TOPOGRAPHICAL DESCRIPTION OF THE EXTERNAL PARTS OF FISHES.
[Sidenote: Form of the body.]
In the body of a fish four parts are distinguished: the _head_,
_trunk_, _tail_, and the _fins_; the boundary between the first and
second being generally indicated by the _gill-opening_, and that
between the second and third by the _vent_. The form of the body and
the relative proportions of those principal parts are subject to much
variation, such as is not found in any other class of Vertebrates.
In fishes which are endowed with the power of steady and more or
less rapid locomotion, a deviation from that form of body, which we
observe in a perch, carp, or mackerel, is never excessive. The body
forms a simple, equally-formed wedge, compressed or slightly rounded,
well fitted for cleaving the water. In fishes which are in the habit
of moving on the bottom, the whole body, or at least the head, is
_vertically depressed_ and flattened; the head may be so enormously
enlarged that the trunk and tail appear merely as an appendage. In
one family of fishes, the _Pleuronectidæ_ or Flat-fishes, the body is
compressed into a thin disk; they swim and move on one side only, which
remains constantly directed towards the bottom, a peculiarity by which
the symmetry of all parts of the body has been affected. A _lateral
compression_ of the body, in conjunction with a lengthening of the
vertical and a shortening of the longitudinal axis, we find in fishes
moving comparatively slowly through the water, and able to remain (as
it were) suspended in it. This deviation from the typical form may
proceed so far that the vertical axis greatly exceeds the longitudinal
in length; generally all the parts of the body participate in this
form, but in one kind of fish (the Sun-fish or _Orthagoriscus_) it is
chiefly the tail which has been shortened, and reduced so much as to
present the appearance of being cut off. An excessive lengthening of
the longitudinal axis, with a shortening of the vertical, occurs in
Eels and eel-like fishes, and in the so-called Band-fishes. They are
bottom-fish, capable of insinuating themselves into narrow crevices and
holes. The form of the body of these long fish is either cylindrical,
snake-like, as in the Eels and many Cod-fishes, or strongly compressed
as in the Band-fishes (_Trichiurus_, _Regalecus_, etc.) It is chiefly
the tail which is lengthened, but frequently the head and trunk
participate more or less in this form. Every possible variation
occurs between these and other principal types of form. The old
ichthyologists, even down to Linnæus, depended in great measure on them
for classification; but although often the same form of body obtains
in the same group of fishes, similarity of form by no means indicates
natural affinity; it only indicates similitude of habits and mode of
life.
[Sidenote: Eye.]
_The external parts of the Head._--The _Eye_ divides the head into the
_ante-orbital_ and _post-orbital_ portion. In most fishes, especially
in those with a compressed head, it is situated on the side and in the
anterior half of the length of the head; in many, chiefly those with a
depressed head, it is directed upwards, and sometimes situated quite
at the upper side; in very few, the eyes look obliquely downwards. In
the Flat-fishes both eyes are on the same side of the head, either the
right or the left, always on that which is directed towards the light,
and coloured.
Fishes in general, compared with other Vertebrata, have large eyes.
Sometimes these organs are enormously enlarged, their great size
indicating that the fish is either nocturnal, or lives at a depth to
which only a part of the sun’s rays penetrate. On the other hand, small
eyes occur in fishes inhabiting muddy places, or great depths to which
scarcely any light descends, or in fishes in which the want of an organ
of sight is compensated by the development of other organs of sense.
In a few fishes, more particularly in those inhabiting caves or the
greatest depths of the ocean, the eyes have become quite rudimentary
and hidden under the skin.
[Sidenote: Snout.]
In the _ante-orbital_ portion of the head, or the _Snout_, are situated
the mouth and the nostrils.
[Sidenote: Mouth.]
The _Mouth_ is formed by the intermaxillary and maxillary bones, or by
the intermaxillary only in the upper jaw, and by the mandibulary bone
in the lower. These bones are either bare or covered by integument, to
which frequently labial folds or lips are added. As regards form, the
mouth offers as many variations as the body itself, in accordance with
the nature of the food, and the mode of feeding. It may be narrow, or
extremely wide and cleft to nearly the hind margin of the head; it may
be semi-elliptical, semicircular, or straight in a transverse line;
it may be quite in front of the snout (_anterior_), or at its upper
surface (_superior_), or at its lower (_inferior_), or extending along
each side _(lateral_); sometimes it is subcircular, organised for
sucking. The jaws of some fishes are modified into a special weapon
of attack (Sword-fish, Saw-fish); in fact, throughout the whole class
of fishes the jaws are the only organ specialised for the purpose of
attacking; weapons on other parts of the body are purely defensive.
