MODIFICATIONS OF THE SKELETON.
The lowermost sub-class of fishes, which comprises one form only, the
Lancelet (_Branchiostoma_ [s. _Amphioxus_] _lanceolatum_), possesses
the skeleton of the most primitive type.
[Illustration: Fig. 28.--Branchiostoma lanceolatum. _a_,
Mouth; _b_, Vent; _c_, abdominal porus.]
[Illustration: Fig. 29.--Anterior end of body of Branchiostoma
(magn.) _d_, Chorda dorsalis; _e_, Spinal chord; _f_,
Cartilaginous rods; _g_, Eye; _h_, Branchial rods; _i_, Labial
cartilage; _k_, Oral cirrhi.]
The vertebral column is represented by a simple _chorda_ _dorsalis_
or _notochord_ only, which extends from one extremity of the fish to
the other, and, so far from being expanded into a cranial cavity,
it is pointed at its anterior end as well as at its posterior. It is
enveloped in a simple membrane like the spinal chord and the abdominal
organs, and there is no trace of vertebral segments or ribs; however, a
series of short cartilaginous rods above the spine evidently represent
apophyses. A maxillary or hyoid apparatus, or elements representing
limbs, are entirely absent.
[J. Müller, Ueber den Bau und die Lebenserscheinungen des
_Branchiostoma lubricum_, in Abhandl. Ak. Wiss. Berlin,
1844.]
The skeleton of the _Cyclostomata_ (or Marsipobranchii) (Lampreys and
Sea-hags) shows a considerable advance of development. It consists of
a notochord, the anterior pointed end of which is wedged into the base
of a cranial capsule, partly membranous partly cartilaginous. This
skull, therefore, is not movable upon the spinal column. No vertebral
segmentation can be observed in the notochord, but neural arches are
represented by a series of cartilages on each side of the spinal chord.
In _Petromyzon_ (Fig. 30) the basis cranii emits two prolongations on
each side: an inferior, extending for some distance along the lower
side of the spinal column, and a lateral, which is ramified into a
skeleton supporting the branchial apparatus. A stylohyal process and a
subocular arch with a palato-pterygoid portion may be distinguished.
The roof of the cranial capsule is membranous in _Myxine_ and in the
larvæ of _Petromyzon_, but more or less cartilaginous in the adult
_Petromyzon_ and in _Bdellostoma_. A cartilaginous capsule on each side
of the hinder part of the skull contains the auditory organ, whilst the
olfactory capsule occupies the anterior upper part of the roof. A broad
cartilaginous lamina, starting from the cranium and overlying part
of the snout, has been determined as representing the ethmo-vomerine
elements, whilst the oral organs are supported by large, very peculiar
cartilages (_labials_), greatly differing in general configuration
and arrangement in the various Cyclostomes. There are three in the
Sea-lamprey, of which the middle one is joined to the palate by an
intermediate smaller one; the foremost is ring-like, tooth-bearing,
emitting on each side a styliform process. The lingual cartilage is
large in all Cyclostomes.
There is no trace of ribs or limbs.
[J. Müller, Vergleichende Anatomie der Myxinoiden. Erster Theil.
Osteologie und Myologie, in Abhandl. Ak. Wiss. Berlin, 1835.]
[Illustration: Fig. 30.--Upper (A) and side (B) views, and
vertical section (C) of the skull of _Petromyzon marinus_.
_a_, Notochord; _b_, Basis cranii; _c_, Inferior, and _d_,
Lateral process of basis; _e_, Auditory capsule; _f_,
Subocular arch; _g_, Stylohyal process; _h_, Olfactory
capsule; _i_, Ethmo-vomerine plate; _k_, Palato-pterygoid
portion of subocular arch; _l-n_, Accessory labial or rostral
cartilages; with _o_, appendage; _p_, lingual cartilage;
_q_, neural arches; _r_, Branchial skeleton; _s_, Blind
termination of the nasal duct between the notochord and
œsophagus.]
[Illustration: Fig. 31.--Heterocercal Tail of Centrina salviani.
_a_, Vertebræ; _b_, Neurapophyses; _c_,
Hæmapophyses.]
The _Chondropterygians_ exhibit a most extraordinary diversity in
the development of their vertebral column; almost every degree of
ossification, from a notochord without a trace of annular structure to
a series of completely ossified vertebræ being found in this order.
Sharks, in which the notochord is persistent, are the _Holocephali_
(if they be reckoned to this order, and the genera _Notidanus_ and
_Echinorhinus_). Among the first, _Chimæra monstrosa_ begins to show
traces of segmentation; but they are limited to the outer sheath of the
notochord, in which slender subossified rings appear. In _Notidanus_
membranous septa, with a central vacuity, cross the substance of the
gelatinous notochord. In the other Sharks the segmentation is complete,
each vertebra having a deep conical excavation in front and behind,
with a central canal through which the notochord is continued; but the
degree in which the primitive cartilage is replaced by concentric or
radiating lamellæ of bone varies greatly in the various genera, and
according to the age of the individuals. In the Rays all the vertebræ
are completely ossified, and the anterior ones confluent into one
continuous mass.
