GENERAL ACCOUNT OF THE SKELETON IN FISHES[39].
EXOSKELETON.
The most primitive type of exoskeleton is that found in Elasmobranchs
and formed of =placoid= scales; these are tooth-like structures
consisting of dentine and bone capped with enamel, and have been
already described (p. 4). In most Elasmobranchs they are small and
their distribution is fairly uniform, but in the Thornback skate,
_Raia clavata_, they have the form of larger, more scattered spines.
In adult Holocephali and in _Polyodon_ and _Torpedo_ there is no
exoskeleton, in young Holocephali, however, there are a few small
dorsal ossifications.
The plates or scales of many Ganoids may have been formed by the
gradual fusion of elements similar to these placoid scales, and often
bear a number of little tooth-like processes. In _Lepidosteus_,
_Polypterus_, and many extinct species, these _ganoid_ scales, which
are rhomboidal in form and united to one another by a peg and socket
articulation, enclose the body in a complete armour. In _Trissolepis_
part of the tail is covered by rhomboidal scales, while rounded scales
cover the trunk and remainder of the tail. _Acipenser_ and
_Scaphirhynchus_ have large dermal bony plates which are not
rhomboidal in shape and do not cover the whole body. In _Acipenser_ a
single row extends along the middle of the back and two along each
side.
The majority of Teleosteans have thin flattened scales which differ
from those of Ganoids in being entirely mesodermal in origin,
containing no enamel. There are two principal types of Teleostean
scales, the cycloid and ctenoid. A =cycloid= scale is a flat thin
scale with concentric markings and an entire posterior margin. A
=ctenoid= scale differs in having its posterior margin pectinate. The
Dipnoi have overlapping cycloid scales. The rounded scales of _Amia_
and of many fossil ganoids such as _Holoptychius_ are shaped like
cycloid scales, but differ from them in being more or less coated with
enamel. In Eels and some other Teleosteans the scales are completely
degenerate and have almost disappeared. Some Teleosteans, like _Diodon
hystrix_, have scales with triradiate roots from which arise long
sharp spines directed backwards. These scales, which resemble teeth,
contain no enamel; they become erect when the fish inflates its body
into a globular form. Many Siluroids have dermal armour in the form of
large bony plates which are confined to the anterior part of the body.
In _Ostracion_ the whole body is covered by hexagonal plates, closely
united together.
The =fin-rays= are structures of dermal origin which entirely or
partially support the unpaired fins, and assist the bony or
cartilaginous endoskeleton in the support of the paired fins.
In Elasmobranchs, Dipnoi, and Chondrosteous ganoids the skeletons of
the fins are, as a rule, about half of exoskeletal, half of
endoskeletal origin, the proximal and inner portion being
cartilaginous and endoskeletal, the distal and outer portion being
exoskeletal, and consisting of horny or of more or less calcified
fin-rays. In bony Ganoids and Teleosteans the endoskeletal parts are
greatly reduced and the fins come to consist mainly of the fin-rays,
which are ossified and frequently become flattened at their distal
ends.
The fin-rays of the ventral part of the caudal fin are carried by the
haemal arches; those of the dorsal and anal fins and of the dorsal
part of the caudal fin generally by interspinous bones, which in adult
Teleosteans alternate with the neural and haemal spines. In Dipnoi
these interspinous bones articulate with the neural and haemal spines.
In many Siluroids the anterior rays of the dorsal and pectoral fins
are developed into large spines which often articulate with the
endoskeleton, or are sometimes fused with the dermal armour plates.
Similar spines may occur in Ganoids in front of both the dorsal and
anal fins. _Polypterus_ has a small spine or _fulcrum_ in front of
each segment of the dorsal fin. Such spines are often found
fossilised, and are known as _ichthyodorulites_.
Similar spines are found in many Elasmobranchs, but they are simply
inserted in the flesh, not articulated to the endoskeleton. They also
differ from the spines of Teleosteans and Ganoids in the fact that
they are covered with enamel, and often have their edges serrated like
teeth. In the extinct Acanthodii they generally occur in front of all
the fins, paired and unpaired.
In _Trygon_, the Sting-ray, the tail bears a serrated spine which is
used for purposes of offence and defence. Many ichthyodorulites may
have been spines of this nature fixed to the tail, rather than spines
situated in front of the fins. The spines, which are always found in
front of the dorsal fin in Holocephali, agree with those of
Elasmobranchs in containing enamel, and with those of Teleosteans in
being articulated to the endoskeleton.
TEETH.
The teeth of fish[40] are subject to a very large amount of variation,
perhaps to more variation than are those of any other class of
animals. Sometimes, as in adult Sturgeons, they are entirely absent,
sometimes they are found on all the bones of the mouth, and also on
the hyoid and branchial arches. The teeth are all originally developed
in the mucous membrane of the mouth, but they afterwards generally
become attached to firmer structures, especially to the jaws. In
Elasmobranchs, however, they are generally simply imbedded in the
tough fibrous integument of the mouth. Their attachment to the jaws
may take place in three different ways.
[Illustration FIG. 14. DIAGRAM OF A SECTION THROUGH THE JAW OF A SHARK
(_Odontaspis americanus_) showing the succession of teeth (Brit. Mus.
from specimen and diagram).
1. teeth in use.
2. teeth in reserve.
3. skin.
4. cartilage of the jaw.
5. encrusting calcification of cartilage.
6. connective tissue.
7. mucous membrane of the mouth.]
(1) By an elastic hinge-joint, as in the Angler (_Lophius_), and the
Pike (_Esox lucius_). In the Angler the tooth is held by a fibrous
band attaching its posterior end to the subjacent bone, in the Pike by
uncalcified elastic rods in the pulp cavity.
(2) By ankylosis, i.e. by the complete union of the calcified tooth
substance with the subjacent bone. This is the commonest method among
fish.