Both jaws may be provided with skinny appendages, _barbels_, which, if
developed and movable, are sensitive organs of touch.
[Sidenote: Nostrils.]
In the majority of fishes the _Nostrils_ are a double opening on
each side of the upper surface of the snout; the openings of each side
being more or less close together. They lead into a shallow groove;
and only in one family (the Myxinoids) perforate the palate. In this
family, as well as in the Lampreys, the nasal aperture is single. In
many Eels the openings are lateral, the lower perforating the upper
lip. In the Sharks and Rays (Fig. 1, p. 34) they are at the lower
surface of the snout, and more or less confluent; and, finally, in the
Dipnoi and other Ganoids, one at least is within the labial boundary of
the mouth.
The space across the forehead, between the orbits, is called the
_interorbital_ space; that below the orbit, the _infraorbital_ or
_sub-orbital_ region.
[Sidenote: Gill-cover.]
In the _post-orbital_ part of the head there are distinguished, at
least in most Teleosteous Fishes and many Ganoids, (Fig. 24) the
_præoperculum_, a sub-semicircular bone, generally with a free and
often serrated or variously-armed margin; the _operculum_, forming
the posterior margin of the gill-opening, and the _sub-operculum_
and _interoperculum_ along its inferior margin. All these bones,
collectively called _opercles_, form the _gill-cover_, a thin bony
lamella covering the cavity containing the gills. Sometimes they are
covered with so thin a membrane that the single bones may be readily
distinguished; sometimes they are hidden under a thick integument.
In some cases the interoperculum is rudimentary or entirely absent
(Siluroids).
[Sidenote: Gill-opening.]
The _Gill-opening_ is a foramen, or a slit behind or below the head,
by which the water which has been taken up through the mouth for the
purpose of breathing is again expelled. This slit may extend from
the upper end of the operculum all round the side of the head to the
symphysis of the lower jaw; or it may be shortened and finally reduced
to a small opening on any part of the margin of the gill-cover.
Sometimes (_Symbranchus_) the two openings, thus reduced, coalesce,
and form what externally appears as a single opening only. The margin
of the gill-cover is provided with a cutaneous fringe, in order to
more effectually close the gill-opening; and this fringe is supported
by one or several or many bony rays, the _branchiostegals_. The space
on the chest between the two rami of the lower jaw and between the
gill-openings is called the _isthmus_.
[Illustration: Fig. 2.--Head of _Mordacia mordax_, showing
the single nostril, and seven branchial openings.]
The Sharks and Rays differ from the Teleosteous and Ganoid fishes in
having five branchial slits (six or seven in _Hexanchus_ and
_Heptanchus_), which are lateral in the Sharks, and at the lower
surface of the head in the Rays (Fig. 1, p. 34). In Myxine only the
gill-opening is at a great distance from the head; it is either single
in this family (Cyclostomi), or there are six and more on each side
(Fig. 2).
[Sidenote: Tail.]
In the _Trunk_ are distinguished the _back_, the _sides_, and the
_abdomen_. It gradually passes in all fishes into the _Tail_; the
termination of the abdominal cavity and the commencement of the tail
being generally indicated by the position of the vent. The exceptions
are numerous: not only certain abdominal organs, like the sexual, may
extend to between the muscles of the tail, but the intestinal tract
itself may pass far backwards, or, singularly, it may be reflected
forwards, so that the position of the vent may be either close to the
extremity of the tail or to the foremost part of the trunk.
In many fishes the greater part of the tail is surrounded by the fins,
leaving only a small portion (between dorsal, caudal, and anal fins)
finless; this part is called the _free portion_ or the _peduncle_ of
the tail.
[Sidenote: Fins.]
The _Fins_ are divided into _vertical_ or _unpaired_, and into
_horizontal_ or _paired fins_. Any of them may be present or absent;
and their position, number, and form are most important guides in
determining the affinities of fishes.
The _vertical_ fins are situated in the median dorsal line, from the
head to the extremity of the tail, and in the ventral line of the
tail. In fishes in which they are least developed or most embryonic,
the vertical fin appears as a simple fold of the skin surrounding the
extremity of the tail In its further progress of development in the
series of fishes, it gradually extends more forwards, and may reach
even the head and vent. Even in this embryonic condition the fin is
generally supported by fine rays, which are the continuations of, or
articulated to, other stronger rays supported by the processes or
apophyses of the vertebral column. This form of the vertical fin is
very common, for instance in the Eels, many Gadoid, Blennioid and
Ganoid fishes in which, besides, the rays have ceased to be simple
rods, showing more or less numerous joints (simple _articulated_
rays; Fig. 3). _Branched_ rays are dichotomically split, the joints
increasing in number towards the extremity.
