Copyright
Richard Lydekker.

The new natural history (Volume 5) online

. (page 58 of 62)
Online LibraryRichard LydekkerThe new natural history (Volume 5) → online text (page 58 of 62)
Font size
QR-code for this ebook


in width. Although it is difficult to form an estimate of the exact size of the fish
to which these teeth belonged, it is thought that the width of the disc must have
been about 15 feet.

In the allied genus Aetobatis, now represented by a single
Other Genera. . n . ,. , . ,, m ,.

widely-spread tropical species but common in the Tertiary forma-
tions, the muzzle carries two fins, and the dentition comprises only a single series
of transversely elongated teeth, corresponding to the central row of the typical
genus. In a third genus (Rhinoptera), of which there are seven living and several
Tertiary species, the so-called fins on the muzzle are likewise double, while the
tesselated teeth form five or more series. Of these the middle one is the largest,
the first, or first and second, lateral series somewhat narrow, and the remainder in
the form of more or less nearly regular hexagons; the dental plates of both jaws
being strongly arched from back to front. The largest existing members of the
family belong to the genera Dicerobatis and Cephaloptera, which are mainly
confined to the tropical seas, and to which the name of devil-fish might well be
restricted. In the former of these the pectoral fins do not extend on to the sides
of the head, which is truncated in front, and furnished with a pair of forwardly-
directed appendages containing fin-rays, the nostrils being widely separated.
Both jaws contain numerous rows of flat or tuberculated teeth ; and the whip-
like tail has a single dorsal fin above and between the pelvic pair, and may be
armed with a spine. In the second genus, the mouth is terminal, and teeth are
present only in the lower jaw. One of the Indian representatives of the first
genus is known to measure fully 18 feet across the disc, and a weight of over
1200 Ibs. has been recorded. Sir W. Elliot states that the horn -like appendages
" are used by the animal to draw its prey into its mouth, which opens like a huge
cavern between them. The fishermen [in India] say they see these creatures
swimming slowly along with their mouths open, and flapping these great sails
inwards, drawing in the smaller crustaceans on which they feed." The capture
of such hideous monsters is a work of no little difficulty and danger, as they are
quite capable of overturning a boat ; and the danger is said to be the greatest in
the case of a female accompanied by its single offspring. We must not leave this



STING-RA VS. 545

family without referring to the curiously-ridged quadrangular teeth from the
Chalk described under the name of Ptychodus, which appear to indicate an extinct
type of eagle-ray. In these teeth the highly-polished crown is ornamented with
large transverse or radiating ridges, surrounded by a more finely -marked marginal
area of variable width. They are arranged in longitudinal rows ; the upper jaw
having the teeth of the middle row the largest, and those of the lateral rows
gradually decreasing in size : while in the lower jaw the middle teeth are rather
small, and the two adjacent rows the largest.

THE STING- RAYS, Family TRYGONID^E.

Apparently the most specialised members of the entire group are the sting-
rays, in which the pectoral fins are continued uninterruptedly round the extremity
of the muzzle, so that the whole of the margin of the very wide disc is formed by
these fins, in the centre of which is the more elevated head and body. The long
and slender tail, which is frequently armed with a serrated spine, is sharply
defined from the body ; and the median fins, if present at all, are either imperfectly
developed, or are modified into serrated spines. The forms with armed tails, to
which the name of sting-ray is alone strictly applicable, inflict very severe wounds,
dangerous not merely from the actual lesion, but apparently also from the pre-
sence of some poisonous substance. In the larger kinds these formidable spines
may be as much as 8 or 9 inches in length ; and, as they wear out, they are from
time to time shed and replaced by new ones growing from behind. Very
numerous in species, and arranged under several genera, the sting - rays are
most abundant in the seas of the tropics, although some range into temperate
waters.

