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GENERAL CHARACTERS.



3 1 ?



are shown in the skeleton represented on p. 316, and occur in both the median and

paired fins, of which the names are also given in the same illustration. In tho

median fins the bases of these rays articulate with the inter spinal bones, or, in
I elasmobranchs, with the radial cartilages. The first

rays of the pectoral and dorsal fins may be developed

into long spines, having the same structure as teeth,
internal In the internal skeleton the back-

Skeleton, bone is divisible only into a trunk and

caudal moiety. In the fringe -finned ganoid fishes
I, the primitive iiotochord persists, although it may be

partly surrounded by rudimental arches ; while in the j

sharks and higher bony fishes the column is divided [

into segments, forming vertebra with doubly-cupped
bodies. In sharks and rays the arches and bodies of
jjthe vertebrae remain separate, but in the other groups

they are fused together ; in the tail, as shown in our \

figure of the skeleton of the perch, there is also an \
(inferior arch and spine to each vertebra. In the more j
I primitive fishes the notochord is continued to the
| hinder extremity of the body, where it is surrounded j
symmetrically by the rays of the caudal fin; this type, i
j which is shown in the accompanying figure of the

skeleton of an extinct fringe -finned shark, being

termed the fringe-tailed, or diphy cereal. Whereas in
Isome fishes with this type of tail the fringes on the i

upper and lower portions of the caudal fin are of

pearly equal depth, in others the lower fringe of rays

jbecomes somewhat deeper than the others, and a

further development of this inequality results in the

martially forked or heterocercal tail of the modern

sharks and sturgeons, where the end of the backbone

is bent upwards into the longer superior lobe of the

tail, the lower lobe of which is formed exclusively

of rays. The lung -fishes and sharks have never

advanced beyond one or other of these types ; but

the bony fishes and ganoids, which started with the

primitive fringed lobate type, by a gradual shortening

of the central part of the tail-fin, accompanied by an

[increasing development of the rays on its lower side,

Biave evolved the completely forked or homocercal tail

pf the perch, in which, as shown in the figure, the

backbone stops short of the fin-rays, and ends in an

expanded, unsymmetrical extremity, from which these rays are given off in a

fan-like manner, so as to produce an appearance of perfect symmetry in the whole

structure.

Turning to the limbs, or paired fins, we find that while in the existing



MSf/ES.



m



elasmobranchs there are no membrane-bones (as the elements of the skeleton not
formed from primitive cartilage are termed), in the higher bony fishes the pectoral
girdle, as shown in the figure on p. 316, comprises a scapula and a coracoid,
flanked by a series of membrane-bones, known as the post-temporal, *ii/>r<~!-

davicular, clavicular, and postclavicular. The pelvis
is generally absent, and is never highly developed. In
all cases the basal and radial bones of the pectoral tins
articulate directly with the pectoral girdle, so that there
are no segments corresponding to the arm and fore-arm
of the higher Vertebrates. In the paired fins the struc-
ture is very similar to that of the tail : and a similar
transition from a fringed to a fan -like type may lu-
traced as we pass from the primitive to the specialised
forma For instance, in the figure of the perch's
skeleton on p. 316, we may notice that the paired tins
are formed of a number of hard rays spreading out in
a fan-like manner from a single point of origin; and
- 5 the same general type obtains in the existing sharks and
rays. In certain extinct sharks, like the one of which
the skeleton is shown on p. 317, as well as in the lung-
fishes and the fringe -finned ganoids, the pectoral fins
have a long central lobe running for some distance up
the middle, and completely covered with scales (where
these are developed), while the rays of these fins form
a kind of fringe radiating on all sides from the central
lobe. The skeleton of such a fin, which is known as
an archipterycjiu m, consists of along cartilaginous axis,
composed of a number of joints, gradually decrea^in^ in
size from the base to the extremity, as shown in the
figure on p. 319. From one or both sides of such
joints there are given off a number of oblique smaller
jointed rods, terminating in the fine rays forming the free
edges of the fins. How different is the structure of this
fin from that of the higher bony fishes will be apparent
by comparing the accompanying figure with that of the,
skeleton of the perch on p. 316. In the lung-fishes this
primitive type of fin has persisted to the present day : in
the sharks it has now totally disappeared ; while among
the bony fishes and ganoids, in the latter of which it \\ ;
the universal type at the period of the Old Red Sandstone
it now only remains in a modified form in the bichir