The continuity of the vertical fin, however, is interrupted in the
majority of fishes; and three fins then are distinguished: one in the
dorsal line--the _dorsal_ fin; one in the ventral line behind the
anus--the _anal_ fin; and one confined to the extremity of the
tail--the _caudal_ fin.
[Illustration: Fig. 3.
1. Simple ray.
2. Spine.
3. Simple articulated ray (soft).
4. Branched ray (soft).]
The _caudal_ fin is rarely symmetrical, so that its upper half would
be equal to its lower; the greatest degree of asymmetry obtains
in fishes with heterocercal termination of the vertebral column
(see subsequently, Figs. 31, 41). In fishes in which it is nearly
symmetrical it is frequently prolonged into an upper and lower _lobe_,
its hind margin being concave or more or less deeply excised; in others
the hind margin is rounded, and when the middle rays greatly exceed in
length the outer ones the fin assumes a pointed form.
[Illustration: Fig. 4.--Labrax lupus (Bass), an Acanthopterygian
with anterior spinous, and posterior soft dorsal fin.]
Many and systematically important differences are observed
in the _dorsal_ fin, which is either spiny-rayed (spinous)
(_Acanthopterygian_), or soft-rayed (_Malacopterygian_). In the
former, a smaller or greater number of the rays are simple and without
transverse joints; they may be flexible, or so much osseous matter
is deposited in them that they appear hard and truly spinous (Fig.
3); these spines form always the anterior portion of the fin, which
is detached from, or continuous with, the remaining jointed rays.
The spines can be erected or depressed at the will of the fish; if
in the depressed position the spines cover one another completely,
their points lying in the same line, the fish is called _homacanth_;
but if the spines are asymmetrical, alternately broader on one side
than on the other, the fish is called _heteracanth_. The spinous
division, as well as the one consisting of jointed rays, may again
be subdivided. In the _Malacopterygian_ type all the rays remain
jointed; indeed, sometimes the foremost ray, with its preceding short
supports, is likewise ossified, and a hard spine, but the articulations
can nearly always be distinctly traced. Sometimes the dorsal fin of
Malacopterygian fishes is very long, extending from the head to the end
of the tail, sometimes it is reduced to a few rays only, and in a few
cases it is entirely absent. In addition to the rayed dorsal fin, many
Malacopterygian fishes (as the Salmonoids, many Siluroids, Scopeloids,
etc.) have another of greater or lesser extent, without any rays; and
as always fat is deposited within this fold, it is called a _fatty_ fin
(_pinna adiposa_).
[Illustration: Fig. 5.--Saurus undosquamis, a Malacopterygian
with anterior soft dorsal, and additional adipose fin.]
The _anal_ fin is built on the same plan as the dorsal, and
may be single or plural, long or short, or entirely absent; in
Acanthopterygians its foremost rays are frequently simple and spinous.
The _horizontal_ or _paired_ fins consist of two pairs: the pectorals
and ventrals.
The _pectoral_ fins (with their osseous supports) are the homologues of
the anterior limbs of the higher Vertebrata. They are always inserted
immediately behind the gill-opening; either symmetrical with a rounded
posterior margin, or asymmetrical, with the upper rays longest and
strongest; in Malacopterygians with a dorsal spine the upper pectoral
ray is frequently developed into a similar defensive weapon.
The _ventral_ fins are the homologues of the hind-limbs, and inserted
on the abdominal surface, either behind the pectorals (_Pisces_ s.
_Pinnæ abdominales_), or below them _(Pisces_ s. _Pinnæ thoracicæ_), or
in advance of them (_Pisces_ s. _Pinnæ jugulares_). They are generally
narrow, composed of a small number of rays, the outer of which is
frequently osseous. In some small groups of fishes, like the Gobies,
the fins coalesce and form a suctorial disk.
[Illustration: Fig. 6.--Salmo salar (Salmon), with abdominal
ventral fins.]
[Illustration: Fig. 7.--Mullus barbatus (Red Mullet), with
thoracic ventral fins.]
[Illustration: Fig. 8.--Burbot (Lota vulgaris), with jugular
ventral fins.]
For the definition of the smaller systematic groups, and the
determination of species, the numbers of the spines and rays are
generally of the greatest importance. This holds good, especially
for the ventral rays, by the number of which the Acanthopterygian
affinities of a fish can nearly always be determined. The numbers of
the dorsal and anal rays generally correspond to the number of vertebræ
in a certain portion of the spine, and are therefore constant specific,
generic, or even family characters; but when their number is very
great, a proportionally wide margin must be allowed for variation, and
the taxinomic value of this character becomes uncertain. The numbers of
the pectoral and caudal rays are rarely of any account.
