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the opposite, or eyeless side is, as a rule, colourless. The bones of the head are
unequally developed and unsymmetrical ; and the dorsal and anal fins are of great
length, and undivided, the former often extending forwards so as to separate the
blind from the eyed side of the head. In the most specialised forms the teeth and
jaws are more developed on the lower or blind side than on the other, and there is
no air-bladder. Dr. Cunningham, who has paid special attention to the structure
of these fishes, writes that " mere dissection of adult specimens shows that the
anomalous position of the eyes is due to a distortion of the facial region of the
skull. The cranial region of the skull is but slightly altered, but the interorbital
parts of the two frontal bones are bent away from their original position in the
dorsal median line down to the side of the head, and they are also compressed into
a thin plate. But the eyes have pretty nearly the same relations to the inter-
orbital septum as in an ordinary fish. There is one eye on each side of the septum
as usual. It is, in fact, the curious condition of the dorsal fin in the flat-fish, even
more than the mere distortion of the eyes, which makes it so different from the
ordinary fish. If the fin terminated some distance behind the eyes, or if it was
prolonged in the direction it ought to follow, that is along the line which divides
the two frontal bones from one another, it would be plain at a glance which was
the left side of the head and which the right. It would then be obvious that the
left eye was still on the left side of the head, and the right eye on the right. But
the dorsal fin does neither of these things. The external ethmoid bone belonging
to the blind side is much enlarged, and sends back a process outside the eye
belonging to that side to meet another process from the cranial region of the skull.
Thus the eye which has migrated the upper eye when the fish is held in a vertical
plane is enclosed in a complete bony orbit, while the lower eye is merely bounded
on its outer side by the jaw muscles. It is on this bony bridge, entirely foreign
to the anatomy of an ordinary fish, that the dorsal fin supports itself in its
advance towards the snout. Properly speaking, the left side of the face in a
plaice, for instance, extends from the ventral edge, or chin, to the line between
the eyes, but the dorsal fin in its anterior extension divides this side of the face
into two parts."

The pigment-bearing elements in the coloration of the dark side of flat- fish
are known as chromatophores ; and while these are absent from the light side, the
so-called silvery layer is present on both. Young flat-fish, which are generally
met with in the open sea, are transparent and perfectly symmetrical, with one eye
on each side of the head, and swim in the vertical plane like ordinary fishes.
That flat-fishes have originated from symmetrical ancestors is quite evident, their
individual metamorphosis indicating the manner in which the evolution took
place. As to the inducing causes of this evolution and metamorphosis, there is still
some difference of opinion; and as it is a subject which does not come within the
province of this work, it need not be further alluded to. There are, however, certain
experiments with regard to the normal absence of coloration on the under surface




FLAT FISH




FLAT-FISH. 441

I' these, fish which are of sufficient interest to merit a brief notice. Knowing, as

n 7

AC <lo, that among plants absence of light leads to the deprivation of colour, it was
nought highly probable that the same might be the case with regard to flat-fish,
more especially since the absence of coloration in the olm among the Amphibians
is clearly due to the same cause. To test this, flounders were kept in a tank, in
which, by the help of a mirror, light was so introduced as to give to the fishes the
unwonted experience of illumination ascending from below instead of coming
down from above. The experiment was conducted for several months; some of
the flounders died, in others no great effect was produced, but in some cases the
white surface became marked over with pigment. This experiment demonstrates
hat the capacity for colorisation existed in the skin, but that light was wanted
o call it into action ; and it may also be inferred that the incidence of light must
11 general be the reason why the upper surfaces of animals are more strongly
igmented than the lower. Occasionally what are known as ' double flat-fish '
re met with, that is specimens in which both sides are coloured, and one eye
ituated on the edge of the head ; such monstrosities having been observed in the
urbot, flounder, plaice, sole, etc. In one turbot the right eye was on the edge of
.he head, so that the dorsal fin, instead of extending, as usual, to the front of the
ead, was separated therefrom by a concavity, the right side being coloured like
he left, although somewhat less strongly. It has been thought that these ' double-
sh ' swam about in a vertical position, M. Giard stating that he has actually seen
turbot so doing. But Dr. Cunningham writes that there is " no satisfactory
vidence at present that the monstrous specimens, whose metamorphosis is per-
nanently arrested, swim about, in whatever position, any more frequently, or rest
>n the ground less constantly than their normal brethren. I have under observa-
ion a living double specimen of the plaice ; its eye is on the edge of the head, the
dorsal fin terminates behind the eye, and the posterior three-fourths of the lower
side are coloured like the upper, the anterior fourth being white. This specimen,
instead of showing a tendency to continue swimming in the water, cannot even be
induced to leave the bottom long enough to enable me to see whether it holds
itself perfectly horizontal or not. I have never seen it leave the bottom of its
own accord ; it lies always buried in the sand up to its eyes, and, when disturbed,
makes violent struggles to bury itself again."

