat-shaped, adapted for swift progress through
the water. They are longer than broad or deep and the greatest width is
in front of the middle, leaving the compressed paddle-like tail as the
chief organ of locomotion.
[Illustration: FIG. 6.--Pine-cone Fish, _Monocentris japonicus_
(Houttuyn). Waka, Japan.]
But to all these statements there are numerous exceptions. Some fishes
depend for protection, not on swiftness, but on the thorny skin or
a bony coat of mail. Some of these are almost globular in form, and
their outline bears no resemblance to that of a boat. The trunkfish
(_Ostracion_) in a hard bony box has no need of rapid progress.
[Illustration: FIG. 7.--Porcupine-fish, _Diodon hystrix_ (Linnæus).
Tortugas Islands.]
[Illustration: FIG. 8.--Thread-eel, _Nemichthys avocetta_ Jordan and
Gilbert. Vancouver Island.]
[Illustration: FIG. 9.--Sea-horse, _Hippocampus hudsonius_ Dekay.
Virginia.]
[Illustration: FIG. 10.--Harvest-fish, _Peprilus paru_ (Linnæus).
Virginia.]
[Illustration: FIG. 11.--Anko or Fishing-frog, _Lophius litulon_
(Jordan). Matsushima Bay, Japan. (The short line in all cases shows the
degree of reduction; it represents an inch of the fish's length.)]
The pine-cone fish (_Monocentris japonicus_) adds strong fin-spines to
its bony box, and the porcupine fish (_Diodon hystrix_) is covered with
long prickles which keep away all enemies.
Among swift fishes, there are some in which the body is much deeper
than long, as in _Antigonia_. Certain sluggish fishes seem to be all
head and tail, looking as though the body by some accident had been
omitted. These, like the headfish (_Mola mola_) are protected by a
leathery skin. Other fishes, as the eels, are extremely long and
slender, and some carry this elongation to great extremes. Usually the
head is in a line with the axis of the body, but in some cases, as the
sea-horse (_Hippocampus_), the head is placed at right angles to the
axis, and the body itself is curved and cannot be straightened without
injury. The type of the swiftest fish is seen among the mackerels and
tunnies, where every outline is such that a racing yacht might copy it.
The body or head of the fish is said to be compressed when it is
flattened sidewise, depressed when it is flattened vertically. Thus the
_Peprilus_ (Fig. 10) is said to be compressed, while the fishing-frog
(_Lophius_) (Fig. 11) has a depressed body and head. Other terms as
truncate (cut off short), attenuate (long-drawn out), robust, cuboid,
filiform, and the like may be needed in descriptions.
=Measurement of the Fish.=--As most fishes grow as long as they live,
the actual length of a specimen has not much value for purposes of
description. The essential point is not actual length, but relative
length. The usual standard of measurement is the length from the tip of
the snout to the base of the caudal fin. With this length the greatest
depth of the body, the greatest length of the head, and the length of
individual parts may be compared. Thus in the Rock Hind (_Epinephelus
adscensionis_), fig. 12, the head is contained 2-3/5 times in the
length, while the greatest depth is contained three times.
Thus, again, the length of the muzzle, the diameter of the eye, and
other dimensions may be compared with the length of the head. In the
Rock Hind, fig. 12, the eye is 5 in head, the snout is 4-2/5 in head,
and the maxillary 2-3/5. Young fishes have the eye larger, the body
slenderer, and the head larger in proportion than old fishes of the
same kind. The mouth grows larger with age, and is sometimes larger
also in the male sex. The development of the fins often varies a good
deal in some fishes with age, old fishes and male fishes having higher
fins when such differences exist. These variations are soon understood
by the student of fishes and cause little doubt or confusion in the
study of fishes.
[Illustration: FIG. 12.--Rock Hind or Cabra Mora of the West Indies,
_Epinephelus adscensionis_ (Osbeck). Family _Serranidæ_.]
=The Scales, or Exoskeleton.=--The surface of the fish may be naked as
in the catfish, or it may be covered with scales, prickles, shagreen,
or bony plates. The hard covering of the skin, when present, is known
as the exoskeleton, or outer skeleton. In the fish, the exoskeleton,
whatever form it may assume, may be held to consist of modified scales,
and this is usually obviously the case. The skin of the fish may be
thick or thin, bony, horny, leathery, or papery, or it may have almost
any intermediate character. When protected by scales the skin is
usually thin and tender; when unprotected it may be ossified, as in
the sea-horse; horny, as in the headfish; leathery, as in the catfish;
or it may, as in the sea-snails, form a loose scarf readily detachable
from the muscles below.
The scales themselves may be broadly classified as ctenoid, cycloid,
placoid, ganoid, or prickly.