[Sidenote: Function of the Fins.]
The fins are organs of motion; but it is chiefly the tail and the
caudal fin by which the fish impels itself forward. To execute
energetic locomotion the tail and caudal fin are strongly bent,
with rapidity, alternately towards the right and left; whilst a
gentle motion forwards is effected by a simple undulating action of
the caudal fin, the lobes of which act like the blades of a screw.
Retrograde motions can be made by fish in an imperfect manner only,
by forward-strokes of the pectoral fins. When the fish wants to
turn towards the left, he gives a stroke of the tail towards the
right, the right pectoral acting simultaneously, whilst the left
remains ad-pressed to the body. Thus the pectoral fins assist in the
progressive motions of the fish, but rather directing its course than
acting as powerful propellers. The chief function of the paired fins is
to maintain the balance of the fish in the water, which is always the
most unsteady where there is no weight to sink it: when the pectoral
of one side, or the pectoral and ventral of the same side are removed,
the fish loses its balance and falls on the side opposite; when both
pectorals are removed, the fish’s head sinks; on removal of the dorsal
and anal fins the motion of the fish assumes a zig-zag course. A fish
deprived of all fins, as well as a dead fish, floats with the belly
upwards, the back being the heavier part of the body.
In numerous groups of fishes which live in mud, or are enabled to pass
a longer or shorter time in soil periodically dried and hardened
during the hot season, forms occur entirely devoid of, or with only
rudimentary, ventral fins (Cyprinodon, Ophiocephalidæ, Galaxiidæ,
Siluridæ). The chief function of these fins being to balance the body
of the fish whilst swimming, it is evident that in fishes moving during
a great part of their life over swampy ground, or through more or less
consistent mud, this function of the ventral fins ceases, and that
nature can readily dispense with these organs altogether.
[Illustration: Fig. 9.--Ventrals of _Gobius_.]
In certain fishes the shape and function of the fins are considerably
modified: thus, in the Rays, locomotion is almost entirely effected
and regulated by the broad and expanded pectoral fins acting with an
undulatory motion of their margins, similar to the undulations of the
long vertical fins of the Flat-fishes; in many Blennies the ventral
fins are adapted for walking on the sea-bottom; in some Gobioids
(_Periophthalmus_), Trigloids, Scorpænioids, and Pediculati, the
pectoral fins are perfect organs of walking; in the Gobies, Cyclopteri,
and Discoboli the ventral fins are transformed into an adhesive disk,
and finally in the Flying-fish, in which the pectorals act as a
parachute. In the Eels and other snake-like fishes, the swimming as
well as the gliding motions are effected by several curvatures of the
body, alternate towards the right and left, resembling the locomotion
of Snakes. In the _Syngnathi_ (Pipe-fishes) and _Hippocampi_,
whose body admits of but a slight degree of lateral curvature, and
whose caudal fin is generally small, if present at all, locomotion is
very limited, and almost wholly dependent on the action of the dorsal
fin, which consists of a rapid undulating movement.
[Illustration: Fig. 10.--Cycloid scale of Gadopsis marmoratus
(magn.)]
[Illustration: Fig. 11.--Cycloid scale of Scopelus resplendens
(magn.)]
[Sidenote: Skin and Scales.]
The _skin_ of fishes is either covered with scales, or naked, or
provided with more or less numerous scutes of various forms and sizes.
Some parts, like the head and fins, are more frequently naked than
scaly. All fishes provided with electric organs, the majority of Eels,
and the Lampreys, are naked. _Scales_ of fishes are very different from
those of Reptiles; the latter being merely folds of the cutis, whilst
the scales of fishes are distinct horny elements, developed in grooves
or pockets of the skin, like hairs, nails, or feathers. Very small
or rudimentary scales are extremely thin, homogeneous in structure,
and more or less imbedded in the skin, and do not cover each other.
When more developed, they are imbricated (arranged in the manner of
tiles), with the posterior part extruded and free, the surface of the
anterior portion being usually covered by the skin to a greater or less
extent. On their surface (Figs. 10 and 11) may be observed a very fine
striation concentric and parallel to the margin, and coarser striæ
radiating from a central point towards the hind margin. Scales without
a covering of enamel, with an entire (not denticulated) posterior
margin, and with a concentric striation, are called _Cycloid_ scales.