(3) By implantation in sockets. This method is not very common among
fish. The teeth are sometimes, as in _Lepidosteus_, ankylosed to the
base of the socket. In this genus there is along each ramus of the
mandible a median row of large teeth placed in perfect sockets, and
two irregular lateral rows of small teeth ankylosed to the jaw.
Dentine, enamel and cement are all represented in the teeth of fishes,
but the enamel is generally very thin, and cement is but rarely
developed. Dentine forms the main bulk of the teeth; it is sometimes
of the normal type, but generally differs from that in higher
vertebrates in being vascular, and is known as _vasodentine_. A third
type occurs, known as _osteodentine_; it is traversed by canals
occupied by marrow, and is closely allied to bone.
[Illustration FIG. 15. PART OF THE LOWER JAW OF A SHARK (_Galeus_)
(from OWEN after ANDRÉ).
1. teeth in use.
2. reserve teeth folded back.
3. part of the caudal spine of a Sting-ray (_Trygon_) which has
pierced the jaw and affected the growth of the teeth.]
The teeth are generally continually renewed throughout life, but
sometimes one set persists.
The teeth of Selachii are fundamentally identical with placoid
scales. They are developed from a layer of dental germs which occurs
all over the surface of the skin, except in the region of the lips. At
this point the layer of tooth-producing germs extends back into the
mouth, being projected by a fold of the mucous membrane (fig. 14, 7).
Here new teeth are successively formed, and as they grow each is
gradually brought into a position to take the place of its predecessor
by the shifting outwards of the gum over the jaw. Owing to this
arrangement sharks have practically an unlimited supply of teeth
(figs. 14 and 15).
Two principal types of teeth are found in ELASMOBRANCHS. In Sharks and
Dogfish, on the one hand, the teeth are very numerous, simple, and
sharp-pointed, and are with or without serrations and lateral cusps.
Many Rays and fossil Elasmobranchs, on the other hand, have broad
flattened teeth adapted for crushing shells. Intermediate conditions
occur between these two extremes. Thus in _Cestracion_ and many
extinct sharks, such as _Acrodus_, while the median teeth are sharp,
the lateral teeth are more or less flattened and adapted for crushing.
In various species belonging to the genus _Raia_ the teeth of the male
are sharp, while those of the female are blunt. A very specialised
dentition is met with in the Eagle-rays (Myliobatidae), in which the
jaws are armed with flattened angular tooth-plates, arranged in seven
rows, forming a compact pavement; the plates of the middle row are
very wide and rectangular, those of the other rows are much smaller
and hexagonal. Lastly, in _Cochliodus_ the individual crushing teeth
are fused, forming two pairs of spirally-coiled dental plates on each
side of each jaw. _Pristis_, the Saw-fish, has a long flat
cartilaginous snout, bearing a double row of persistently-growing
teeth planted in sockets along its sides. Each tooth consists of a
number of parallel dentinal columns, united at the base, but elsewhere
distinct.
In the HOLOCEPHALI--_Chimaera_, _Hariotta_ and _Callorhynchus_--only
three pairs of teeth or dental plates occur, two pairs in the upper
jaw, one in the lower. These structures persist throughout life and
grow continuously. The upper tooth structures are attached
respectively to the ethmoid or vomerine region of the skull, and to
the palato-pterygoids. The vomerine teeth are small, while those
attached to the mandible and the palato-pterygoid region are large and
bear several roughened ridges adapted for grinding food. The teeth of
the two opposite sides of the jaw meet in a median symphysis. The
teeth of _Chimaera_ are more adapted for cutting, those of
_Callorhynchus_ for crushing. Many extinct forms are known, some of
whose teeth are intermediate in structure between those of _Chimaera_
and _Callorhynchus_.
The teeth of GANOIDS are also extremely variable. Among living forms,
the Holostei are more richly provided with teeth than are any other
fishes, as they may occur on the premaxillae, maxillae, palatines,
pterygoids, parasphenoid, vomers, dentaries, and splenials. Among the
Chondrostei, on the other hand, the adult Acipenseridae are toothless;
small teeth however occur in the larval sturgeon, and in _Polyodon_
many small teeth are found attached merely to the mucous membrane of
the jaws. Many fossil Ganoids have numerous flattened or knob-like
teeth, borne on the maxillae, palatines, vomers and dentaries. Others
have a distinctly heterodont dentition. Thus in _Lepidotus_ the
premaxillae bear chisel-like teeth, while knob-like teeth occur on the
maxillae, palatines and vomers. In _Rhizodus_ all the teeth are
pointed, but while the majority are small a few very large ones are
interspersed.
In TELEOSTEANS, too, the teeth are eminently variable both in form and
mode of arrangement. They may be simple and isolated, or compound, and
may be borne on almost any of the bones bounding the mouth cavity, and
also as in the Pike, on the hyoid and branchial arches. The splenial
however never bears teeth and the pterygoid and parasphenoid only
rarely, thus differing from the arrangement in the Holostei.
The isolated teeth are generally conical in form and are ankylosed to
the bone that bears them. Such teeth are, with a few exceptions such
as _Balistes_, not imbedded in sockets nor replaced vertically.
In some fish beak-like structures occur, formed partly of teeth,
partly of the underlying jaw bones. These beaks are of two kinds: (1)
In _Scarus_, the parrot fish, the premaxillae and dentaries bear
numerous small, separately developed teeth, which are closely packed
together and attached by their proximal ends to the bone, while their
distal ends form a mosaic. Not only the teeth but the jaws which bear
them are gradually worn away at the margins, while both grow
continuously along their attached edge. (2) In Gymnodonts, e.g.
_Diodon_, the beaks are formed by the coalescence of broad calcified
horizontal plates, which when young are free and separated from one
another by a considerable interval.