the Nile, having been developed in the modern bony fishes into the fan-type.
may be mentioned that the latter modification of fin is obviously the one
adapted for quick-swimming fishes, the fringe-finned type partaking more of tl
nature of clumsy paddles, and being adapted for slowly-moving forms like
lung-fishes, which pass most of their time among the mud at the bottom of rivers.



GENERAL STRUCTURE.



It must not be supposed, however, that even the fringed type is the most
primitive form of fin known, since in an extinct armoured shark (Cladoselache)
from the Carboniferous formation, we meet with what may be called the fold-type
of paired fins. In these fishes (one of which is figured in the sequel) the pectoral
and pelvic fins are placed far apart, but in the same longitudinal line, and are
formed of a series of parallel cartilaginous rods arising from an extended base,
and projecting at right angles to the body; the pectoral pair being considerably
the larger. Assuming that fishes originally possessed on each side of the body a
continuous fold of skin, strengthened by parallel cartilaginous rods projecting at
right angles, this fold-type is exactly what we should expect to find in the
evolution of pectoral and pelvic fins, by the disappear-
ance of a considerable portion of the original fold, and
the development and basal contraction of the remaining
moieties.

Although the structure of the skull is of the
highest importance in the classification of fishes, our
remarks on this subject must be very brief. In the
skulls of the higher bony fishes the original cartilaginous
cranium, which persists in the sharks, is overlain by a
number of membrane-bones, the names and position of
which are indicated in the figure of the perch's skeleton
on p. 316. Among these the pterotic and sphenotic are
peculiar to the class, and there is always a large para-
sphenoid underlying the base of the skull. The inter-
vention of the elements known as the hyomandibidar
and symplectic between the squamosal and the quadrate is
unknown elsewhere, although it is by no means universal

among fishes. Among other bones may be mentioned the infraorbital ring
beneath the eye ; as well as the preopercular, subopercular, inter opercular, and
<>l>crcidar, collectively constituting the gill-cover or operculum of the bony fishes.
In the gill (or branchiostegal) membrane, which joins with the gill-cover in closing
in the gill-chamber, there may be developed a number of gill (or branchiostegal)
rays ; but these may be partially or entirely replaced by jugular plates, occupying
the space between the two branches of the lower jaw, It will be unnecessary to
mention by name the various bones constituting the hyoid arch, which is attached
to the inner side of the hyomandibular by the stylohyal, and extends forwards to
support the tongue ; and it will suffice to state that behind this arch are
situated the branchial or gill-arches, to the inner margins of which are attached
the spine-like gill-rakers. In the lower jaw, or mandible, there is usually both a
dentary and an articular piece ; but an angular, and more rarely a 8plenial or
<:<n'<nioid element may likewise be developed.

The teeth of fishes present a greater degree of variation than

is found among any other class of Vertebrates. While in some

cases they may be totally wanting, in others they may be developed on all the

bones of the mouth, and even on the hyoid bones and gill-arches ; and they may

be attached only to the membrane lining the cavity of the mouth. Frequently




SKELETON OF PECTORAL FIN OK
AN EXTINCT FRINGE-FINNED

SHARK. (From Fritsch.)



320 FISHES.

they are welded to the underlying bone or cartilage by a broad basis; but, as in
the saw-fishes, they may be emplanted in distinct sockets. Usually the coating of
enamel is very thin; and the ivory, or dentine, is more vascular than in the otl id-
classes. In rare instances the ivory may be penetrated by branching prolongations
from the central pulp-cavity, as well as by similar infoldings from the exterior,
thus producing a structure similar to that obtaining in those of the primeval
salamanders. As a general rule the teeth are being constantly renewed through-
out life, but in a few instances a single set persists.

