Copyright
Richard Lydekker.

The new natural history (Volume 5) online

. (page 55 of 62)
Online LibraryRichard LydekkerThe new natural history (Volume 5) → online text (page 55 of 62)
Font size
QR-code for this ebook


forms the basis of isinglass. In a fossil state sturgeons are unknown before the

upper part of the Eocene period. All the members of the genus are exceedingly

voracious fishes, and the majority are mainly carnivorous. During the winter

many or all of them crowd together, either in inlets of the sea, estuaries, or the

deep pools of rivers, where they undergo a kind of hibernation; and it is stated

that in some localities they bury their noses in the mud, with their bodies and tails

standing vertically upwards like a series of posts. They increase very rapidly in

size ; and the eggs are hatched in five days. Although still abundant in the

northern rivers, in those of Central Europe sturgeon have greatly decreased in

numbers, and few really big fish are now taken. In the beginning of the year.

when they are still torpid, sturgeon are captured by breaking the ice, and stirring

up the mud at the bottom of their haunts with very long poles armed with barbed

prongs. As the fish seek to escape, some are stabbed with the spears ; and it is

said that half a score of large fish may be thus taken by a single fisherman. In

summer regular fishing-stations are established on the Russian rivers, where the

approach of a shoal is heralded by a watchman. Upwards of fifteen thousand

sturgeon have been taken in a day at one of these stations; and when the fishing

is suspended for a short time, a river of nearly four hundred feet in width, and

five-and-twenty in depth has been known to be completely blocked by a solid

mass of fish.

The common sturgeon (Acipenser sturio), of which a small

True Sturgeons. . . . _! . nr, AI. i

example is shown m the illustration racing p. 510, is the typical

representative of the first genus, in which the rows of bony plates remain distinct
from one another on the tail, spiracles are present on the head, the upper lobe of



STURGEON TRIBE. 5 , 5

the tail is completely surrounded by the tin-rays, and the muzzle is either short or
developed into a narrow beak of moderate length. There is some doubt as to the
exact number of species of sturgeons, as these fish vary considerably according to
their age, but it is probable that nearly twenty different kinds may be admitted
Among the better known forms one of the most esteemed is the sterlet (A rutkvenus)
which although rarely exceeding a yard in length, yields better flavoured flesh and
hiier caviare than any of the others. It is characterised by its narrow, pointed
snout, and by the great number of bony plates on the sides of the body- these
varying from sixty to seventy. Common in the Black Sea and Caspian, as well as
m their influent rivers, the sterlet is likewise found in the Siberian rivers while it




STERLET (^ nat. size).

ascends the Danube as far as Vienna. In contrast to this species, may be noticed
the giant sturgeon, or hausen (8. huso}, shown in our full-page illustration.
Having from forty to forty-five lateral bony plates, this species may be readily
distinguished by the absence of shields on the muzzle, which is rather short and
pointed. It is found in the Black Sea, Caspian, Sea of Azov, and their tributaries,
and occasionally enters the Mediterranean. At one time this sturgeon was to be
met with in the Danube by thousands, among which specimens of upwards of
24 feet in length were by no means uncommon: but relentless slaughter has
greatly reduced not only their numbers but likewise their size, although even now
fish of from 1200 to 1500 Ibs. weight are occasionally taken. These, however, are
mere pigmies to certain Russian examples, one of which is stated to have weighed



GANOIDS.



2760, and a second 3200 Ibs. Migratory in its habits, this sturgeon crowds into
the Russian rivers as the ice is breaking up, when many individuals are more or
less severely injured by being jammed against the floes. It appears that only full-
grown fish ascend some rivers, as no small ones are found in the Danube ; but in
the Volga these sturgeon are stated to remain during the winter in a semi-torpid
condition. Although extremely powerful, the hausen is an inactive and timid fish,
fleeing even from the diminutive sterlet, and passing much of its time on the mud
at the river-bottom, but rising occasionally to swim near the surface. In diet it is
both carnivorous and herbivorous, feeding on vegetable substances, other fish,
especially various kind of carp, and even water-fowl. Its isinglass is inferior to
that of the common sturgeon. Rarely visiting the British coasts, where it is a
" royal " fish, the latter species has only from twenty -six to thirty-one lateral plates,
and from eleven to thirteen down the middle of the back ; the muzzle peing pointed,
and about equal to one-half the length of the head. It is a widely distributed
form, frequenting the coasts of both sides of the Atlantic, but absent from the
Caspian, although found in the Black Sea. In Italy it ascends the rivers from
March to May ; and while in that country it does not commonly exceed 5 or 6 feet
in length, specimens of upwards of 18 feet are on record.