In some Teleosteans the differentiation of the teeth into biting teeth
and crushing teeth is as complete as in _Lepidosteus_. Thus in the
Wrasse (_Labrus_), the jaws bear conical slightly recurved teeth
arranged in one or two rows, with some of the anterior ones much
larger than the rest. The bones of the palate are toothless, while
both upper and lower pharyngeal bones are paved with knob-like
crushing teeth; such pharyngeal teeth occur also in the Carp but are
attached only to the lower pharyngeal bone, the jaw bones proper being
toothless.
In DIPNOI the arrangement of the teeth is very similar to that in
Holocephali. The mandible bears a single pair of grinding teeth
attached to the splenials, and a corresponding pair occur on the
palato-pterygoids. In front of these there are a pair of small conical
vomerine teeth loosely attached to the ethmoid cartilage. The
palato-pterygoid teeth of _Ceratodus_ are roughly semicircular in
shape with a smooth convex inner border, and an outer border bearing a
number of strongly marked ridges. The teeth of the extinct Dipteridae
resemble those of _Ceratodus_ but are more complicated.
ENDOSKELETON.
SPINAL COLUMN[41].
The spinal column of fishes is divisible into only two regions, a
caudal region in which the haemal arches or ribs meet one another
ventrally, and a precaudal region in which they do not meet.
The various modifications of the spinal column in fishes can be best
understood by comparing them with the arrangement in the simplest type
known, namely _Amphioxus_. In _Amphioxus_ the notochord is immediately
surrounded by a structureless cuticular layer, the _chordal sheath_.
Outside this is the _skeletogenous layer_, which in addition to
surrounding the notochord and chordal sheath embraces the nerve cord
dorsally, and laterally sends out septa forming the _myomeres_.
The CARTILAGINOUS GANOIDS[42] _Acipenser_, _Polyodon_ and
_Scaphirhynchus_ are the simplest fishes as regards their spinal
column. The notochord remains permanently unconstricted and is
enclosed in a chordal sheath, external to which is the skeletogenous
layer. In this layer the development of cartilaginous elements has
taken place. In connection with each _neuromere_, or segment as
determined by the points of exit of the spinal nerves, there are
developed two pairs of ventral cartilages, the ventral arches
(basiventralia) and intercalary pieces (interventralia); and at least
two pairs of dorsal pieces, the neural arches (basidorsalia) and
intercalary pieces (interdorsalia). The lateral parts of the
skeletogenous layer do not become converted into cartilage, so there
are no traces of vertebral centra. The ventral or haemal arches meet
one another ventrally and send out processes to protect the ventral
vessels. The neural arches do not meet, but are united by a
longitudinal elastic band.
In Cartilaginous ganoids the only indications of metameric
segmentation are found in the neural and haemal arches. The case is
somewhat similar with the Holocephali and Dipnoi.
In the HOLOCEPHALI the notochord grows persistently throughout life,
and is of uniform diameter throughout the whole body except in the
cervical region and in the gradually tapering tail. The chordal sheath
is very thick and includes a well-marked zone of calcification which
separates an outer zone of hyaline cartilage from an inner zone. There
are also a number of cartilaginous pieces derived from the
skeletogenous layer which are arranged in two series, a dorsal series
forming the neural arches and a ventral series forming the haemal
arches. These do not, except in the cervical region, meet one another
laterally round the notochord and form centra. To each neuromere there
occur a pair of basidorsals, a pair of interdorsals, and one or two
supradorsals. In the tail the arrangement is irregular.
In the DIPNOI as in the Holocephali the notochord grows persistently
and uniformly, and the chordal sheath is thick and cartilaginous
though there are no metamerically arranged centra. The neural and
haemal arches and spines are cartilaginous and interbasalia
(intercalary pieces) are present. The basidorsalia and basiventralia
do not in _Ceratodus_ meet round the notochord and enclose it except
in the anterior part of the cervical and posterior part of the caudal
region.
In ELASMOBRANCHII the chordal sheath is weak and the skeletogenous
layer strong. Biconcave cartilaginous vertebrae are developed, and as
is the case in most fishes, constrict the notochord _vertebrally_.
Two distinct types of vertebral column can be distinguished in
Elasmobranchs[43]:
1. In many extinct forms and in the living Notidanidae, _Cestracion_,
and _Squatina_, the dorsal and ventral arches do not meet one another
laterally round the centrum, and consequently readily come away from
it.
2. In most living Elasmobranchs the arches meet laterally round the
centrum.
The vertebrae are never ossified but endochondral calcification nearly
always takes place, though it very rarely reaches the outer surface of
the vertebrae. Elasmobranchs are sometimes subdivided into three
groups according to the method in which this calcification takes
place:
1. =Cyclospondyli= (_Scymnus_, _Acanthias_), in which the calcified
matter is deposited as one ring in each vertebra.
2. =Tectospondyli= (_Squatina_, _Raia_, _Trygon_), in which there are
several concentric rings of calcification.
3. =Asterospondyli= (Notidanidae, _Scyllium_, _Cestracion_), in which
the calcified material instead of forming one simple ring, extends out
in a more or less star-shaped manner.
In _Heptanchus_ the length of the vertebral centra in the middle of
the trunk is double that in the anterior and posterior portions, and
as the length of the arches does not vary, the long centra carry more
of them than do the short centra.
In many Rays the skull articulates with the vertebral column by
distinct occipital condyles.
In BONY GANOIDS the skeletogenous layer becomes calcified
ectochondrally in such a way that the notochord is pinched in at
intervals, and distinct vertebrae are produced. Ossification of the
calcified cartilage rapidly follows. In _Amia_ the vertebrae are
biconcave, in _Lepidosteus_ they are opisthocoelous, cup and ball
joints being developed between the vertebrae in a manner unique among
fishes. The notochord entirely disappears in the adult _Lepidosteus_,
but at one stage in larval life it is expanded vertebrally and
constricted intervertebrally in the manner usual in the higher
vertebrata, but unknown elsewhere among fishes.