In beauty, variety, and changeability, the colours of fishes
cannot be exceeded by those of any of the other vertebrate classes;
metallic tints and almost all the colours of the rainbow being very commonly
displayed ; while the beauty of the coloration is often enhanced by the rapid
changes it undergoes. In many cases the coloration is of a protective nature.
An example of this is afforded by the colouring of the upper surface of many
flat-fishes, such as flounders, which exactly harmonises with the tints of the sea-
bottom on which they dwell; while another equally marked instance presents
itself in the case of so-called pelagic fishes, like the mackerels and flying-fish,
which live near the surface of the sea, and have the under-parts silvery white, and
the back mottled with dark green and black. When viewed from below against
the light sky such a fish is practically invisible, while it is equally inconspicuous
when seen from above among the dark waters.

The body of fishes is made up of a great lateral muscle on each side,
Soft Parts. J

divided into a number of segments corresponding with the vertebras,

and also separated into a dorsal and ventral moiety by a median longitudinal
groove. On its surface the lateral muscle is marked by a number of white zig/.ag
stripes, generally forming three angles, of which the middle one is directed
forwards; these stripes being formed by the edges of the tendinous divisions
between the segments. Generally the muscles are glistening white in colour;
but in some instances they are " salmon-coloured," this tint being due to the
colouring matter of the crustaceans on which such fish subsist, which is turned
red by the action of the digestive fluids in the same manner as by boiling. The
electric organs possessed by certain fishes are considered to be probably formed
from specially developed muscles. With regard to the brain, it will suffice to say
that it is of an exceedingly low type; and that the ear lacks the tympanum and
tympanic cavity of the higher classes. In the bony fishes there occurs at the
base of the brain-cavity a sac, often divided into two unequal-sized chambers, and
each of which contains an ear bone, or otolith, of very dense structure. These,
otoliths, which are very constant in form in the different groups, frequently have
scalloped margins and groove-like markings, formed by the ramifications of the
auditory nerve. The tongue is frequently absent, and if present is of very simple
structure and incapable of protrusion; and since fishes generally bolt their food
without mastication, it is probable that they have little, if any, sense of taste.
With regard to the digestive organs, it will suffice to mention that in the sharks,
rays, and chimseroids the intestine for a large portion of its length is provided
with a spiral valve, a similar structure occurring in the ichthyosauriau reptiles.

The gills of sharks, rays, and chimseroids are contained in pouches, usually



GENERAL CHARACTERS. 321

five in number on each side ; each pouch opening externally by a slit, and also
communicating by a separate aperture with the pharynx. In the embryos
filamentous gills protrude externally from the slits. The spiracles found on the
top of the head in the groups mentioned are the external openings of a canal
leading on each side into the pharynx, and represent what is known as the first
visceral cleft in the embryo. In the bony fishes the gills, which are generally
four in number, lie in one undivided cavity on each side of the head, supported
by their gill-arches, and covered over by the gill-cover, which is open behind.
In ordinary respiration the water is taken in by the mouth, and by a kind of
swallowing action driven over the gills, after which it is expelled by the gill-
opening. Allusion must be made in this place to certain organs known as
false gills, or pseudobranchice. These are remains of a gill situated in front of
the persistent ones, which was functional during embryonic life, but in the adult
appears merely as a plexus of blood-vessels. Although the majority of fishes
breathe in the manner indicated above, in many forms this is by no means the
sole method of respiration. On this subject Day observes that respiration in
fishes is carried out normally, either by their using the air which is held in water
to oxygenate the blood at the gills, or " by taking in atmospheric air direct, which
is employed at a special organ, where it oxygenates the blood, which can be
returned for use into the general circulation without going through the gills.
The true amphibious fishes respire by the latter method. No doubt we observe that
fishes which normally oxygenate their blood solely at their gills do rise to the surface
in very hot weather, when the water is foul or insufficiently charged with air, and
take in air by the mouth ; likewise we find that those which mainly take in
atmospheric air direct by the mouth may, to a certain extent, be able to use their
gills. If fishes having these two different modes of respiration are placed in a
<j;lobe of water, across which a diaphragm of net is inserted below the surface, so
as to prevent their obtaining access to the atmosphere, those of the class which
oxygenate their blood at the gills are unaffected, whereas those which have accessory
breathing-organs and take in air direct die from blood-poisoning."