_Ctenoid and Cycloid Scales._--Normally formed scales are rounded in
outline, marked by fine concentric rings, and crossed on the inner
side by a few strong radiating ridges and folds. They usually cover
the body more or less evenly and are imbricated like shingles on a
roof, the free edge being turned backward. Such normal scales are of
two types, ctenoid or cycloid. Ctenoid scales have a comb-edge of
fine prickles or cilia; cycloid scales have the edges smooth. These
two types are not very different, and the one readily passes into the
other, both being sometimes seen on different parts of the same fish.
In general, however, the more primitive representatives of the typical
fishes, those with abdominal ventrals and without spines in the fins,
have cycloid or smooth scales. Examples are the salmon, herring,
minnow, and carp. Some of the more specialized spiny-rayed fishes,
as the parrot-fishes, have, however, scales equally smooth, although
somewhat different in structure. Sometimes, as in the eel, the cycloid
scales may be reduced to mere rudiments buried in the skin.
_Ctenoid_ scales are beset on the free edge by little prickles or
points, sometimes rising to the rank of spines, at other times soft and
scarcely noticeable, when they are known as ciliate or eyelash-like.
Such scales are possessed in general by the more specialized types of
bony fishes, as the perch and bass, those with thoracic ventrals and
spines in the fins.
[Illustration: FIG. 13.--Scales of _Acanthoessus bronni_ (Agassiz).
(After Dean.)]
_Placoid Scales._--Placoid scales are ossified papillæ, minute,
enamelled, and close-set, forming a fine shagreen. These are
characteristic of the sharks; and in the most primitive sharks the
teeth are evidently modifications of these primitive structures. Some
other fishes have scales which appear shagreen-like to sight and
feeling, but only the sharks have the peculiar structure to which
Agassiz gave the name of placoid. The rough prickles of the filefishes
and some sculpins are not placoid, but are reduced or modified ctenoid
scales, scales narrowed and reduced to prickles.
_Bony and Prickly Scales._--Bony and prickly scales are found in
great variety, and scarcely admit of description or classification.
In general, prickly points on the skin are modifications of ctenoid
scales. Ganoid scales are thickened and covered with bony enamel, much
like that seen in teeth, otherwise essentially like cycloid scales.
These are found in the garpike and in many genera of extinct Ganoid
and Crossopterygian fishes. In the line of descent the placoid scale
preceded the ganoid, which in turn was followed by the cycloid and
lastly by the ctenoid scale. Bony scales in other types of fishes may
have nothing structurally in common with ganoid scales or plates,
however great may be the superficial resemblance.
[Illustration: FIG. 14.--Cycloid Scale.]
The distribution of scales on the body may vary exceedingly. In some
fishes the scales are arranged in very regular series; in others they
are variously scattered over the body. Some are scaly everywhere on
head, body, and fins. Others may have only a few lines or patches. The
scales may be everywhere alike, or they may in one part or another
be greatly modified. Sometimes they are transformed into feelers or
tactile organs. The number of scales is always one of the most valuable
of the characters by which to distinguish species.
=Lateral Line.=--The lateral line in most fishes consists of a series
of modified scales, each one provided with a mucous tube extending
along the side of the body from the head to the caudal fin. The canal
which pierces each scale is simple at its base, but its free edge is
often branched or ramified. In most spiny-rayed fishes it runs parallel
with the outline of the back. In most soft-rayed fishes it follows
rather the outline of the belly. It is subject to many variations. In
some large groups (_Gobiidæ_, _Pæciliidæ_) its surface structures are
entirely wanting. In scaleless fishes the mucous tube lies in the skin
itself. In some groups the lateral line has a peculiar position, as in
the flying-fishes, where it forms a raised ridge bounding the belly.
In many cases the lateral line has branches of one sort or another. It
is often double or triple, and in some cases the whole back and sides
of the fish are covered with lateral lines and their ramifications.
Sometimes peculiar sense-organs and occasionally eye-like luminous
spots are developed in connection with the lateral line, enabling the
fish to see in the black depths of the sea. These will be noticed in
another chapter.
_The Lateral Line as a Mucous Channel._--The more primitive condition
of the lateral line is seen in the sharks and chimæras, in which fishes
it appears as a series of channels in or under the skin. These channels
are filled with mucus, which exudes through occasional open pores. In
many fishes the bones of the skull are cavernous, that is, provided
with cavities filled with mucus. Analogous to these cavities are the
mucous channels which in primitive fishes constitute the lateral line.
[Illustration: FIG. 15.--Singing Fish (with many lateral lines),
_Porichthys porosissimus_ (Cuv. and Val.). Gulf of Mexico.]