The typical genus includes some twenty-five species, one of which (Trygon
pastinaca) ranges from the south of England westwards to America and east-
wards to Japan. In this group the greatly elongated and tapering tail is armed
with a barbed arrow-shaped spine, while the skin is either smooth or dotted over
with tubercles, the nasal valves unite to form a quadrangular flap, and the teeth
are flattened. Mainly characteristic of tropical latitudes, these rays are most
abundant in the Indian and Atlantic Oceans, although some species are inhabitants
of fresh-water lakes in Eastern Tropical America. The rough ray (Urogymnus
asperrimus), of the Reel Sea and Indian Ocean, which may measure from 4 to 5
feet in length from the head to the root of the tail, is the sole representative of
a second genus, characterised by the long tail being devoid of either fin or spine,
although sometimes furnished with a narrow fold of skin below. The whole
of the body is thickly covered with teeth-like tubercles, the teeth themselves
being flattened. The third genus ( Urolopkus) in which the tail is of medium length,
furnished with a distinct terminal rayed fin, armed with a barbed spine, and some-
times with a rudimental dorsal fin, while the teeth are flattened contains several
rather small-sized species from the tropical seas, and likewise an extinct one from
the Eocene rocks of Italy. A fourth genus (Pteroplatea), of which there are some
half-dozen representatives from temperate and tropical seas, is characterised by
the great width of the disc, which is at least twice as long as wide, and also

VOL. v. 35



S4 6 SHARKS AND RAYS.

by the shortness of the thin tail, which always bears a serrated spine, and may
have a rudimental fin ; the minute teeth being either singly or triply cuspidate.
The oldest representative of the family seems to be the extinct Cyclobatis from
the Cretaceous rocks of Palestine, in which the disc is either circular or oval in
form, the tail very short, only slightly projecting beyond the margin of the disc,
and devoid of either spine or fin, while the upper surface of the body has one or
more longitudinal series of large spiny tubercles running backwards from the
pectoral girdle, the remainder of the body and disc being more or less sparsely
covered with minute prickles.

THE EXTINCT LOBE-FINNED AND FOLD-FINNED SHARKS, Orders ICHTHYOTOMI

and CLADODONTIA.

The whole of the preceding members of the subclass are included in a single
order, the characters of which have been already described ; but in the Palaeozoic
strata of both Europe and the United States there occur remains of extinct sharks,
indicating two perfectly distinct ordinal groups.

Lobe-Finned The essential characteristic of this group, as shown in the restored

Group. skeleton figured on p. 317, is the lobed structure of the pectoral fins,

which consist internally of a long tapering segmented axis, from which are given

off a double series of cartilaginous rays,
as shown in the figure on p. 319. The
internal skeleton of these sharks shows
granular calcifications in the cartilage ;
but the notochord is never or but seldom
constricted into distinct vertebras, the
calcification, except in the tail, stopping
short at an incomplete stage, when the
body of each segment of the backbone
consists of three separate pieces, as in

TEETH OF A LOBB-F^BD SHARK. -After Fritsch. the example figured on p. 312. The

upper and lower arches and spines of the

backbone are tall and slender ; the upper spines having no intercalary cartilages
between them. As represented by the genus Pleur ucanthus, common to the
Permian and Carboniferous rocks of both sides of the Atlantic, these sharks are
further characterised by the slender and slightly depressed form of the body, the
terminal position of the mouth, and the diphycercal tail. The long and low
dorsal fin is continued along the whole of the back from a short distance behind
the head, and its cartilages are more numerous than the subjacent spines of the
vertebras ; immediately behind the head is a long barbed spine, and 'the body was
probably devoid of shagreen. The teeth, as shown in the annexed illustration,
are very peculiar, consisting of two divergent and generally unequal-sized cones,
supported on an expanded base.

Fold-Pinned The oldest and most primitive representatives of the entire

Group. subclass are the armoured sharks of the Devonian and lower

Carboniferous epochs, especially characterised by the simple structure of their fins,




SPINE-FINNED SHARKS. 547

which, as explained on p. 319, are of the fold-type, and consist simply of a series of
parallel cartilaginous rods arising from a broad base. In many, but not all of them,
the granules con- ~
stituting the sha-
green of modern
sharks coalesced so
as to form large
shields protecting
the body ; and
these fish were
also armed with
more or less mark-
edly triangular
spines inserted in
the skin by their
bases. In some
cases the teeth con-




RESTORATION OF A FOLD-FINNED SHARK (Cladosdache).Mizr Dean.



sisted of a single cone, with one small basal cusp ; but in other forms they seem
to have coalesced into a pavement-like structure. No traces of calcification have
been detected in the notochord. As might have been expected, these primitive
sharks were of comparatively small size, averaging from 3 to 4 feet in length.

THE EXTINCT SPINE-FINNED SHARKS, Order ACANTHODII.