Shovel-Beaked The four species of the genus Scaphirhynchus (which must not

sturgeons. k e confused with the toothless sturgeons) differ from the preceding

'genus by the production of the muzzle into a spatulate beak, by the narrow and

depressed hinder portion of the tail being completely covered by the bony plates,

as well as in the absence of spiracles, and by the fin-rays not surrounding the

extremity of the upper lobe of the tail, which terminates in a long filament. Of

the four species, one is restricted to the Mississippi river-system, while the others

inhabit the rivers of Central Asia ; all being exclusively fluviatile in their habits.

Allied Extinct The genera Chondrosteus and Belonorhynchus from the European

Families. Lias severally represent two families differing from all the modern

sturgeons in the absence of a median unpaired series of bones in the head-shield,

and also in the possession of branchiostegal rays. In the latter family the tail is

diphycercal, and there are longitudinal series of bony plates on the body; whereas

in the former the tail is heter-
ocercal, and the body is either
naked or with a small series of
scales on the upper lobe of the
tail ; both being furnished witli
teeth.

The scaled
types of this sub-
order are so utterly unlike the
sturgeons in external appear-
ance that it is only by a study
of their internal structure that
their true affinities have been
determined. They are all
extinct, and mainly character-




Scaled Types.



AN EXTINCT ACIPENSEROIU Vlsll (/'liiti/SOMUs), from the

Magncsian Limestone.



FRINGE-PINNED GROUP. 5 r 7

istic of the Secondary period, their remains being especially common in the British
Lias. In both of the two principal families the tail is of the heterocercal type.
In one family, as typified by the genus Palceoniscus, the body is elongated fusiform,
and the teeth are slender and conical or straight. On the other hand, Platysomus
represents a second family (Platysomatidcv), in which the body is rhomboidal,
and the teeth in the upper jaw mainly confined to the pterygoid bones obtuse.'
In both groups the scales are of the ganoid type.

THE FRINGE-FINNED GANOIDS, Order CROSSOPTERYGII.

The whole of the members of the subclass under consideration described in
the foregoing pages constitute one great order (Actinopterygii), characterised, as
mentioned on p. 334, by the fan-like structure of the paired fins, and frequently
also of the caudal fin ; the scales being generally of the cycloid or ctenoid type.
These fishes form, indeed, the dominant group at the present day ; whereas the group
now to be considered is represented only by two existing species referable to as
many genera, and is mainly characteristic of the earlier epochs of the earth's




THE BICHIR.



history, being abundant even in the Devonian and Carboniferous epochs, since
which time it has been steadily decreasing in numbers. These fringe-finned
ganoids, as they may be called, have the paired fins lobate, with an internal
longitudinal axis belonging to the true skeleton more or less fringed with dermal
rays, the caudal fin being either of the diphycercal or heterocercal type. A pair
of large jugular plates, bounded in some instances by a series of smaller lateral
ones, and an anterior unpaired element, are developed in the branchiostegal
membrane to fill up the space between the two branches of the lower jaw, and
thus representing the branchiostegal rays of the first order. In all the scales are
coated with ganoine, although they may be thin, overlapping, and rounded, or thick
and quadrangular. The existing forms have the optic nerves simply crossing one
another, a spiral valve in the intestine, and a duct to the air-bladder ; the presence
of the latter being also shown in certain extinct types. Next to the sharks and
rays, this group is one of the oldest, being well represented in the Devonian.