When lying on the sandy bottom of the sea and they prefer sandy to muddy
situations flat-fish are almost indistinguishable from their inanimate surroundings,
the spots with which the bodies of many of them are marked harmonising exactly
with the bright-coloured pebbles strewing the sand. This resemblance is, perhaps,
carried to the fullest extent in the flounder, as anyone who visits a large aquarium
may ascertain for himself. Occasionally rising to the surface, they swim with an
undulating lateral movement of the body, which is decidedly graceful ; and they
are found in shallow water, or at moderate depths. They are inhabitants of all
seas, except those of the polar regions, and where the coast is precipitous and
rocky; and although more numerous in the tropics, they attain their greatest
development in point of size in the temperate regions. Many species, such as
flounders, ascend rivers to a considerable distance; and a few have become
accustomed to a fresh-water existence. As regards food, the whole of the species



442 SOFT-FINNED GROUP.

are exclusively carnivorous. In a fossil state the flat-fish arc 1 tut poorly repre-
sented, and it is probable that they were not evolved till the commencement of
the Tertiarv period: the earliest known form being a turbot from the middle
Eocene of Monte Bolca, while a fossil sole has been described from the Miocene of
Wiirtemberg. As a food-supply the flat-fish are of especial value, not only on
account of the large size, and abundance of their numerous representatives, but
likewise from the excellent quality and flavour of the flesh of the majority of
these. It is on the coasts of the temperate regions of the Northern Hemisphere
that the pursuit of these fish is carried out with the greatest energy and success.

The least specialised member of the family (Psettodes erarnei),

which ranges from the Red Sea through the Indian Ocean to China,
and also occurs on the West Coast of Africa, belongs to a group in which the teeth
and jaws are nearly equally developed on both sides, and is specially distinguished
by the dorsal fin commencing on the nape of the neck, whereas in all the other forms
it starts from above or in front of the eyes. In the Indian fish, which attains a
length of about 16 inches, the eyes are as frequently on the right as on the left
side, and the transposed one is situated nearly in the line of the dorsal fin. This
species is a connecting link between the other members of the family and ordinary
fishes, and is reported to swim at times in a vertical position.

Having the jaws nearly equally developed on both sides, and the

dorsal fin commencing above the eyes, the holibut (Hippogloesus
vulgaris) is one of two species forming a genus characterised by the eyes being on
the right side, and the teeth of the upper jaw arranged in a double series, those in
the front of the upper and on the sides of the lower jaw being enlarged. The
mouth is relatively wide. In colour the holibut is dusky brown, frequently
inclining to olive, on the dark side ; the opposite side being white arid smooth. It
is the largest member of the family, ranging usually from 3 to 6 feet in length ;
one specimen with the latter length having a breadth of 30 inches, and a weight of
161 Ibs. It is stated, however, that an example taken in the early part of this
century off the Isle of Man was nearly double that weight. Holibut are found
near all the northern coasts of Europe, as well as those of Kamschatka and Cali-
fornia, generally frequenting banks at some distance from the shore, in water of
from fifty to one hundred fathoms in depth, where they often associate in consider-
able numbers. The flesh is coarse, and of inferior flavour. This fish is shown in
the upper figure of the coloured Plate.