Whereas the two preceding groups contain the most primitive and generalised
representatives of the subclass, the order now to be considered, which is likewise
confined to the Palaeozoic epoch, comprises sharks of a more specialised type than
any existing forms. Indeed, these spine-finned sharks bear much the same
relationship to the lobe-finned group, as is presented by the bony fishes to the
fringe-finned ganoids ; and in a strictly natural arrangement these forms should

stand at the head of the
class, although it is
more convenient to con-
sider them in this place.
One of the essential
features of the group
is to be found in the
development of mem-
brane - bones overlying
the original cartilagin-
ous skull ; the socket

of the eye being also frequently surrounded with a ring of bones of similar
origin. In the internal skeleton the notochord is persistent, and the cartilages are
superficially calcified, frequently with a granular structure. When teeth are
present, these are firmly fixed upon membrane-bones overlying the cartilages corre-
sponding to the functional jaws of other sharks. The gill-arches bear a series of




RESTORATION OF A SPINE-FINNED SHARK (Acdnthodes).

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



548 SHARKS AND RAYS.

appendages which during life were probably furnished with membranous expan-
sions similar to those of the existing frill-gilled shark. In the fins the cartilages
of the internal skeleton are greatly reduced, and the membranous portions are
almost destitute of cartilaginous rays ; while each of the paired and most of the
median fins are provided with a large spine on the front edge. The tail is of
the heterocercal type, and the males lack the claspers characterising the existing
forms. Externally the body is covered with small and closely-arranged quad-
rangular granules, between two series of which runs the lateral line. Three
families constitute the order; the first of these, as represented by the genus
Acanthodes, having but a single dorsal fin ; while in the other two respectively
typified by Ischnacanthus and Diplacanthus there are two of these fins.



THE LOWEST VERTEBRATES AND
THEIR ALLIES.



CHAPTER I.
THE LAMPREY GROUP, Class Cyclostomata.

TILL within recent years both the lampreys and the strange little creature known as
the lancelet were generally included among the class of fishes, which was also taken
to comprise a number of armoured extinct forms, of which a brief notice is given
below. On the other hand, the marine animals commonly termed sea-squirts, but
technically known as ascidians, together with certain aberrant worm-like creatures,
were classed with the great assemblage of so-called Invertebrates. Anatomical
and palseontological investigations have, however, revolutionised our ideas concern-
ing the creatures in question, with the result that while the lampreys are now
separated from the fishes to form a class by themselves in the vertebrate subking-
doin, the lancelet and sea-squirts, together with the above-mentioned worm -like
creatures are now regarded as forming a subkingdom by themselves, known as the
Semivertebrates, or Protochordata. The reason for the separation of the lampreys
from the fishes will be gathered when we corne to that group ; but we must briefly
notice in this place the considerations which have induced naturalists to brigade in
one group such very dissimilar creatures as the lancelet, sea-squirts, and the afore-
said worms.

In the introduction to the Vertebrates given in the first volume we have indicated
the leading structural features of that group more especially as developed in its
higher members ; among these one of the most important being the dorsal position
of the great nervous system, or spinal marrow, which in the higher forms is under-
lain by the bodies of the vertebrae. In our description of the fishes we have, how-
ever, seen that in some of the lower forms the vertebrae are represented only by the
original cartilaginous rod known as the notochord, from which they are developed
by constriction in the higher types. To this we have to add that in the earlier
stages of their development all vertebrates possess gill-slits, which persist in their
original condition only in the fishes and lampreys. Now the result of anatomical
investigations has been to show that the lancelet, sea-squirts, and the aforesaid worm-
like creatures agree with the Vertebrates in the possession of a dorsally-situated
nervous system, of a notochord, and of gill - slits ; and thereby differ from all



550 LOWEST

other known animals. Consequently we may classify the animal kingdom as
follows :

I. CHORDATE ANIMALS Division CHORDATA 4 Nervous s y stem uonaija

Notocfaord, and Gill-Slits.

1. Vertebrates Subkingdom VERTEBRATA.

(1) Mammals Class MAMMALIA.

(2) Birds Class AVES.

(3) Reptiles Class REPTILIA.

(4) Frogs and Salamanders Class AMPHIBIA.

(5) Fishes Class PISCES.

(6) Lampreys and Hag-Fishes Class CYCLOSTOMATA.

2. Semi vertebrates Subkingdom PROTOCHORDATA.

(1) Lancelots Class LEPTOCARDII.