The sole existing survivors of this great group of fishes are the
Existing Species.

bichir (Polypterus bichir) of the Nile, and other rivers of Tropical

Africa, and the reed-fish (Calamoichtkys calabaricus) from Old Calabar; these
constituting the family Polypteridce, \vhich has no fossil representatives, and
probably forms a subordinal group by itself. In this family the notochord is more
or less constricted and replaced by ossified vertebrae ; the baseosts, or superior
supporting elements, are rudimentary, or wanting, in the median fins ; whereas the



5i8 FRINGE-PINNED GANOIDS.

axonosts, or inferior supports, form a regular series equal in number to the dermal
fin-rays with which they articulate. The scales are ganoid, and the fins without
fulcra. The dorsal tin is divided into a number of tinlets, each formed by a spine
in front and a series of rays behind; the anal fin being situated close to the
diphycercal caudal, and the vent near the end of the tail, while the whole caudal
region is very short. In the bichir the body is moderately elongated ; the teeth
are rasp-like, and arranged in broad bands in the jaws and on the vomers and
palatines, the jaws also bearing an outer series of larger pointed teeth : and the
pelvic fins are well developed, but do not show the obtusely lobate structure
characterising the front pair. The large air-bladder is double. The bichir is
found in the Upper Nile and the rivers on the west coast of Tropical Africa,
examples being occasionally carried down into the Lower Nile. The number of
finlets varies from eight to eighteen, and in size this fish grows to as much as
4 feet. Nothing is known of its habits. The reed-fish is a smaller form, charac-
terised by the great elongation of the body, and the absence of pelvic fins.

. Very little can be said here as to the numerous extinct repre-

Extmct Families.

sentatives of this group. One subordinal group (Actinistia) is

represented by the hollow-spined ganoids (Ccelacanthidcv), which range from the




SKELETON OF A HOLLOW-SPINED FRINGE-FINNED GANOID,

(From A. S. Woodward, Cat. Foss. Fish, Brit. Mus.)

Carboniferous to the Jurassic, and are best known by the genera C&lacantJnts and
Undina. In these fishes (as shown in the accompanying figure) the notochord
persists ; the axonosts of the anal and two dorsal fins are fused into a single piece :
in the caudal fin the dermal fin-rays are each supported by a series of axonosts,
equal in number to the upper and lower spines of the vertebrae ; and each pelvic
has a single axonost, which is not united with that of the opposite side. In these
fishes the body is deeply and irregularly fusiform, with the scales overlapping,
rounded, and more or less coated with ganoine. There is a gill-cover and a single
pair of jugular plates ; the paired fins are obtusely lobate ; the tail is diphycercal,
frequently with a small supplemental fin at the extremity; and the air-bladder
is ossified. A third suborder (Rhipidistia) includes most of the other forms,
especially those from the Devonian formation, and while agreeing with the
preceding group in having a more or less completely persistent notochord, and
the axonosts of the anal and two dorsal fins each fused into a single piece, diti'ers
in that in the caudal and other median fins the baseosts are fewer in number than



EXTINCT FAMILIES. 519

the dermal fin-rays, by which they are overlapped. The suborder is represented
by three well-defined families. In the first, which is typified by the genus
Holoptychius, the lobes of the pectoral fins are long and acute, while the teeth
have complex infoldirigs of the outer layer, somewhat after the manner of those
of the primeval salamanders, and the scales are thin arid cycloidal. The second
family, of which Rhizodus is the typical genus, differs by the lobes of the pectoral
fins being shorter and blunter, and also by the less complicated infoklings of the
teeth. To this family belongs Gyroptychius, from the Devonian or Old Red
Sandstone of Scotland. While agreeing with the last in the obtusely lobate
pectoral fins, the third family, as represented typically by Osteolepis of the
Old Red Sandstone, is characterised by the walls of the teeth being slightly
infolded only at their bases, and by the scales being of the true quadrangular,
ganoid type. Remains of these fishes occur in extraordinary abundance in the
Old Red Sandstone of Scotland ; and as this deposit is of fresh-water origin, it is
evident that they were either fluviatile or lacustrine forms. The reason why these
and so many other ancient creatures were enveloped in coats-of-mail has not yet
been discovered.



CHAPTER IV.
SHARKS AND RAYS, Subclass ELASMOBRANCHII.

THE last subclass of the fishes is represented by the existing sharks and rays,
together with a number of more or less closely allied extinct forms ; some of the
latter being the most primitive members of the order yet known. Indeed, taking
these primitive types into consideration, and remembering that sharks and their
allies are the oldest fishes with which we are acquainted dating from the lower
Ludlow beds of the Silurian epoch it seems probable that the present subclass
may have been the stock whence all other fishes were derived. Agreeing with
the bony fishes and ganoids in having the suspending apparatus of the lower jaw
movably articulated to the skull (generally with the intervention of a distinct
hyomandibular element), the sharks and rays have the skeleton entirely cartila-
ginous throughout life ; membrane-bones except in one extinct group being
entirely wanting. The gills open by separate external clefts, and have no cover.
When bony elements are developed in the skin, these agree in structure with teeth,
and have nothing to do with true bone. In all the living members of the subclass
the optic nerves cross one another without giving off any mutually interlacing
fibres, the arterial bulb of the heart is furnished with three valves, the intestine
has a spiral valve, the eggs are large and detached, and an air-bladder is wanting.

