(II.) GNATHOSTOMATA.
THE mouth is supported by definite jaws.
ICHTHYOPSIDA.
The epiblastic exoskeleton is generally unimportant, the mesoblastic
exoskeleton is usually well developed.
The notochord with its membranous sheath (1) may remain unmodified, or
(2) may be replaced by bone or cartilage derived from the
skeletogenous layer, or (3) may be calcified to a varying extent.
The first vertebra is not homologous throughout the whole series and
so is not strictly comparable to the atlas of Sauropsids and Mammals.
The centra of the vertebrae have no epiphyses. The skull may be (_a_)
incomplete and membranous, or (_b_) more or less cartilaginous, or
(_c_) bony. Membrane bones are not included in the cranial walls, and
there are large unossified tracts in the skull. When membrane bones
are developed in connection with the skull, a large parasphenoid
occurs. The basisphenoid is always small or absent. The skull may be
immovably fixed to the vertebral column, or may articulate with it by
a single or double occipital condyle. When the occipital condyle is
double, it is formed by the exoccipitals, and the basi-occipital is
small or unossified. The mandible may be (_a_) cartilaginous, (_b_)
partially ossified, or (_c_) membrane bones may be developed in
connection with it,--if so, there is usually more than one membrane
bone developed in connection with each half.
There are at least four pairs of branchial arches present during
development. The sternum, if present, is not costal in origin.
CLASS I. PISCES.
The exoskeleton is in the form of scales, which may be entirely
mesoblastic or dermal in origin (e.g. _cycloid_ and _ctenoid_ scales),
or may be formed of both mesoblast and epiblast (e.g. _placoid_ and
_ganoid_ scales). Large bony plates may be derived from both these
types of scale. In general fish with a greatly developed dermal armour
have the endoskeleton poorly developed; and the converse also holds
good.
The integument of the dorsal and ventral surfaces is commonly
prolonged into longitudinal unpaired fins, supported by an internal
skeleton. These fins are distinguished according to their position as
dorsal, caudal and anal fins. The dorsal and anal fins are used
chiefly as directing organs, the caudal fin is however a most
important organ of propulsion.
Three types of tail are found in fishes, viz.:--
1. The =diphycercal=, in which the axis is straight and the tail is
one-bladed and symmetrical, an equal proportion of radiale[29] being
attached to the upper and lower surfaces of the axis.
2. The =heterocercal=, in which the tail is asymmetrical and the axis
is bent upwards, the proportion of radiale or of fin-rays attached to
its upper surface being much smaller than that attached to its lower
surface.
3. The =homocercal=, in which the tail though externally symmetrical,
so far resembling the diphycercal type, is internally really
heterocercal, the great majority of the radiale or of the fin-rays
being attached to the lower surface of the axis.
The cranium in the simplest cases (e.g. Selachii) forms a
cartilaginous box enclosing the brain and sense organs; in bony fishes
it is greatly complicated. When palatine or pterygoid bones are
present they are formed by the ossification of cartilage; in
Sauropsida and Mammalia they are laid down as membrane bones. There is
no tympanic cavity or auditory ossicle in relation to the ear.
There are two principal types of suspensorium by means of which the
jaws are attached to the cranium:--
(1) The =Autostylic=. This is the primitive condition in which the
mandibular arch articulates with the base of the cranium in front of
the hyoid and in a similar manner.
(2) The =Hyostylic=. In this case the mandibular arch becomes
connected with the hyomandibular and supported by the hyoid arch.
These terms are more fully discussed in Chapter VIII.
There is always an internal framework supporting the gills; it usually
consists of the hyoid arch and five, rarely six or seven, pairs of
branchial arches. The limbs are represented by two pairs of fins, the
pectoral and the pelvic; they are not divided into proximal, middle
and distal portions. The ribs do not unite with a median ventral
sternum, or meet in the mid-ventral line in any other way in the trunk
region.
_Order I._ ELASMOBRANCHII.