_Ctenoid_ scales (Figs. 12–15) are generally thicker, and provided
with spinous teeth on the posterior edges of the layers of which the
scale consists. In some species only the layer nearest to the margin
is provided with denticulations (Fig. 14). Scales, the free surface of
which is spiny, and which have no denticulation on the margin, have
been termed _Sparoid_ scales; but their distinction from ctenoid scales
is by no means sharp, and there are even intermediate forms between
the cycloid and ctenoid types. Both kinds of scales may occur not only
in species of the same genus of fishes, but in the same fish.
[Illustration: Fig. 12.--Ctenoid scale of Scatophagus
multifasciatus (magn.)]
[Illustration: Fig. 13.--Ctenoid scale of Platycephalus
cirrhonasus (magn.)]
[Illustration: Fig. 14.--Ctenoid scale of Gobius ommaturus
(magn.)]
[Illustration: Fig. 15.--Ctenoid scale of Lethrinus (magn.)]
[Illustration: Fig. 16. Ganoid Scales.]
_Ganoid_ scales are hard and bony, covered with a layer of enamel; they
are generally rhombic or quadrangular, rarely rounded and imbricate;
and arranged in oblique rows, those of one row being linked together
by an articulary process. This type of scales, common in fossil Ganoid
fishes, occurs among recent fishes in _Lepidosteus_ and _Polypterus_
only.
Finally, in Sharks, the Balistidæ, and others, true scales are absent
and replaced by the ossified papillæ of the cutis, which give the
surface the appearance of fine-grained chagreen. These generally small
bodies, as well as the large osseous scutes of the Rays, Sturgeons,
etc., have been comprised under the common name _Placoid_ scales; a
term which deservedly is being abandoned.
[Illustration: Fig. 17.--Dermal papillæ of Monacanthus trossulus.]
[Illustration: Fig. 18.--Dermal papillæ of Monacanthus
hippocrepis (magn.)]
[Illustration: Fig. 19.--Cycloid scale from the lateral line of
Odax lineatus (magn.)]
Along the side of the body of osseous fishes runs a series of
perforated scales, which is called the _lateral line_ (Fig. 21).
The perforating duct is simple at its base, and may be also simple at
its outer opening (Fig. 19), or (and this is frequently the case) the
portion on the free surface of the scale is ramified (Fig. 20). The
lateral line runs from the head to the tail, sometimes reaching the
caudal fin, sometimes stopping in front of it, sometimes advancing over
its rays. It is nearer to the dorsal profile in some fishes than in
others. Some species have several lateral lines, the upper one coasting
the dorsal, the lower the abdominal outline, one running along the
middle as usual. The scales of the lateral line are sometimes larger
than the others, sometimes smaller, sometimes modified into scutes,
sometimes there are no other scales beside them, the rest of the body
being naked. The foramina of the lateral line are the outlets of a
muciferous duct which is continued on to the head, running along the
infraorbital bones, and sending off a branch into the præopercular
margin and mandible. In many fishes, as in many Sciænoids, Gadoids,
and in numerous deep-sea fishes, the ducts of this muciferous system
are extraordinarily wide, and generally filled with mucus, which
is congealed or contracted in specimens preserved in spirits, but
swells again when the specimens are immersed in water. This system is
abundantly provided with nerves, and, therefore, has been considered
to be the seat of a sense peculiar to fishes, but there cannot be any
doubt that its function is the excretion of mucus, although probably
mucus is excreted also from the entire surface of the fish.
[Illustration: Fig. 20.--Cycloid scale from the lateral line of
Labrichthys laticlavius (magn.)]
The scales, their structure, number and arrangement, are an important
character for the determination of fishes; in most scaly fishes they
are arranged in oblique transverse series; and as the number of scales
in the lateral line generally corresponds to the number of transverse
series, it is usual to count the scales in that line. To ascertain
the number of longitudinal series of scales, the scales are counted
in one of the transverse series, generally in that running from the
commencement of the dorsal fin, or the middle of the back to the
lateral line, and from the lateral line down to the vent or ventral
fin, or middle of the abdomen.[4]
[Illustration: Fig. 21.--Arrangement of scales in the Roach
(Leuciscus ratilus): _Ll_ = Lateral line; _tr_ =
Transverse line. _a_, Transverse line from lateral line to
ventral fin.]
The scales of many fishes are modified for special purposes, especially
to form weapons of defence or a protective armour, but the details of
such modifications are better mentioned under the several families in
which they occur. All scales are continually growing and wasting away
on the surface, and it seems that some fish, at least,--for instance,
Salmonoids--“shed” them periodically; during the progress of this
shedding the outlines of the scales are singularly irregular.