In the genus typically represented by the turbot (Rhombus
Turbot, etc.

the dorsal mi commences on the muzzle in advance of the



eyes; the eyes are on the left side; the mouth is wide; and the jaws are furnished
with a single series of equal-sized villiform teeth, while there are also teeth on the
vomer. Scales are either very small or wanting. The genus includes seven species.
ranging over the North Atlantic and Mediterranean, but those on the two sides of
the Atlantic are different. The turbot. which attains a yard in length, and is by
far the best food-fish of its tribe, is exclusively European, and has the pelvic tins
distinct from the anal, and no scales; the general colour being greyish or brownish,
sometimes spotted with a darker tint. On the other hand the brill (Hit. l<t<i'i*),
which is likewise Knropean. is a smaller fish, of more oval shape, with the body



FLAT-FISH.



443



and all the head, except the muzzle, covered with minute scales ; its colour being
greyish brown, with reddish brown spots. Turbot commonly weigh from 5 to
10 Ibs., and occasionally reach 20 or even 30, while considerably greater weights
have been recorded. Another British representative of the genus is the Mary-sole
(R/i. aquosus), which may be distinguished by its ciliated scales ; while a fourth,
known as Block's top-knot (Rk. pu nctatus), differs from all the foregoing by having
the pelvic fins confluent with the anal. The true top-knot (Phrynorhombus
unimaculatus), which is a small form not uncommon on the southern coasts of
England, and abundant in the Mediterranean, is referred to a distinct genus on
account of the absence of vomerine teeth. A turbot is shown in the central figure
of the coloured Plate.

Plaice and The plaice (Pleuronectes platessa) and flounder (PI. flesus), of

Flounder. which examples are shown in the right lower corner of the coloured
Plate, are examples of a genus pertaining to a group characterised by the narrow-
ness of the cleft of the mouth, and by the jaws and teeth being much more developed
on the light than on the dark side. Unlike the turbot and its allies, where the
upper is somewhat behind the lower, the two eyes are in the same transverse line,
and generally situated on the right side. The dorsal fin commences above the
eyes ; the scales are minute or wanting ; and there are no teeth on the palate,
while those in the jaws are of medium size, and may be arranged in either a double
>r a single row. The genus, which is common to the Northern, Temperate, and
tic seas of both hemispheres, contains over a score of species, which may be
livicled into groups according to the form of the teeth, the number of rays in the
lorsal fin, and the conformation of the lateral line. The plaice, which ranges from
the French coasts to Iceland, and -is represented by an allied form on the opposite
side of the Atlantic, belongs to a group with compressed, lanceolate, or truncate
3eth, and no fewer than ninety dorsal rays ; it has the brownish upper surface
narked with bright yellow spots. This species is exclusively marine, but the
flounder is almost as much a fresh-water as a sea fish, ascending rivers to a con-
siderable distance. Distinguished from the plaice by the dark mottlings on the
brownish or brownish yellow skin of the upper surface, it belongs to a group in
ch the teeth are conical ; the lateral line being very slightly curved in front,
and the scales minute. Its distribution is practically the same as that of the
plaice, and it is represented by an allied species in the Mediterranean.

In the plaice and its allies the pectoral fins are always well
developed, but in the group to which the common sole (Solea vulgaris)
belongs these may be wanting, while the upper eye is always somewhat in advance
of the lower one, both being on the right side. As a genus the numerous species
of soles (somewhere about forty in number) are characterised by the median fins
being separate from one another, and the ctenoid scales ; the dorsal fin commencing
on the muzzle, and the lateral line being straight. The cleft of the mouth is very
narrow, and twisted round to the left, or blind side ; and it is on this side only that
villiform teeth are developed in the jaws, the palate being toothless. With the
exception of the lower south temperate zone, soles are distributed over all temperate
and tropical coasts in localities suited to their habits ; many of the species entering,
or even dwelling permanently in fresh waters. The common sole, which is found