The exoskeleton is in the form of placoid scales which are sometimes
so numerous as to give the whole skin a rough surface forming
shagreen. In some cases the placoid scales are enlarged to form plates
or spines capped or coated with enamel. These spines may be imbedded
in the flesh in front of the paired or unpaired fins, or may be
attached to the tail. They are specially characteristic of the
suborder Acanthodii. The endoskeleton is cartilaginous and true bone
is never found. Much of the skeleton, especially of the vertebral
column, is however often calcified, this being especially well seen in
the anterior part of the vertebral column of Rays (Raiidae). In living
forms cartilaginous biconcave vertebrae are always well developed, but
in some extinct forms the notochord persists unconstricted. Neural and
haemal arches are however always developed; they sometimes remain
separate, sometimes fuse with the centra. Ribs are often wanting and
when present are often not separated off from the vertebrae. The
cranium is a simple cartilaginous box whose most prominent parts are
the capsules which enclose the sense organs. The skull is sometimes
immovably fixed to the vertebral column, sometimes articulates with it
by means of two condyles. There is no operculum and no representative
of the maxilla or premaxillae. The teeth are very variable. Large
pectoral and pelvic fins always occur.
The Elasmobranchii may be divided into four suborders:--
(1) Ichthyotomi.
(2) Pleuropterygii.
(3) Selachii.
(4) Acanthodii.
_Suborder_ (1). ICHTHYOTOMI[30].
The members of this suborder range from the Devonian to the Permian
and so have long been extinct.
The endoskeletal cartilage has granular calcifications evenly
distributed throughout it. The notochord is unconstricted, but the
neural and haemal arches are well-developed, and the neural spines are
long and slender. There is a continuous dorsal fin with separate
basalia and radiale. The tail is diphycercal, and the pectoral fins
are typical archipterygia[31]. The pelvic fins of the male are
prolonged to form claspers.
The best known of these primitive Elasmobranchs are the
Pleuracanthidae.
_Suborder_ (2). PLEUROPTERYGII.
This suborder was formed for the reception of _Cladoselache_, an
Elasmobranch found in the Lower Carboniferous of Ohio[32].
The exoskeleton is in the form of small, thickly-studded dermal
denticles. The vertebral centra are unossified, and the tail is
strongly heterocercal. There were certainly five, perhaps seven gill
slits, and the suspensorium is apparently hyostylic. The paired fins
are, according to the view which derives them by concentration from
continuous lateral folds, the most primitive known (see p. 129) and
claspers are absent.
_Suborder_ (3). SELACHII.
Cartilaginous or partially calcified biconcave vertebrae are always
well developed; they constrict the notochord intervertebrally. The
neural and haemal arches and spines are stout and intercalary
cartilages (interdorsalia) are present. The tail is heterocercal, but
in some cases (_Squatina_) approaches the diphycercal condition. In
most cases the suspensorium is hyostylic, the jaws being attached to
the cranium by means of the hyomandibular, and the palato-pterygo
quadrate bar not being fused to the cranium. There are generally five
pairs of branchial arches, and gill rays are borne on the posterior
surface of the hyoid arch, and on both the anterior and posterior
surfaces of the first four branchial arches. The Notidanidae differ
from most Selachians in two respects, first as regards the
suspensorium,--Meckel's cartilage articulating directly with the
palato-pterygo-quadrate bar, and not being connected with the hyoid
arch; and secondly as regards the number of branchial arches,--six
pairs occurring in _Hexanchus_ and seven in _Heptanchus_.
The pectoral fins are without the segmented axis of the
archipterygium. In most cases they are sharply marked off from the
body and lie almost at right angles to it; but in the Rays they have
the form of lateral expansions in the same plane as the body, from
which they are not sharply marked off. The pelvic fins in the male
bear long grooved cartilaginous rods which are accessory copulatory
organs or claspers.