The tail of _Amia_ is remarkable from the fact that as a rule to each
neuromere, as determined by the exit of the spinal nerves, there are
two centra, a posterior one which bears nothing, and an anterior one
which bears the neural and haemal arches, these being throughout the
vertebral column connected with the centra by cartilaginous discs.
In most TELEOSTEANS but not in the Plectognathi the neural arches are
continuous with the centra, which are nearly always deeply biconcave.
In some cases many of the anterior vertebrae are ankylosed together
and to the skull. The vertebrae often articulate with one another by
means of obliquely placed flattened surfaces, the zygapophyses. The
centrum in early stages of development is partially cartilaginous, but
the neural arches and spines in the trunk at any rate, pass directly
from the membranous to the osseous condition.
FINS.
The most primitive fins are undoubtedly the unpaired ones, which
probably originally arose as ridges or folds of skin along the
mid-dorsal line of the body, and passed thence round the posterior end
on to the ventral surface, partially corresponding in position and
function to the keel of a ship.
In long 'fish' which pass through the water with an undulating motion
such simple continuous fins may be the only ones found, as in
_Myxine_. To support these median fins skeletal structures came to be
developed; these show two very distinct forms, viz. cartilaginous
endoskeletal pieces, the _radiale_, and horny exoskeletal fibres, the
_fin-rays_. Mechanical reasons caused the fin to become concentrated
at certain points and reduced at intervening regions. Thus a terminal
caudal fin arose and became the chief organ of propulsion, and the
dorsal and ventral fins became specialised to act as balancing organs.
In some of the earlier Elasmobranchs, the Pleuracanthidae, the
endoskeletal cartilaginous radiale are directly continuous with
outgrowths from the dorsal and ventral arches of the vertebrae, and
form the main part of the fin. In later types of Elasmobranchs the
horny exoskeletal fin-rays have comparatively greater prominence. In
bony fish, as has been already stated, the horny fibres are replaced
by bony rays of dermal origin, and at the same time complete reduction
and disappearance of the cartilaginous radiale takes place.
THE CAUDAL FIN.
The caudal region of the spinal column in fishes is of special
importance. It is distinctly marked off from the rest of the spinal
column by the fact that the ventral or haemal arches meet one another
and are commonly prolonged into spines, while in the trunk region they
do not meet but commonly diverge from one another.
In some fish the terminal part of the caudal region of the spinal
column retains the same direction as the rest of the spinal column.
The blade of the caudal fin is then divided into two nearly equal
portions, and is said to be =diphycercal=. This condition is generally
regarded as the most primitive one; it occurs in the Ichthyotomi,
Holocephali, all living Dipnoi, _Polypterus_ and some extinct
Crossopterygii, and a few Selachii and Teleostei. It occurs also in
deep-sea fish belonging to almost every group, and under these
conditions obviously cannot be regarded as primitive, but must be
looked on as a feature induced by the peculiar conditions of life.
In the great majority of fish the terminal part of the caudal region
of the spinal column is bent dorsalwards, and the part of the blade of
the caudal fin which arises on the dorsal surface is much smaller than
is that arising on the ventral surface. Such a fin is said to be
=heterocercal=.
Strictly speaking all fish whose tails are not diphycercal have
heterocercal tails, but the term is commonly applied to two-bladed
tails in which the spinal column forms a definite axis running through
the dorsal blade, while the ventral blade is enlarged and generally
forms the functional part of the tail. Such heterocercal tails are
found in nearly all Elasmobranchii, together with the living
cartilaginous Ganoidei, and many extinct forms belonging to the same
order; _Lepidosteus_, _Amia_, and the Dipteridae among Dipnoi, have
tails which, though obviously heterocercal, are not two-bladed.
The vast majority of the Teleostei and some extinct Ganoidei have
heterocercal tails of the modified type to which the term =homocercal=
is applied. The hypural bones which support the lower half of the tail
fin become much enlarged, and frequently unite to form a wedge-shaped
bone which becomes ankylosed to the last ossified vertebral centrum.
The fin-rays then become arranged in such a way as to produce a
secondary appearance of symmetry. Some homocercal fish such as the
Perch have the end of the notochord protected by a calcified or
completely ossified sheath, the =urostyle=, to which several neural
and haemal arches may be attached, and which becomes united with the
centrum of the last vertebra; in others such as the Salmon the end of
the notochord is protected only by laterally placed bony plates.
THE SKULL.
It is often impossible to draw a hard and fast line between the
cranium and the vertebral column. This is the case for instance in
_Acipenser_ (fig. 18, 16) among Chondrostei, in _Amia_ among Holostei,
and in _Ceratodus_ and _Protopterus_ among Dipnoi. The occipital
region of the skull in _Amia_ is clearly formed of three cervical
vertebrae whose centra have become absorbed into the cranium, while
the neural arches and spines are still distinguishable.
The simplest type of cranium is that found in ELASMOBRANCHS: it
consists of a simple cartilaginous box, which is generally immovably
fixed to the vertebral column, though in some forms, like _Scymnus_
and _Galeus_, a joint is indicated, and in others, such as the
Rays, one is fairly well developed. The cranium in Elasmobranchs
is never bony, though the cartilage is sometimes calcified. It is
drawn out laterally into an antorbital process in front of the eye,
and a postorbital process behind it. The nasal capsules are always
cartilaginous, and the eye, as a general rule, has a cartilaginous
sclerotic investment. The cranium is often prolonged in front into
a rostrum which is enormously developed in _Pristis_ and some Rays.
The cartilaginous roof of the cranium is rendered incomplete by the
presence of a large hole, the anterior fontanelle.