One of the most characteristic organs of fishes is the air-bladder, which is a
ong sac filled with gas lying in the abdominal cavity, which may be either com-
pletely closed, or may communicate with the alimentary tract by means of a duct.
Vs it is susceptible of compression, its usual function appears to be to regulate the
specific gravity, or to change the centre of gravity of the fish ; but in the lung-
fishes it assumes the characters and functions of the lungs of the higher Verte-
brates, to which, indeed, it corresponds.

Although in the few fishes which produce living young, as well
Reproduction.

as in the sharks and rays, an actual connection takes place between

the two sexes, in the great majority of the class the ova are deposited by the
female, after which they are fertilised by the male. The bony fishes lay numerous
ggs which are of relatively small size and may be extremely minute, those of the
el being almost microscopic; but there is a considerable degree of variation in
.his respect. In the herring the number of eggs in the "roe" has been estimated
it twenty-five thousand, and in the cod at over nine millions. Only in a cat-fish
\Axpredo), where they are pressed into the skin of the under surface of the body,
VOL. v. 21



322 FISHES.

and a pipe-fish (Solenoetoma), where they are carried in a pouch formed by the
coalescence of the broad pelvic fins with the skin of the body, is the female known
to take any care of her eggs after spawning. Among the bony fishes there are,
however, several instances where the young are more or less carefully tended by
the male parent; some, like the sticklebacks, building a nest, while others, like
certain pipe-fishes, have an abdominal pouch in which the eggs are hatched.
The eggs of sharks, rays, and chima3roids differ remarkably from those of bony
fishes, being large in size, few in number, and laid singly instead of in masses.
They are invested in a hard horny envelope, which is generally oblong in i'orii-,
with the four corners produced, and frequently elongated into tendrils by means of
which the egg is moored to some foreign substance. The males of these fishes are
armed with organs known as claspers, which are partially ossified processes arising
from the pubis, and are evidently connected with the function of reproduction.
The young of many fishes differ markedly from the adult; and certain peculiar
creatures with long ribbon -like bodies and small heads, for which the name of
Leptocephali has been proposed, are believed to be the young of littoral fishes which
have been carried out to sea, where they have undergone an altogether abnormal
development. The changes which take place in the flat-fishes during development
may be more conveniently noticed under the heading of that group. Although
male and female rays differ remarkably from one another in the structure of their
teeth, while both in this group and in the sharks and chimaeroids the males arc
distinguished by the possession of the aforesaid claspers, there is generally but
little sexual difference among fishes. In the bony fishes, however, the females are
larger than the males ; among the cyprinodonts the difference between the two
being occasionally as much as six times.

Fishes exhibit a remarkable degree of difference in regard to
Tenacity of Life.

their power or bearing changes from their normal environment. On

this subject Dr. Giinther writes that, " some \vill bear suspension of respiration-
caused by removal from water, or by exposure to cold or heat for a long time.
whilst others succumb at once. Nearly all marine fishes are very sensitive to
changes in the temperature of the water, and will not bear transportation from one
climate to another. This seems to be much less the case with some fresh-water
fishes of the temperate zone; since carp may survive after being frozen in a solid
block of ice, and will thrive in the southern parts of the temperate zone. On f he-
other hand, some fresh- water fishes are so sensitive to a change in the water that
they perish when transferred from their native river into another apparently
offering the same physical conditions. Some marine fishes may be abruptly trans-
ferred from salt into fresh-water, like sticklebacks; others survive the change
when gradually effected, as many migratory fishes; whilst others, again, cannot
bear the least alteration in the composition of the salt-water (all pelagic fishes).
On the whole, instances of marine fishes voluntarily entering brackish or fresh-
water are very numerous, whilst fresh-water fishes proper but rarely descend into
salt water."