There are two principal groups of Selachii, the Squalidae or Sharks
and Dogfish, and the Batoidei or Skates and Rays. The Squalidae have
the shape of ordinary fish, the pectoral fins are vertically placed
and the body ends in a powerful heterocercal tail. The Batoidei have
flattened bodies owing to the great size and horizontal position of
the pectoral fins. The tail is long and thin and is often armed with
spines. The teeth in Selachii differ much in character in the
different forms, and are always arranged in numerous rows. They are
generally pointed and triangular or conical in the Squalidae, while in
the Batoidei they are often broad and flattened.
_Suborder_ (4). ACANTHODII.
The fishes included in this group are all extinct and in some respects
are intermediate between Elasmobranchii and Ganoidei. The body is
elongated and closely covered with small scales consisting of dentine
enamelled at the surface. The notochord is persistent and the
calcification of the endoskeletal cartilage is only superficial. The
tail is heterocercal. The jaws bear small conical teeth, or in some
cases are toothless. The skeleton of all the fins differs from that of
modern Elasmobranchs in having the cartilaginous radiale much reduced,
and the fins are nearly always each provided with an anterior spine,
which except in the case of the pectoral fins is merely inserted
between the muscles. These spines are really enormous dermal fin-rays;
the pectoral fin-spine is articulated to the pectoral girdle.
The suborder includes many well-known extinct forms like _Acanthodes_
and _Diplacanthus_; it ranges from the Devonian to the Permian.
PISCES, HOLOCEPHALI.
_Order II._ HOLOCEPHALI.
This order includes a single suborder only.
_Suborder._ CHIMAEROIDEI.
[Illustration FIG. 5. SKULL OF A MALE _Chimaera monstrosa_ (after
HUBRECHT).
1. nasal capsule.
2. cartilaginous appendage to the fronto-nasal region.
3. erectile appendage.
4. foramen by which the ophthalmic nerves leave the orbit.
5. foramen by which the ophthalmic branch of the Vth nerve enters
the orbit.
6. auditory capsule.
7. interorbital septum.
8. mandible articulating with an outgrowth from the posterior part
of the palato-pterygo-quadrate.
9. teeth.
10. labial cartilage.
II. III. V. VII. IX. X. foramina for the passage of cranial nerves.]
These singular fish have the skin smooth and in living forms almost or
quite scaleless. The palato-pterygo-quadrate bar and hyomandibular are
fused to the cranium, and Meckel's cartilage articulates directly with
the part corresponding to the quadrate. The skull is distinctly
articulated with the spinal column, the notochord is persistent and
unconstricted, and the skeletogenous layer shows no trace of metameric
segmentation, though in the neural arches this segmentation is
readily traceable. The neural arches of the first few vertebrae are
fused together and completely surround the notochord, while they do
not in other parts of the body. The tail is diphycercal. Of the living
genera, in _Callorhynchus_ there is no trace of calcification in the
skeletogenous layer, while in _Chimaera_ rings of calcification are
found, there being three to five for each vertebra as indicated by the
foramina for the exit of the spinal nerves. The pelvic fins are
produced into claspers. Besides the living genera _Chimaera_,
_Harriotta_ and _Callorhynchus_ a fair number of fossil forms are
known, e.g. _Ischyodus_.
_Order_ III. GANOIDEI.
The fishes included under the term Ganoidei form a very heterogeneous
group, some of which closely approach the Dipnoi, others the
Elasmobranchii, others the Teleostei. The great majority of them are
extinct, only eight living genera being known; these are all
inhabitants of the northern hemisphere, and with the exception of
_Acipenser_, which is both fluviatile and marine, are entirely
confined to fresh water.
The following is a list of the living genera of Ganoids with their
respective habitats:--
_Acipenser._ Rivers and seas of the northern hemisphere.
_Scaphirhynchus._ Mississippi and rivers of Central Asia.
_Polyodon_ (_Spatularia_). Mississippi.
_Psephurus._ Yan-tse-kiang, and Hoangho.
_Polypterus._ Rivers of tropical Africa.
_Calamoichthys._ Some rivers of West Africa.
_Lepidosteus._ Freshwaters of Central and North America and
Cuba.
_Amia._ Rivers of Carolina.