[Illustration FIG. 16. A. SKULL OF _Notidanus_ × 1/2 (Brit. Mus.). B.
SKULL OF _Cestracion_ × 1/3 (after GEGENBAUR). In neither case are the
branchial arches shown.
1. rostrum.
2. olfactory capsule.
3. ethmo-palatine process.
4. palatine portion of palato-pterygo-quadrate bar.
5. quadrate portion of bar.
6. Meckel's cartilage.
7. teeth.
8. labial cartilage.
9. hyomandibular.
10. postorbital process.
II. optic foramen.]
Two pairs of labial cartilages (fig. 16, B, 8) are often present. They
lie imbedded in the cheeks outside the anterior region of the jaws,
and are specially large in _Squatina_.
As regards the visceral arches[44] the simplest and most primitive
condition of the jaws is that of the Notidanidae, in which the
mandibular and hyoid arches are entirely separate. In these primitive
fishes the palato-pterygo-quadrate bar articulates with the
postorbital process (fig. 16, 10), while further forwards it is united
to the cranium by the ethmo-palatine ligament. The hyoid arch is small
and is broadly overlapped by the mandibular arch. The term
=autostylic= is used to describe this condition of the suspensorium.
From this condition we pass in the one direction to that of
_Cestracion_ (fig. 16, B), in which the whole of the palato-pterygo
quadrate bar has become bound to the cranium, and in the other to that
of _Scyllium_. In _Scyllium_ (fig. 6), while the ethmo-palatine
ligament is retained, the postorbital articulation of the palato
pterygo-quadrate has been given up, so that the palato-pterygo
quadrate comes to abut on the hyomandibular and is attached to it by
ligaments. The pre-spiracular ligament (fig. 16, 20) running from the
auditory capsule also assists in supporting the jaws.
Lastly we come to the purely =hyostylic= condition met with in Rays,
in which the mandibular arch is entirely supported by the
hyomandibular. In some Rays the hyoid is attached to the posterior
face of the hyomandibular near its proximal end, and may even come to
articulate with the cranium.
The =visceral arches of Elasmobranchs= may be summarised as follows:--
1. The =mandibular arch=, consisting of a much reduced dorsal portion,
the pre-spiracular ligament, and a greatly developed ventral portion
from which both upper and lower jaws are derived. The mandible
(Meckel's cartilage) is the original lower member of the mandibular
arch, and from it arises an outgrowth which forms the upper jaw or
palato-pterygo-quadrate bar. In _Scymnus_ this bears a few
branchiostegal rays.
2. The =hyoid arch=, which consists of the hyomandibular and the
hyoid, and bears branchiostegal rays on its posterior face.
[Illustration FIG. 17. DORSAL VIEW OF THE BRANCHIAL ARCHES OF
_Heptanchus_. (From GEGENBAUR).
1. basi-hyal.
2. cerato-hyal.
3. second hypo-branchial.
4. first cerato-branchial.
5. first epi-branchial.
6. first pharyngo-branchial.
7. pharyngo-branchial, common to the sixth and seventh arches.
8. basibranchial of second arch.
9. basibranchial, common to the sixth and seventh arches.]
3. The =branchial arches=, generally five in number, all of which
except the last bear branchiostegal rays. In the Notidanidae the
number of branchial arches is increased beyond the normal series,
thus in _Hexanchus_ there are six, and in _Heptanchus_ seven. There
are six also in _Chlamydoselache_ and _Protopterus_.
4. The so-called external branchial arches which are cartilaginous
rods attached to all the visceral arches. They are especially large in
_Cestracion_.
The skull in HOLOCEPHALI is entirely cartilaginous. The
palato-pterygo-quadrate bar is fixed to the cranium, and to it the
mandible articulates. There is a well-marked joint between the skull
and the spinal column.
In living Cartilaginous GANOIDS the primitive cartilaginous cranium is
very massive, and is greatly prolonged anteriorly, while posteriorly
it merges into the spinal column. Although it is mainly cartilaginous
a number of ossifications take place in the skull, and membrane bones
are now found definitely developed, especially in connection with the
roof of the cranium. In _Acipenser_ (fig. 18) the ossifications in the
cartilage include the pro-otic, which is pierced by the foramen for
the fifth nerve, the alisphenoid, orbitosphenoid, ectethmoid,
palatine, pterygoid, meso-pterygoid, hyomandibular (fig. 18, 11),
cerato-hyal, all the cerato-branchials, and the first two
epi-branchials. Most of these structures are, however, partly
cartilaginous, though they include an ossified area. The membrane
bones too of _Acipenser_ are very well developed, they include a bone
occupying the position of the supra-occipital, and form a complete
dorsal cephalic shield. Resting on the ventral surface are a vomer and
a very large parasphenoid (fig. 18, 3). There is a bony operculum
attached to the hyomandibular, and membrane bones representing
respectively the maxilla and dentary are attached to the jaws. The
suspensorium is most markedly hyostylic. The palato-pterygo-quadrate
bar has a very curious shape and is quite separate from the cranium.
It is connected to the hyomandibular by a thick symplectic ligament
containing a small bone homologous with the symplectic of
Teleosteans.
_Polyodon_ differs much from _Acipenser_, the membrane bones not being
so well developed though they cover the great cartilaginous snout.
[Illustration FIG. 18. LATERAL VIEW OF THE SKULL OF A STURGEON
(_Acipenser sturio_). Nearly all the membrane bones have been removed
(Brit. Mus.).
1. nasal cavity.
2. orbit.
3. parasphenoid.
4. vomer.
5. pterygoid.
6. maxilla. (The dotted line running from 6 passes into the mouth
cavity.)
7. dentary.
8. symplectic.
10. palatine.
11. hyomandibular.
12. pharyngo-branchial.
13. epi-branchial.
14. cerato-branchial.
15. hypo-branchial.