444 TUBE-BLADDERED GROUP.

on the coasts of the greater part of Europe, lias both pectoral fins well developed,
and the nostrils of the blind side very narrow; the general colour being dark
brown, with the tips of the pectoral tins blackish. Large specimens may weigh as
much as 5 or 6 Ibs., and a tish of 9 Ibs. in weight is on record. Soles are taken by
trawling ; the best ground in England being along the south coast from Dover to
Devonshire. The lemon-sole (X. a/wrantiaca), which is a more southern form,
ranging from the south of England to Portugal, and living in deeper water, is one
of a group characterised by one of the nostrils of the blind side being dilated and
surrounded with a fringe of papillae It is smaller and wider than the common
species, and orange or light brown in colour, dotted over with numerous small
brown spots. Other British species are the banded sole (S. va/riegata) and the
dwarf sole (8. minuta), both belonging to a group characterised by the small si/e
of the pectoral fins. The Mediterranean sole (S. mionochir) is peculiar in lacking
a pectoral fin on the blind side ; while the Japanese sole (8. japonica) is one of two
species in which both these fins are absent. The common species is shown in the
left lower corner of the coloured Plate.

On account of the rudimentary condition of their eyes we must
not omit mention of the blind soles, which are divided into two
genera, the one (Soleotalpa) characterised by the separation of the median fins,
which are confluent in the other (Apioniclithys) ; pectoral fins being wanting in
both. Each genus is represented only by a single species; Soleotalpci coming from
the West Indies, while the habitat of the other species appears to be unknown.

THE TUBE-BLADDERED FISHES, Suborder PHYSOSTOMI.

It has been already stated on p. 334 that the whole of the four preceding sub-
ordinal groups of the bony fishes are regarded by Professor Cope as constituting
but a single suborder (Physoclysti) characterised by the absence of a duct to the
air-bladder, the separation of the parietal bones of the skull b}^ the supraoccipital,
and by the pelvic fins being usually thoracic or jugular in position. The group to
which we now come, including the whole of the remaining representatives of the
existing bony fishes, differs from the above in that the air-bladder, when present,
has a duct communicating with the stomach or oesophagus, while the pelvic tins
are always abdominal in position, and the parietal bones are usually in contact
with each other. With regard to the constancy and importance of these characters
of the present suborder, Professor Cope writes that the presence of the duct from
the air-bladder which characterises it, " is always associated with an abdominal
position of the pelvic fins and cycloid scales, and mostly with the presence of the
precoracoid arch, the entrance of the maxillary bone into the border of the mouth,
and the non-separation of the parietal bones by the supraoccipital. Yet none of
these characters are precisely associated at the point of change in each, for there
are Physostomous fishes with separated parietals and ctenoid scales (some Cyprino-
(luiifidw), and there are Physoclysti with abdominal pelvic fins." In the present
suborder, with the exception of the first in the dorsal and pectorals, which may be
ossified into spines, all the fin-rays are soft and jointed. Very different views
obtain as to the best mode of arranging the families constituting the suborder, and



EEL TRIBE. 445

a tinal classification is still a desideratum. By Professor Cope the families have
been arranged in a number of sectional groups, mainly distinguished by the structure
of the skeleton ; and a modification of this arrangement is adopted here, although
fewer groups are recognised. It is, however, impossible to enter here into the
consideration of the osteological features by which these sections are distinguished,
and we are accordingly compelled to rely mainly on external characters.

THE EELS AND THEIR ALLIES, Families MURJSNID+-E,
SYMBRANCHID^:, and



The whole of the members of these three families are characterised by the
longated, " eel-like " form of the body ; but it is quite probable that this external
imilarity is due to parallelism in development, and that the three families have
een independently derived from very different types of more normally formed
shes. The first family, which includes the true eels, mursenas, and congers, is
characterised by the normal structure of the upper jaw, which is formed in front
by the premaxillae (more or less confluent with the vomer and ethmoid) and
laterally by the toothed maxillae. The median fins, when present, are either
confluent or separated by the projecting tail ; the pectorals may or may not be
developed ; but the pelvic pair is invariably wanting. There are no accessory
breathing -organs; the stomach has a blind appendage; the vent is generally
situated far back, but may be near the pectoral fins; and the ovaries have no
ducts. Externally the skin may be either completely naked, or may contain
rudimental scales. In the skeleton the pectoral arch is unconnected with the
skull, and attached to one of the earlier vertebrae. Eels are found in the fresh
waters and seas of the greater part of the temperate and tropical regions ; some
living at abyssal depths in the ocean. The young of some forms are pelagic for
a portion of their existence; and it is believed that a large number of the so-
called Leptocephali (see p. 322), or glass-eels, are abnormal larvse of this family.
Geologically the family is a comparatively ancient one, true eels having been
discovered in the Chalk of the Lebanon, as well as in the Tertiaries of Europe.
Congers referred to the existing genus Ophichthys have been described from the
middle Eocene of Monte Bolca, and there is also an extinct genus from the latter
deposits, and a second from the London Clay.