The exoskeleton is very variable, thus the body may be:--
(_a_) Naked or with minute stellate ossifications as in the
Polyodontidae. (_b_) Partially covered with large detached bony
plates as in _Scaphirhynchus_ and _Acipenser_. (_c_) Entirely covered
with rhomboidal ganoid scales as in _Lepidosteus_, _Polypterus_,
_Palaeoniscus_ and many extinct forms. (_d_) Covered with rounded
scales shaped like the cycloid scales of Teleosteans as in _Amia_.
(_e_) Having the trunk and part of the tail covered with rhomboidal
scales, and the remainder of the tail with rounded scales as in
_Trissolepis_.
The teeth also are very variable. The endoskeleton shows every stage
of transition from an almost entirely cartilaginous state as in
_Acipenser_ to a purely bony state as in _Lepidosteus_. Sometimes, as
in _Acipenser_, the notochord persists, and its sheath is unsegmented;
sometimes, as in _Lepidosteus_, there are fully formed vertebrae. The
tail may be heterocercal, as in _Acipenser_, or diphycercal as in
_Polypterus_. The cartilaginous cranium is always covered with
external membrane bone to a greater or less extent, and the
suspensorium is markedly hyostylic. The pectoral girdle is formed of
two parts, one endoskeletal and cartilaginous, corresponding with the
pectoral girdle of Elasmobranchs, and one exoskeletal and formed of
membrane bones, corresponding with the clavicular bones of
Teleosteans. The pelvic fins are always abdominal. The fins often, as
in _Polypterus_, have spines (fulcra) attached to their anterior
borders.
The order Ganoidei may be divided into three suborders:
(1) CHONDROSTEI. Living genera _Acipenser_, _Scaphirhynchus_,
_Polyodon_ and _Psephurus_.
(2) CROSSOPTERYGII. Living genera _Polypterus_ and
_Calamoichthys_.
(3) HOLOSTEI. Living genera _Lepidosteus_ and _Amia_.
_Suborder_ (1). CHONDROSTEI.
The skull is immovably fixed to the vertebral column. By far the
greater part of the skeleton is cartilaginous. The notochord is
persistent and unconstricted, its sheath is membranous, but
cartilaginous neural and haemal arches are developed. Intercalary
pieces (interdorsalia) occur between the neural arches, and similar
pieces (interventralia) between the haemal arches. The cranium is
covered with membrane bone, and teeth are but slightly developed. The
tail is heterocercal. Gill rays occur on the hyoid arch, and the gills
are protected by a bony operculum attached to the hyomandibular. The
skin (1) may be almost or quite naked, (2) may carry bony plates
arranged in rows, or may be covered (3) with rhomboidal scales, or (4)
partly with rhomboidal, partly with cycloidal scales.
_Suborder_ (2). CROSSOPTERYGII.
The exoskeleton has the form of cycloidal or rhomboidal scales. The
condition of the vertebral column differs in the different genera.
Sometimes, as in _Polypterus_, there are well-developed ossified
vertebrae; sometimes, as in many extinct forms, the notochord persists
and is unconstricted. The tail may be diphycercal or heterocercal. The
pectoral and sometimes the pelvic fins consist of an endoskeletal axis
bearing a fringe of dermal rays.
_Suborder_ (3). HOLOSTEI.
The exoskeleton has the form of cycloidal or rhomboidal scales. The
notochord is constricted and its sheath is segmented and ossified,
forming distinct vertebrae, which are generally biconcave, sometimes
opisthocoelous (_Lepidosteus_). The cartilaginous cranium is largely
replaced by bone, and in connection with it we find not only membrane
bone, but cartilage bone, as the basi-occipital, exoccipitals, and
pro-otic are ossified. The tail is heterocercal. The suspensorium
resembles that of Teleosteans, consisting of a proximal ossification,
the hyomandibular, which is movably articulated to the skull and a
distal ossification, the symplectic. The two are separated by some
unossified cartilage. The cartilaginous upper and lower jaws are to a
great extent surrounded and replaced by a series of membrane bones.