16. coalesced anterior vertebrae.
17. inter-hyal.
18. cerato-hyal.
19. rib.]
The skull in _Polypterus_ (Crossopterygii) shows a great advance
towards the condition met with in Teleostei. The cranium remains to a
great extent unossified, and large dorsal and ventral fontanelles
pierce its walls. It is covered by a great development of membrane
bones, paired nasals, frontals, parietals, supra- and post-temporals,
and dermo-supra-occipitals among others being present. The
palato-pterygo-quadrate bar is fused to the cranium, and in connection
with it the following paired membrane bones appear, palatine, ecto-,
meso- and meta-pterygoid, and further forwards jugal, vomer, maxilla
and premaxillae. The membrane bones developed in connection with each
ramus of the mandible are the dentary, angular, and splenial, in
addition to the cartilage bone the articular. Several large opercular
bones occur. There are also a pair of large jugular or gular plates,
and several large opercular bones.
In Bony Ganoids both cartilage bone and membrane bone is well
developed. The pro-otics and exoccipitals are well ossified, but the
supra-occipital and pterotics are not. Lateral ethmoids are developed,
and there are ossifications in the sphenoidal region which vary in
different forms. The place of the cartilaginous palato-pterygo
quadrate is taken by a series of bones, the quadrate behind and the
palatine, ecto-, meso- and meta-pterygoids in front. In _Lepidosteus_,
however, the palatine and pterygoid are membrane bones, as they are in
_Polypterus_ and the Frog. Paired maxillae, premaxillae, vomers and a
parasphenoid occur forming the upper jaw and roof of the mouth, and a
series of membrane bones are found investing the mandible and forming
the operculum.
In _Amia_[45] membrane bones are as freely developed as they are in
Teleosteans; they include on each side a squamosal, four opercular
bones, a lachrymal, a pre-orbital, one or two suborbitals, two large
postorbitals and a supratemporal; while investing the mandible,
besides the dentary, splenial, angular, and supra-angular, there is an
unpaired jugular. The articular too is double and a mento-meckelian
occurs. In _Amia_ teeth are borne on the premaxillae, maxillae, vomers,
palatines and pterygoids.
Bony Ganoids are the lowest animals in which squamosal bones are
found, and they do not occur in Teleosteans.
The suspensorium in bony Ganoids, as in the Chondrostei, is hyostylic,
and there are two ossifications in the hyomandibular cartilage, viz.
the hyomandibular, and the symplectic.
The skull of TELEOSTEI is very similar to those of _Lepidosteus_ and
_Amia_. Although the bony skull is greatly developed and very
complicated, much of the original cartilaginous cranium often
persists. Membrane bones are specially developed on the roof of the
skull where they include the parietal, frontal, and nasal bones. The
same bones are developed in connection with the upper jaw and roof of
the mouth as in bony Ganoids, but only two membrane bones occur in the
lower jaw, viz. the angular and dentary. A number of large
ossifications take place in the cartilage of the auditory capsules. In
some forms parts of the last pair of branchial arches are broadened
out and form the pharyngeal bones which bear teeth. The opercular
bones and those of the upper and lower jaws are quite comparable to
those of bony Ganoids.
A full account of the Teleostean skull has been given in the case of
the Salmon (pp. 87-96) and the Cod (pp. 96-101).
In DIPNOI the skull is mainly cartilaginous, but both cartilage- and
membrane-bone occur also. Cartilage-bone is found in the ossified
exoccipitals, while of membrane-bones _Protopterus_ has among unpaired
bones a fronto-parietal, a median ethmoid, and a parasphenoid, and
among paired bones nasals and large supra-orbitals. The skull of
_Ceratodus_ (fig. 19) has an almost complete roof of membrane bones,
including some whose homology is doubtful. The ethmo-vomerine region
is always cartilaginous, but bears small teeth. The palato-pterygo
quadrate bar is ossified and firmly united to the cranium, and the
mandible articulates directly with it (autostylic). Membrane bones are
freely developed in connection with the mandible, dentary, splenial,
and angular bones being all present. There are two opercular bones.
In the extinct Dipteridae the cranium is very completely covered with
plates of dermal bone, and the skeleton in general is more ossified
than is the case in recent Dipnoi.
Six pairs of branchial arches occur in _Protopterus_; _Ceratodus_ and
_Lepidosiren_ have five, like most other fish. The branchial arches
bear gill rakers.
[Illustration FIG. 19. DORSAL (TO THE LEFT) AND VENTRAL (TO THE RIGHT)
VIEWS OF THE CRANIUM OF _Ceratodus miolepis_ (after GÜNTHER).
1. cartilaginous part of the quadrate with which the mandible
articulates.
2. scleroparietal.
3. frontal.
4. ethmoid.
5. nares.
6. orbit.
7. pre-opercular (squamosal).
8. second rib.
9. first rib.
10. vomerine tooth.
11. palato-pterygoid tooth.
12. palato-pterygoid.
13. parasphenoid.
14. interopercular.]
RIBS.
As has been already mentioned (p. 24), although ribs commonly appear
to be the cut-off ends of the transverse processes, they are really
elements derived from the ventral or haemal arch.
In Elasmobranchii and other cartilaginous fish they have the form of
small cartilaginous structures imperfectly separated from the
diverging halves of the ventral arch, and are often absent.
In Teleostei and bony Ganoids they often have different attachments in
different parts of the body. In the tail region they are not
differentiated from the two halves of the ventral arch, which meet in
the middle line, and are prolonged into a haemal spine. In the
posterior trunk region they sometimes form distinct processes
diverging from the two halves of the ventral arch; while further
forward they may shift their attachment so as to arise from the dorsal
side of the two halves of the ventral arch and at some distance from
their ends, which now diverge as ventri-lateral processes.
APPENDICULAR SKELETON.