The muraenas are large marine eels, remarkable for their bright
spotted or mottled coloration, and taking their name from the species
here figured (Murcena helena), which was so called by the ancient Romans.
Belonging to a small section of the family characterised by the gill-openings into
the pharynx being in the form of narrow slits, they are specially distinguished by
the median fins being well developed, and the total absence of pectorals. The
skin is scaleless; the mouth is well furnished with teeth; and there are two
nostrils on each side of the muzzle, the front pair being tubular, while the hinder
ones may be either tube-like or mere flat openings. The muranas, of which there
are more than eighty species, are distributed over all tropical and temperate seas,
and a few ascend tidal rivers. The majority of them are armed with formidable
teeth which frequently alter considerably with age adapted for seizing the fish



446



TUBE-BLADDERED GROUP.



on which they feed. " Large specimens thus armed," writes Dr. Giinther, " readily
attack persons in and out of the water ; and as some species attain a length of
6 or 8 feet, they are justly feared by fishermen. The minority of species have
obtuse and molar-like teeth, their food consisting chiefly of crustaceans and other
hard-shelled animals. Most of the muraenas are beautifully coloured and spotted,
some in a regular and constant manner, whilst in others the pattern varies in a
most irregular fashion; they have quite the appearance of snakes." The figured
species, which ranges from the Mediterranean to the Indian Ocean and Australia,
has the ground-colour a rich brown, upon which are large yellowish spots, each
dotted with smaller spots of brown. The finless mursenas (Gymnomurcena), of
which half a dozen species have been described from the Indian and Pacific Oceans,
differ in the reduction of the fins to a rudiment near the end of the tail.




True Eels.



MEDITERRANEAN' MUH.KNA (i liat. size).

The typical eels, familiar to all in the form of the common
European species (Anguilla vulgaris), agree with the great majority
of the family in having the gill-openings into the pharynx as wide slits. The
skin contains small scales embedded in its substance ; the upper jaw does not
project beyond the lower; the small teeth are arranged in bands; the narrow
external gill-openings are situated at the base of the well-developed pectoral fins,
and the dorsal fin commences at a considerable distance behind the back of the
head. Eels, of which there are numerous species, appear to be distributed throughout
the fresh waters of the habitable portions of the globe, being reputed to be absent
only from those of the Arctic regions, and probably also from cold elevated
districts like Turkestan and Tibet. The common European eel is spread over
the greater part of Europe and the Mediterranean area although unknown in the
l>;mul)p -;md ivapprars in the United States. The so-called grig, or glat-cd.
characterised by its lighter colour, broader head, and snout, and the more backward



EEL TRIBE.



447



position of the front of the dorsal tin, has been generally regarded as a distinct
species, although this is not admitted by Day. About a yard is a good size for an
eel, although much longer specimens are on record. Few subjects have given rise
to more discussion than the mode of propagation of eels, and as this must be
noticed in some detail, we take the following extracts from a paper published a
few years ago by the last-named observer. After mentioning the difficulties that




EELS IN THE MUD (\ nat. size).

took place in the identification of female eels, the author states that when this
point was cleared up, naturalists became rather puzzled where to look for the
male element, so the idea took root that these fishes might be hermaphrodites.
It was observed that in addition to the frill-shaped band forming the undoubted
ovaries, there was a second fatty band running along one side of the intestines,
in which milt was erroneously stated to have been discovered. In 1873, however,
an organ was discovered in an eel of 16 inches in length, which was correctly
identified with the male element, since which date numerous males, which are



448 TUBE-BLADDERED GROUP.

generally of smaller size than the females, have been recognised. " For the



Online LibraryRichard LydekkerThe new natural history (Volume 5) → online text (page 47 of 62)