(2) Sea-Squirts Class TUNICATA.

(3) Worm -Like Forms Class ENTEROPNEUSTA.

II. NON-CHORD ATE ANIMALS Division lNVERTEBRATA{ NervousSystem Ventral; no

< Notochord or Gill-Slits.

We shall consider briefly the suggestions that have been made concerning the
relationships between the semichordates and nonchordates at the close of this
volume, and therefore proceed at once to the lampreys. Before doing so it may,
however, be as well to mention that to rightly understand the peculiarities of all
these matters requires a considerable amount of anatomical knowledge on the part
of the reader ; and structural features will accordingly be alluded to as simply and
shortly as possible.

THE LAMPREYS AND HAG-FISHES, Subclass Marsipobranchii.

As a class, the lampreys and their near allies the hag-fishes, with which may
probably be grouped certain armoured extinct forms, are distinguished not only
from the fishes, but likewise from all the vertebrates hitherto described, by the
absence of true jaws, by the single aperture of the nostrils, as well as by the rasp-
ing tongue ; there being no limbs or ribs, and the notochord either persisting in its
original form or being merely surrounded by a series of calcified rings. Probably
many or all of these characters are applicable to certain extinct forms now con-
sidered as more or less nearly allied to the lampreys, and we may accordingly
provisionally regard these as distinctive of the subclass. On the other hand, we may
consider the under-mentioned features distinctive of the lampreys as the represen-
tatives of a subclass (Marsipobranchii), apart from the aforesaid extinct forms. In
the existing members of the group the skeleton is cartilaginous ; the skull, as in
ths chimseroid fishes and some of the sharks, is immovably joined to the vertebral
column ; and the gills are in the form of fixed pouches (hence the name of the sub-
class), without gill-arches, and either six or seven in number, with their external
apertures usually opening on the sides of the neck. Anterior in position, and
adapted for sucking, the mouth is surrounded by a circular or subcircular lip
supported by cartilages. The naked body is provided with median fins, having
cartilaginous rays like those of many fishes. Internally, the heart is devoid of
the anterior expansion known as the bulbus arteriosus ; the intestinal canal is



LAMPREYS AND HAG-FISHES. 55 i

straight and simple ; and the reproductive organs discharge into the cavity of the
body. The place of teeth is taken in some forms by horny structures, while in
others the mouth is completely unarmed. Some difference of opinion exists among
naturalists as to whether the absence of the true jaws in the lampreys is an original
or an acquired feature ; but, to our mind, the apparent want of these organs in
the primitive extinct lampreys seems to be strongly in favour of the former view.

True Lampreys. The true lam P re y s > f which the sea -lamprey (Petromyzum

' marinus}, river-lamprey (P. fluviatilis), and the small lamprey

(P.branchialis) occur in Britain, are the typical representatives of a family




SEA-LAMPREY, RIVER-LAMPKEY, AND SMALL LAJ1PKEY ( liat. size).

(PetromyzidcB) characterised by the nasal duct terminating in a closed sac
behind, without perforating the palate. As in all the other members of the group,
the naked body is eel-like in form ; but the family is peculiar in that its members
undergo a metamorphosis, the young being devoid of teeth, and furnished with a
single median fin, whereas in the adult the sucking-rnouth is furnished with horny
teeth resting on a soft cushion, and the median fin is divided. In the adult the
tongue is furnished with rasping teeth, while above and below the aperture of the
mouth there are a series of upper and lower teeth, and the sucking-disc is likewise
provided with smaller isolated teeth. Eyes are present in the adult; and the
aperture of the nostrils is situated in the middle of the head. The seven-gill
pouches open externally by as many apertures on each side of the neck, but com-



552 LOWEST VERTEBRATES.

numicate with the pharynx by a common opening to which the ducts of all con-
verge. The intestine is furnished with a spiral valve, and the eggs are minute.