PECTORAL GIRDLE.
The simplest type of pectoral girdle is found in Elasmobranchs. It is
entirely cartilaginous and consists of a curved ventrally-placed rod,
ending dorsally in two horn-like scapular processes which are
sometimes attached to the cranium or vertebral column. In Rays the
shoulder girdle is very large, and has a distinct suprascapular
portion forming a broad plate attached to the neural spines of the
vertebrae. There is often a cup-like glenoid cavity for the
articulation of the limb; this cavity is specially large in Rays and
is much pierced by holes. In Dipnoi the cartilaginous girdle still
occurs, but on it there is a deposit of membrane bone forming the
clavicle, infraclavicle, and supra-clavicle. These bones, which with
the exception of the clavicle, are unknown in higher vertebrates, are
better developed in Ganoids, and best of all in Teleosteans. They are
connected by the supratemporal with the epi-otic and opisthotic
regions of the cranium. Owing to this development of dermal bone, the
original cartilaginous arch becomes much reduced, but ossifications
representing the scapula and coracoid occur in bony Ganoids and
Teleosteans.
PELVIC GIRDLE.
In Elasmobranchs the pelvic girdle consists of a short ventral rod of
cartilage representing the ischium and pubis, which does not send up
dorsal iliac processes. In _Chimaera_ the pelvic girdle has a
flattened pointed iliac portion, and ventrally an unpaired movable
cartilaginous plate which bears hooks and is supposed to be copulatory
in function. Claspers of the usual type are present as well. The
Dipnoi have a primitive kind of pelvis in the form of a cartilaginous
plate lying in the mid-ventral line and drawn out into three horns
anteriorly. In Ganoids the pelvis has almost entirely disappeared,
though small cartilaginous vestiges of it remain in _Polypterus_. In
Teleosteans even these vestiges are gone, and in these fish and
Ganoids the place of the pelvis is taken by the enlarged basi-pterygia
(meta-pterygia) of the fins.
PAIRED FINS[46].
As regards the origin of the limbs or paired fins of fishes there are
two principal views. One view, that of Gegenbaur, considers that limbs
and their girdles are derived from visceral arches which have migrated
backwards. The other view, which probably now has the greater number
of supporters, considers that the paired fins of fishes are of
essentially the same nature as the median fins.
According to Gegenbaur's view[47] the =archipterygium= of _Ceratodus_
(fig. 20) represents the lowest type of fin; it consists of a central
cartilaginous axis bearing a large number of radiale. The dorsal or
pre-axial radiale are more numerous than the ventral or postaxial, and
at the margin of the fin[48] the cartilaginous endoskeletal radiale
are replaced by horny exoskeletal fin-rays.
[Illustration FIG. 20. LATERAL VIEW OF THE SKELETON OF _Ceratodus
miolepis_ (after GÜNTHER).
1. ethmoid.
2. scleroparietal.
3. frontal.
4. cartilaginous posterior part of cranium.
5. pre-opercular (squamosal).
6. opercular.
7. suborbital.
8. orbit.
9. pectoral girdle.
10. proximal cartilage of pectoral fin.
11. pectoral fin.
12. pelvic girdle.
13. pelvic fin.
14. spinal column.
15. caudal fin (diphycercal).]
It is impossible here to give a full discussion of the rival views,
but some of the points which support Gegenbaur's view may be
mentioned. The fact that migration of visceral arches has to be
assumed is no difficulty, as it is obvious that migration in the
opposite direction has taken place in many Teleosteans such as the
Cod, whose pelvic fins are attached to the throat in front of the
pectorals. If migration did take place, the pelvic fins being older
than the pectoral should be the more modified, and this is the case.
Again, if the pectoral girdle is a modified branchial arch, it must at
some period have carried a gill, and in _Protopterus_ it does bear a
vestigial gill.
According to the view more prevalent at the present time, the paired
fins have been derived from two continuous folds of skin and their
skeletal supports running forward from the anal region along the sides
of the body, their character being similar to the fold that gave rise
to the median fins. In support of this view it may be argued that the
paired and unpaired fins are often identical in structure, and that
some Elasmobranch embryos do show a ridge running between the pectoral
and pelvic fins. Then from this continuous fold two pairs of smaller
folds may have been specialised off, and in each a number of
cartilaginous radiale may have been developed. The fin of
_Cladoselache_ from the Carboniferous of Ohio apparently illustrates
this condition. It consists of certain basal pieces which do not
project beyond the body wall and bear a number of unsegmented
cartilaginous radiale, which show crowding together and are sometimes
bifurcated distally; they extend throughout the whole fin from the
body wall to the margin. From this fin the archipterygium might be
easily derived by the enlargement of one of the middle radiale and the
segmentation and partial fusion of them all.
Whether the archipterygium be a primitive or secondary type of fin,
when it is once reached it is easy to derive all the other types from
it. The fins of the other living Dipnoi,--_Protopterus_ and
_Lepidosiren_--are simply archipterygia from which the radiale have
almost or completely disappeared, leaving only the segmented axes.
Archipterygia too are found in the pectoral fins of the Ichthyotomi,
but the postaxial radiale are much reduced.
The =ichthyopterygium=, or type of fin, characteristic of many modern
Elasmobranchs such as _Scyllium_, may have been derived from the
archipterygium by the gradual reduction of the rays on the postaxial
side of the axis and their condensation on the pre-axial side. The
Ichthyotomi such as _Xenacanthus_ show one stage in the reduction of
the postaxial rays, and a further stage is seen in the Notidanidae and
some other sharks like _Scymnus_ and _Acanthias_, in which a few
postaxial rays still remain. The condensation of the pre-axial rays
when further continued leads to one of the rays getting an attachment
to the girdle. Thus the fin comes to articulate with the girdle by two
basalia or basal pieces; a third attachment is formed in the same way
and the three basalia are called respectively pro-, meso-, and
meta-pterygia. By some authors the meta-pterygium and by others the
meso-pterygium is regarded as homologous with the axis of the
archipterygium.