The whole of the existing representatives of the subclass form an order
(Selachii) characterised by the cartilaginous internal skeleton being, as a general
rule, only superficially calcified ; while, except in some of the earlier extinct types,
the notochord is constricted at the centre of each vertebra. The superior and
inferior arches of the vertebrae are short and stout, and intercalary cartilages are
very generally developed. The pectoral fin has not a segmented longitudinal
central axis, its cartilaginous rays forming a fan-shaped structure radiating from
an abbreviated base, into the anatomical details of which it will be unnecessary to
enter here; and the axis of each pelvic fin is developed in the males into a
" clasper," connected with the reproductive function. With regard to the structure
of the skull, it may be mentioned that the hyomandibular usually intervenes
between the palatopterygoid bar (forming the functional upper jaw, and carrying
the teeth) and the cranium proper ; but in the genus Notidanus the hyomandibular
takes no share in the support of the jaws, the palatopterygoid bar articulating
directly with the cranium by means of a facet behind the socket of the eye ; this
structure being probably the original one. We have already said that the tooth-
bearing palatopterygoid bar serves the function of an upper jaw, by which name
it may be conveniently referred to ; and similarly the functional lower jaw is in
reality the element known as Meek el's cartilage. The gills are attached to the



DISTINCTIVE CHARACTERS. 521

skin by their margins, and usually communicate with the exterior by means of
five vertical slits on the sides of the neck, although occasionally the number of
these clefts is increased to six or seven. Very generally the mouth is situated on
the inferior aspect of the head; and the teeth carried on the functional jaws may
be either sharply-pointed and separate, or blunt and articulated together, so as to
form a more or less pavement-like structure. In the former case there is a




HAMMER-HEADED SHARK ( T \j liat. size).

continuous succession of new teeth to replace the old ones as they are worn away
and shed. As a rule, the tail-fin is heterocercal, with the upper lobe greatly
elongated ; the pelvic fins are always abdominal in position ; and the dorsal fins
of many extinct and a few living types bear large spines on their front edge,
which, unlike those of the bony fishes, are simply imbedded in the flesh, without
articulating with the internal skeleton, and are consequently immovable. Spiracles
are frequently developed on the upper surface of the head ; and the intercalary
cartilages already alluded to are ovoid or diamond-shaped structures occurring



5 22 SHARKS AND RAYS.

between the superior arches of the vertebrae. The eggs are generally invested in
horny rhomboidal capsules, furnished at the four corners with long tendril-like
H laments, by which they attach themselves to the stems of seaweeds and other
bodies, as shown in the figure of the lesser spotted dog-tish given on p. 529. In
some species, however, the eggs are hatched within the body of the female ; and
in all cases the embryos are furnished with external gills, which are shed before
birth. All the members of the order subsist on animal substances, but whereas the
typical sharks are highly predaceous creatures, seizing and devouring everything
they come across, some of the largest species are armed only with small teeth, and
feed on molluscs and other invertebrates. The rays, too, are largely shell-fish
eaters, and most of them differ from the sharks in living on or near the bottom,
instead of swimming about actively at or just below the surface. All the species
are typically marine, but many ascend tidal rivers, and in the Viti Levu Lake in
Fiji, as well as in the Nicaragua Lake in South America, there are sharks dwelling
permanently in fresh water. The species inhabiting the former lake, which is cut
off from the sea by a cataract, is Carcharias gangeticus, common alike in the
Ganges and in the Tigris, and ascending in the latter river to a distance of three
hundred and fifty miles from the sea in a straight line. Then, again, a species of
saw-fish is found in a fresh-water lake in the Philippines. It has been commonly
stated that sharks have the power of scenting their prey from a distance, since
they rapidly congregate whenever animal refuse or other decomposing matter is
cast overboard from a ship ; but it may be suggested that such assemblages, as in
the case of vultures, are rather due to one shark following the movements of
another, and thus being attracted to the central point. The order was formerly
divided into two subordinal groups, based upon the conformation of the body ;
the one group including all the sharks and dog-fishes, and the other the rays and
their immediate allies. It has been found, however, that although this difference
in bodily form is of considerable importance in classification, yet that it does not
constitute the essential line of distinction, which is based upon a difference in the
internal structure of the bodies of the vertebrae. Taking this character as a basis,
the members of the order may be arranged in two subordinal groups, the first of
which comprises the true sharks and dog-fishes, while the second includes the
spiny dog-fishes, saw-fishes, eagle-rays, and rays.