_Function of the Lateral Line._--The general function of the lateral
line with its tubes and pores is still little understood. As the
structures of the lateral line are well provided with nerves, it has
been thought to be an organ of sense of some sort not yet understood.
Its close relation to the ear is beyond question, the ear-sac being an
outgrowth from it.
"The original significance of the lateral line," according to Dr.
Dean,[2] "as yet remains undetermined. It appears intimately if not
genetically related to the sense-organs of the head and gill region
of the ancestral fish. In response to special aquatic needs, it may
thence have extended farther and farther backward along the median line
of the trunk, and in its later differentiation acquired its metameral
characters." In view of its peculiar nerve-supply, "the precise
function of this entire system of organs becomes especially difficult
to determine. Feeling, in its broadest sense, has safely been admitted
as its possible use. Its close genetic relationship to the hearing
organ suggests the kindred function of determining waves of vibration.
These are transmitted in so favorable a way in the aquatic medium that
from the side of theory a system of hypersensitive end-organs may well
have been established. The sensory tracts along the sides of the body
are certainly well situated to determine the direction of the approach
of friend, enemy, or prey."
=The Fins of Fishes.=--The organs of locomotion in the fishes are
knows as fins. These are composed of bony or cartilaginous rods or
rays connected by membranes. The fins are divided into two groups,
paired fins and vertical fins. The pectoral fins, one on either side,
correspond to the anterior limbs of the higher vertebrates. The ventral
fins below or behind them represent the hinder limbs. Either or both
pairs may be absent, but the ventrals are much more frequently abortive
than the pectorals. The insertion of the ventral fins may be abdominal,
as in the sharks and the more generalized of the bony fishes, thoracic
under the breast (the pelvis attached to the shoulder-girdle) or
jugular, under the throat. When the ventral fins are abdominal, the
pectoral fins are usually placed very low. The paired fins are not in
general used for progression in the water, but serve rather to enable
the fish to keep its equilibrium. With the rays, however, the wing-like
pectoral fins form the chief organ of locomotion.
The fin on the median line of the back is called the dorsal, that on
the tail the caudal, and that on the lower median line the anal fin.
The dorsal is often divided into two fins or even three. The anal is
sometimes divided, and either dorsal or anal fin may have behind it
detached single rays called finlets.
The rays composing the fin may be either simple or branched. The
branched rays are always articulated, that is, crossed by numerous
fine joints which render them flexible. Simple rays are also sometimes
articulate. Rays thus jointed are known as soft rays, while those rays
which are neither jointed nor branched are called spines. A spine is
usually stiff and sharp-pointed, but it may be neither, and some spines
are very slender and flexible, the lack of branches or joints being the
feature which distinguishes spine from soft ray.
The anterior rays of the dorsal and anal fins are spinous in most
fishes with thoracic ventrals. The dorsal fin has usually about ten
spines, the anal three, but as to this there is much variation in
different groups. When the dorsal is divided all the rays of the first
dorsal and usually the first ray of the second are spines. The caudal
fin has never true spines, though at the base of its lobes are often
rudimentary rays which resemble spines. Most spineless fishes have such
rudiments in front of their vertical fins. The pectoral, as a rule,
is without spines, although in the catfishes and some others a single
large spine may be developed. The ventrals when abdominal are usually
without spines. When thoracic each usually, but not always, consists
of one spine and five soft rays. When jugular the number of soft rays
may be reduced, this being a phase of degeneration of the fin. In
writing descriptions of fishes the number of spines may be indicated
by Roman numerals, those of the soft rays by Arabic. Thus D. XII-I, 17
means that the dorsal is divided, that the anterior portion consists
of twelve spines, the posterior of one spine and seventeen soft rays.
In some fishes, as the catfish or the salmon, there is a small fin on
the back behind the dorsal fin. This is known as the adipose fin, being
formed of fatty substance covered by skin. In a few catfishes, this
adipose fin develops a spine or soft rays.
=Muscles.=--The movements of the fins are accomplished by the muscles.
These organs lie along the sides of the body, forming the flesh of the
fish. They are little specialized, and not clearly differentiated as in
the higher vertebrates.
With the higher fishes there are several distinct systems of muscles
controlling the jaws, the gills, the eye, the different fins, and the
body itself. The largest of all is the great lateral muscle, composed
of flake-like segments (myocommas) which correspond in general with the
number of the vertebræ. In general the muscles of the fish are white
in color. In some groups, especially of the mackerel family, they are
deep red, charged with animal oils. In the salmon they are orange-red,
a color also due to the presence of certain oils.
In a few fishes muscular structures are modified into electric organs.
These will be discussed in a later chapter.
FOOTNOTES:
[2] Fishes Recent and Fossil, p. 52.