. _ The foregoing remarks lead naturally to the subject of the dis-

Distnbution.

tribution of fishes; a subject which the limits of space compel us to

dismiss with a few sentences. In the first place, we find that many marine fishes



DISTRIBUTION.



323



have a much less wide geographical distribution than might at first sight be
expected;, while, on the other hand, we find families and genera, and even species,
of fresh-water fishes inhabiting widely separated areas of the earth's surface. The
primary division into fresh-water and marine fishes does not form such a sharply
; defined boundary as is commonly supposed ; the transition being formed by the
brackish- water types, species or even individuals of which can accustom themselves
to live in either salt- or fresh-water. Then, again, we have certain essentially
fresh-water fishes, like the salmon and some kinds of cat-fish, which pass a certain
period of their existence in the ocean ; while, on the other hand, some marine
forms, such as sturgeons, periodically ascend rivers for the purpose of spawning.
To a certain extent such habits will help to explain the occurrence of peculiar
families of fresh-water fishes (such as the chromids of Africa, South America, and
India) in widely separated areas, although this must probably be supplemented by
dispersal from a common northern centre.

After the separation of the fresh-water and brackish-water types, the marine
fishes are divided by Dr. Giinther into a littoral, a pelagic, and a deep-sea group,
although here, again, no hard-and-fast lines can be drawn. The littoral or shore-
lishes are those found in the immediate neighbourhood of land or sunken shoals;
the majority living close to the surface, and very few descending as deep as three
hundred fathoms. Their distribution is determined not only by the temperature of
the surface-water, but likewise by the nature of the neighbouring land, and its
animal and vegetable products ; some of these fishes being suited to inhabit flat
coasts with muddy or sandy bottoms, while others frequent rock-bound shores
where the water is deep, and others, again, congregate round coral-reefs. Cod,
rays, and flat-fish are well-known examples of this group. Pelagic fishes, such as
tunnies, flying-fish, sword-fishes, and sun-fishes, inhabit the superficial layers of the
>p"ii ocean, approaching the shores only by accident, or in some cases in search of
food, or for the purpose of spawning, Dr. Giinther writes that, " with regard to
their distribution, they are still subject to the influences of light and the tempera-
ture of the surface-water; but they are independent of the variable local conditions
\vhich tie the shore-fish to its original home, and therefore roam freely over a space
rt'hich would take a fresh- water or shore-fish thousands of years to cover in its
.gradual dispersal. Such as are devoid of rapidity of motion are dispersed over
similarly large areas by the oceanic currents, more slowly than, but as surely as, the
itroii"' swimmers."

c5

In marked contrast to the last are the deep-sea fishes, inhabiting the abyssal
lepths of the ocean, where they are undisturbed by tides or currents, and live for
he most part in total darkness ; their organisation, in consequence of the great
ressure of the medium in which they live, preventing them from coming to the
urface in a healthy condition. From the similarity in the physical conditions of
he ocean-depths in all parts of the world, there seems no reason why a single
pecies of deep-sea fish should not range from the Equator to the Poles ; and the
byssal fauna is probably more or less nearly the same throughout the globe.
L'hese fishes belong for the most part to pelagic families, and especially to such
yprs as are of nocturnal habits; and are characterised by their generally black or
ilvery colour, although in a few instances the fin-rays and certain filaments are



3 24 FISHES.

scarlet. Writing of those fishes, Dr. Giinther observes that, " the organ of sight is
the first to be affected by a sojourn in deep water. Even in fishes which habitually
live at a depth of only eighty fathoms, we find the eye of a proportionately larger
si/e than in their representatives at the surface. In such fishes the eyes increase
in size with the depth inhabited by them, down to the depth of two hundred
fathoms ; the large eyes being necessary to collect as many rays of light as possible.
Beyond that depth, small-eyed as well as large-eyed fishes occur; the former
having their want of vision compensated by tentacular organs of touch, while the



Online LibraryRichard LydekkerThe new natural history (Volume 5) → online text (page 34 of 62)