THE BLUE SHARK AND ITS ALLIES, Family CARCHARIID^E.

The well-known blue shark (Carcharias glancus), of which examples are
depicted in our coloured Plate, may be taken as the typical representative of the
leading family of the first suborder. Before, however, indicating the characters
of the family, we must refer to those of the suborder, for which the name of
Asterospondyli has been suggested. The essential feature of this group is to be
found in the circumstance that when the bodies of the vertebne are fully calcified,
the radiating plates in the interior predominate over the circular ones, so that a
transverse section presents -,\ star-like arrangement. All these fishes have an anal
iin, and the form of the body elongated and subcylindrical, while the tail is
powerful and well adapted for swimming. In no case are the pectoral fins




o:
<

x

CO

LU

D
_J

CO



TYPICAL GROUP.



5 2 3



expanded ; and the spiracle is always small and may be wanting. The front
teeth, and very frequently also those on the sides of the jaws, are formed on the
type of a laterally compressed cone with cutting edges, at the base of which two
or more smaller cones may be developed ; but in one existing and many extinct
genera the hinder teeth have blunt crowns adapted for crushing. The two branches
of the jaws do not run parallel to each other, in consequence of which the teeth
form oblique rows, whereas in the rays they are set in straight longitudinal rows.
From the other sharks the members of the present family may be distinguished
by the absence of spines in both the dorsal fins, of which the first is situated above
the interval between the pectoral and pelvic pairs ; by the presence of a nictitating
membrane to the eye ; and by the teeth, when fully formed, being hollow, and
usually pointed. The bony elements in the skin take the form of minute granules,
thus constituting the well-known "shagreen," as the dried skin is termed.

In all the members of the typical ;enus (Carcharias)
Typical Genus. i *? i i

the muzzle is produced forwards, and the inferiorly-placed mouth

is crescentic and armed with large, flat, triangular, single-coned teeth, of
which the upper ones differ considerably in form from those of the lower jaw.
Spiracles are absent, and there is a pit at the root of the caudal fin, which has a
distinct lower lobe. At the present day these sharks are represented by between
thirty and forty species, of which the blue shark is one of the commonest and
most widely distributed ; while in a fossil state the genus is known from the
Tertiary formations. The blue shark frequently attains a length of from 12 to
15 feet, but some of the other species are stated to grow to as much as 25 feet.
In common with the other larger members of the suborder, all these sharks are
more abundant in tropical than in temperate seas ; but the blue shark is by no
means an uncommon visitor to British waters, more especially on the southern and
western coasts of Ireland. Mr. J. T. Carrington writes that they more usually
wander to the British coasts " in warm weather, especially in autumn, but they
have been seen in June, and even in the month of March. They are nocturnal in
their more active habits, taking rest and sleep in the daytime, often on the surface
of the water, with a portion of the dorsal fin and extremity of the tail exposed
above in the air. So gentle are they in their movements that, unlike many other
monsters of the deep, they do not disturb the luminous creatures, which at the
same time will be lighting every wavelet with their phosphorescence. Blue
sharks are not very particular as to what fish they take as food, though those
which are gregarious in their habits, like mackerel, pilchards, and herring, are
most commonly hunted by them. It is on record that big fish, such as congers
and the larger dog-fish, were found in a dead specimen from Cornwall. Occasion-
ally they become entangled in the drift-nets set by the pilchard-fishers, but these
sharks will also take a bait. Great care is necessary in landing a hooked specimen,
in case it gives a blow with its tail, which may result in serious consequences, such



Online LibraryRichard LydekkerThe new natural history (Volume 5) → online text (page 55 of 62)