The pectoral fins of Elasmobranchs vary very much in their mode of
attachment. In some of the sharks, including the Notidanidae and
_Scyllium_, all three basalia articulate with the pectoral girdle,
while in others such as _Cestracion_ the meta-pterygium is excluded.
In Rays the propterygium and the meta-pterygium are long and narrow
and diverge much from one another; other basalia work their way in
between the meso-pterygium and meta-pterygium, and come to articulate
with the pectoral girdle. Sometimes they fuse and form a second
meso-pterygium. The radiale are greatly elongated and are segmented.
In _Chimaera_ all three basalia are present, but the meso-pterygium
is shifted and does not articulate with the pectoral girdle[49].
In _Acipenser_ and _Polyodon_ the pectoral fin is built on the same
type as in Elasmobranchs, but becomes modified from the fact that the
propterygium is replaced by dermal bone which forms a large =marginal
ray=. Extra meso-pterygia are formed in the same way as in Rays.
In _Polypterus_ the pro-and meta-pterygia have ossified while the
meso-pterygium remains chiefly cartilaginous; the fin-rays are also
chiefly ossified.
In _Amia_, _Lepidosteus_, and certain Teleosteans like _Salmo_, not
only the propterygium but the meso-pterygium is almost suppressed by
the marginal ray.
In the great majority of Teleosteans a still further stage is reached,
the endoskeletal elements, the basalia and radiale are almost entirely
suppressed and the fin comes to consist entirely of ossified fin-rays
of dermal origin.
In some Teleosteans--_Exocaetus_, a herring, and _Dactylopterus_, a
gurnard--the pectoral fins are so enormously developed that by means
of them the fish is able to fly through the air for considerable
distances. The skeleton of these great fins is almost entirely
composed of dermal bone.
PELVIC FIN.
The pelvic fin is almost always further removed from the
archipterygial condition, and is in general more modified than is the
pectoral. Thus in the Ichthyotomi, while the pectoral fins are
archipterygia similar to those of _Ceratodus_, the pelvic fins consist
of an axis bearing rays on the postaxial side only, and prolonged
distally into a clasper. In Dipnoi however the pelvic fins are very
similar to the pectoral. In Elasmobranchs the meso-pterygium is
missing, the propterygium is small or absent, and the fin is mainly
composed of the meta-pterygium (generally called basi-pterygium) and
its radiale. The males in Elasmobranchii and Holocephali have the
distal end of the meta-pterygium prolonged into a clasper.
In Ganoids and in Teleosteans the loss of the pelvic girdle causes the
pelvic fin to be still further removed from the primitive state. There
is always a large basi-pterygium which lies imbedded in the muscles
and meets its fellow at its proximal end. In Cartilaginous Ganoids it
has a secondary segmentation. Its relation to its fellow is subject to
much variation in Teleosteans, sometimes as in the Perch the two are
in contact throughout, sometimes as in the Salmon they meet distally
as well as proximally, but are elsewhere separated by a space,
sometimes as in the Pike and Bony Ganoids they diverge widely. The
radiale are articulated to the basi-pterygium. In Cartilaginous
Ganoids and _Polypterus_ they are well developed, in other Ganoids and
in Teleosteans they are in the main replaced by dermal fin-rays.
In some Teleosteans such as the Cod the pelvic fins have migrated from
their usual position and come to be attached to the throat in front of
the pectoral fins. Fish with this arrangement are grouped together as
=jugulares=.
FOOTNOTES:
[39] The following general works on fishes may be referred to:
Bashford Dean, _Fishes, Living and Fossil_, New York, 1895. A.
Günther, _An Introduction to the Study of Fishes_, Edinburgh, 1880.
A.A.W. Hubrecht and M. Sagemehl, _Fische_ in Bronn's _Classen und
Ordnungen des Thierreichs_, Band VI. Leipzig, 1876.
[40] See W.G. Ridewood, _Nat. Sci._ vol. VIII. 1896, p. 380. Full
references are there given to the literature of the subject.
[41] See H. Gadow and E.C. Abbott, _Phil. Trans._ vol. 186 (1895) B.
pp. 163-221.
[42] C. Hasse, _Zeitschr. wiss. Zool._ LVII. 1893, p. 76.
[43] C. Hasse, _Das natürliche System der Elasmobranchier auf
Grundlage des Baues und der Entwickelung ihrer Wirbelsäule_, Jena,
1879 and 1885, and "Die fossilen Wirbel, Morph. Studien I.-IV.,"
_Morphol. Jahrb. Bd._ II., III. and IV. 1876-78.
[44] See H.B. Pollard, _Anat. Anz._ X. 1894.
[45] T.W. Bridge, "The Cranial Osteology of _Amia calva_," _J. Anat.
Physiol. norm. path._ 1876, vol. XI. p. 605. R. Shufeldt, "The
Osteology of _Amia calva_," _Ann. Rep. of the Commissioner for Fish
and Fisheries_, Washington, 1885.
[46] A. Smith Woodward, _Nat. Sci._ vol. I. 1892, p. 28. Further
references are here given on the literature of the subject.
[47] C. Gegenbaur, Ueber das Archipterygium, _Jena Zeitschr. der
Wirbelthiere_, 2^e Heft, 1873, vol. 7, and _Morphol. Jahrb._ XXII.
1894, p. 119.
[48] The fins of _Ceratodus_ are very variable, no two being exactly
alike. Sometimes even the main axis bifurcates. See W.A. Haswell,
_Linn. Soc. N. S. Wales_, vol. VII. 1882.
[49] Some of these views with regard to the homologies of the parts of
the fins are not accepted by all anatomists.