_Order_ IV. TELEOSTEI.
The exoskeleton is sometimes absent but generally consists of
overlapping cycloid or ctenoid scales. Bony plates are sometimes
present, as in the Siluridae, or the body may be encased in a complete
armour of calcified plates, as in _Ostracion_. Enamel is however never
present, and the plates are entirely mesodermal. The skeleton is bony,
but in the skull much cartilage generally remains. The vertebral
centra are usually deeply biconcave, and the tail is of the masked
heterocercal type distinguished as _homocercal_. In the skull the
occipital region is always completely ossified, while the sphenoidal
region is generally less ossified. The skull has usually a very large
number of membrane bones developed in connection with it. The teeth
vary much in character in the different members of the order, but are
as a rule numerous and pointed, and are ankylosed to the bone. The
suspensorium is hyostylic and the jaws have much the same arrangement
as in the Holostei. There are five pairs of branchial arches, of which
all except the last bear gill rays. A series of dermal opercular bones
is developed in connection with these arches. The pectoral girdle
consists almost entirely of dermal clavicular bones. The pelvic girdle
has disappeared, its place being taken by the enlarged and ossified
dermal fin-rays of the pelvic fins.
The group includes the vast majority of living fish (see p. 33).
_Order_ V. DIPNOI.
The exoskeleton is of two types; dermal bones are largely developed in
the head region, while the tail and posterior part of the body may be
naked or may be covered with overlapping scales. The cranium remains
chiefly cartilaginous, the palato-pterygo-quadrate bar is fused with
the cranium, and the suspensorium is autostylic. The gill clefts are
feebly developed and open into a cavity covered by an operculum. The
notochord is persistent and unconstricted, and the limbs are
archipterygia. The pelvic fins are without claspers.
_Suborder_ (1). SIRENOIDEI[33].
The head has well developed membrane bones. The trunk is covered with
overlapping scales and bears no bony plates. Three pairs of teeth are
present, two in the upper and one in the lower jaw, the two principal
pairs of teeth are borne on the palato-pterygoids and splenials, while
the third pair are found in the vomerine region. The tail is
diphycercal in living forms. In the extinct Dipteridae it is
heterocercal. The pectoral girdle includes both membrane and cartilage
bones. The pelvic girdle consists of a single bilaterally symmetrical
piece of cartilage.
This suborder is represented by the living genera _Ceratodus_,
_Protopterus_ and _Lepidosiren_, and among extinct forms by the
Dipteridae and others.
_Suborder_ (2). ARTHRODIRA.
Bony plates are developed not only on the head but also on the
anterior part of the trunk, where they consist of a dorsal, a ventral,
and a pair of lateral plates which articulate with the cranial shield.
The posterior part of the trunk is naked. The tail is diphycercal. The
jaws are shear-like, and their margins are usually provided with
pointed teeth whose bases fuse with the tissue of the jaw and
constitute dental plates. There seem to have been three pairs of these
plates, arranged as in the Sirenoidei, the principal ones in the upper
jaw being borne on the palato-pterygoids. Small pelvic fins are
present, but pectoral fins are unknown.
The Arthrodira occur chiefly in beds of Devonian and Carboniferous
age. Two of the best known genera are _Coccosteus_ from the European
Devonian and _Dinichthys_, a large predatory form from the lower
Carboniferous of Ohio.
FOOTNOTES:
[29] See p. 79.
[30] For this and other groups of extinct fish see A. Smith Woodward,
_Catalogue of Fossil Fish in the British Museum_, Parts I.-III.
London, 1889-95.
[31] See p. 127.
[32] See B. Dean, _J. Morphol._ vol. IX. pp. 87-114, 1894, and _Nat.
Sci._ vol. VIII. p. 245, 1896.
[33] A. Günther, _Phil. Trans._ vol. 161, Part II. 1871, p. 511. T.H.
Huxley, "On Ceratodus and the classification of fishes," _P.Z.S._
1876, p. 24.