The true lampreys are characterised in the adult condition by having two
dorsal fins, the hindmost of which is continuous with the caudal; and likewise by
the upper series of oral teeth consisting either of a doubly -cusped trans verse ridge,
or of two closely-placed separate teeth ; while the teetli on the tongue are serratcil.
The genus appears to be represented by four species, which are confined to the
coasts and fresh waters of the Northern Hemisphere, ranging as far south as West
Africa. The largest of these is the sea-lamprey, represented in the upper figure of
our illustration, which may grow to as much as a yard in length, and is common to
Europe, North America, and West Africa. On the other hand, the river-lamprey,
or lampern, which at certain seasons ascends the rivers of Europe, North America,
and Japan in innumerable hosts, is somewhat less than two feet in length, and differs
from the last species in being uniformly coloured, instead of marbled with black.
Still smaller is the small lamprey, also known as the pride or sand-piper, which is
likewise common to Europe and Western North America, and scarcely reaches one
foot in length ; its coloration being uniform. The young of this form was long
regarded as a distinct genus, under the name of Ammoccetes ; but its true nature
was discovered by watching the transformation into the adult. The larva, writes
Dr. Glinther, requires three or four years for its full development. At first the
head is very small, and the cavity of the mouth " surrounded by a semicircular
upper lip, the separate lower lip being very small. There are no teeth, but several
fringed barbels surround the mouth. The extremely small eyes are hidden in a
shallow grove ; but there is a median single nasal opening, and seven gill-openings,
as in the adult. The vertical fins form a continuous fringe, in which the later
divisions are more or less distinctly indicated." When open, the mouth of lampreys
is nearly circular in shape, but when closed forms a narrow slit.

Much has still to be learned regarding the habits of lampreys, but it appears
that all the members of the present genus ascend rivers for the purpose of spawn-
ing, and that some of them pass the whole of their larval conditions in fresh waters.
They are all carnivorous, and in the adult state attach themselves by their mouths
to the bodies of fishes, from which they rasp off the flesh with their horny teeth ;
fish being not unfrequently met with bearing the scars of wounds thus inflicted,
and a salmon has been taken high up in the Rhone with a sea-lamprey tightly
adhering to its side. Bathers have also been known to be attacked by the same
species. Commonly keeping to the bottom, the sea-lamprey may at times be seen
swimming near the surface with a serpentine movement of the body. In the
Severn the capture of this species lasts from February to May, while in the Thames
the season is May and June ; but in the Scottish rivers the lampreys do not ascend
till the end of June, remaining till the beginning of August. During the spawn-
ing-season these fishes excavate furrows in the river-bottoms for the reception of
their eggs, and are said to remove impeding stones by lifting them up with thru-
sucking-mouths. Being much exhausted by the function of spawning, at its con-
clusion they make their way with all speed to the sea. The river-lamprey was
at one time thought to be a permanent inhabitant of fresh waters, but it has been
taken in the sea, and it has even been considered that it may undergo its meta-



LAMPREYS AND HAG-FISHES.



553



morphosis in salt water. Always restricted to low-lying countries, this lamprey
may be found alike in rivers, streams, lakes, and marshes, although it only spawns
where the water is clear and Hows swiftly over a stony bed. During the spawning-
season, which takes place in March and April, the lampreys acquire a brilliant
metallic lustre ; while at the conclusion of the function they generally perish.
Formerly these lampreys occurred in enormous quantities in many of the English
rivers, upwards of three thousand having been taken at Newark in a single night ;
but the numbers in the Thames are now considerably diminished. Their chief use
is as bait for cod and other fish ; for which they are specially adapted on account
of the ease with which they can be kept alive. There is nothing calling for special
notice with regard to the habits of the small lamprey.

southern In the Southern Hemisphere the family is represented by three

Lampreys, genera, in one of which there is a single species (Mordacia mordax)

common to the coasts of Chili and Tasmania ; while in a second (Geotria) there is

one Chilian and another South Australian species. The first of these two genera

agrees with the typical representatives of the family in the continuity between




HAG-FISH ( nat. size).

the second dorsal and caudal fins, but differs in having two groups of three-cusped
teeth above the aperture of the mouth ; whereas in the second genus the two fins
above-mentioned are separate, and there is a four-lobed plate above the mouth.
Some of these lampreys grow to a length of a couple of feet ; and in the adults of
some or all of them the skin of the throat is so much expanded as to form a kind
of pouch. The third genus (Exomegas) appears to be known only by two
examples from the Atlantic side of South America, one of which was picked up
in the streets of Buenos Aires in 1867, while the second was obtained from the
Bay of Monte Video in 1890. With the exception that the dentition is of a
peculiar type, very little is known as to the structure of this rare form. It will



Online LibraryRichard LydekkerThe new natural history (Volume 5) → online text (page 58 of 62)