In the majority of Chondropterygians the extremity of the vertebral
column shows a decidedly heterocercal condition (Fig. 31), and only a
few, like _Squatina_ and some Rays, possess a diphycercal tail
The advance in the development of the skeleton of the Chondropterygians
beyond the primitive condition of the previous sub-classes, manifests
itself further by the presence of neural and hæmal elements, which
extend to the foremost part of the axial column, but of which the hæmal
form a closed arch in the caudal region only, whilst on the trunk they
appear merely as a lateral longitudinal ridge.
[Illustration: Fig. 32. Fig. 33. Fig. 34.
Fig. 32.--Lateral view. Fig. 33.--Longitudinal section. Fig.
34.--Transverse section of Caudal vertebra of Basking Shark
(Selache maxima). (After Hasse.) _a_, Centrum; _b_,
Neurapophysis; _c_, Intercrural cartilage; _d_,
Hæmapophysis; _e_, Spinal canal; _f_, Intervertebral
cavity; _g_, Central canal for persistent portion of
notochord; _h_, Hæmal canals for blood-vessels.]
The neural and hæmal apophyses are either merely attached to the
axis, as in Chondropterygians with persistent notochord, the Rays
and some Sharks; or their basal portions penetrate like wedges into
the substance of the centrum, so that, in a transverse section, in
consequence of the difference in their texture, they appear in the
form of an X.[6] The interspaces between the neurapophyses of the
vertebræ are not filled by fibrous membrane, as in other fishes, but
by separate cartilages, _laminæ_ or _cartilagines intercrurales_, to
which frequently a series of terminal pieces is superadded, which
must be regarded as the first appearance of the interneural spines
of the Teleostei and many Ganoids. Similar terminal pieces are
sometimes observed on the hæmal arches. _Ribs_ are either absent or but
imperfectly represented (_Carcharias_).
The substance of the _skull_ of the Chondropterygians is cartilage,
interrupted especially on its upper surface by more or less extensive
fibro-membranous fontanelles. Superficially it is covered by a more or
less thick chagreen-like osseous deposit. The articulation with the
vertebral column is effected by a pair of lateral condyles. In the
Sharks, besides, a central conical excavation corresponds to that of
the centrum of the foremost vertebral segment, whilst in the Rays this
central excavation of the skull receives a condyle of the axis of the
spinous column.
The cranium itself is a continuous undivided cartilage, in which the
limits of the orbit are well marked by an anterior and posterior
protuberance. The ethmoidal region sends horizontal plates over
the nasal sacs, the apertures of which retain their embryonic
situation upon the under surface of the skull. In the majority of
Chondropterygians these plates are conically produced, forming the base
of the soft projecting snout; and in some forms, especially in the
long-snouted Rays and the Saw-fishes (_Pristis_) this prolongation
appears in the form of three or more tubiform rods.
As separate cartilages there are appended to the skull a suspensorium,
a palatine, mandible, hyoid, and rudimentary maxillary elements.
The suspensorium is movably attached to the side of the skull. It
generally consists of one piece only, but in some Rays of two. In the
Rays it is articulated with the mandible only, their hyoid possessing
a distinct point of attachment to the skull. In the Sharks the hyoid
is suspended from the lower end of the suspensorium together with the
mandible.
What is generally called the upper jaw of a Shark is, as Cuvier has
already stated, not the maxillary, but palatine. It consists of two
simple lateral halves, each of which articulates with the corresponding
half of the lower jaw, which is formed by the simple representative of
Meckel’s cartilage.
Some cartilages of various sizes are generally developed on each side
of the palatine, and one on each side of the mandible. They are called
_labial_ cartilages, and seem to represent maxillary elements.
The _hyoid_ consists generally of a pair of long and strong lateral
pieces, and a single mesial piece. From the former cartilaginous
filaments (representing branchiostegals) pass directly outwards.
Branchial arches, varying in number, and similar to the hyoid, succeed
it. They are suspended from the side of the foremost part of the
spinous column, and, like the hyoid, bear a number of filaments.
The vertical fins are supported by interneural and interhæmal
cartilages, each of which consists of two and more pieces, and to which
the fin-rays are attached without articulation.
The _scapular arch_ of the Sharks is formed by a single coracoid
cartilage bent from the dorsal region downwards and forwards. In some
genera (_Scyllium_, _Squatina_) a small separate scapular cartilage
is attached to the dorsal extremities of the coracoid; but in none of
the Elasmobranchs is the scapular arch suspended from the skull or
vertebral column; it is merely sunk, and fixed in the substance of
the muscles. Behind, at the point of its greatest curvature, three
carpal cartilages are joined to the coracoid, which Gegenbaur has
distinguished as _propterygium_, _mesopterygium_, and _metapterygium_,
the former occupying the front, the latter the hind margin of the fin.
Several more or less regular transverse series of styliform cartilages
follow. They represent the phalanges, to which the horny filaments
which are imbedded in the skin of the fin are attached.
In the Rays, with the exception of _Torpedo_, the scapular arch is
intimately connected with the confluent anterior portion of the
vertebral column. The anterior and posterior carpal cartilages are
followed by a series of similar pieces, which extend like an arch
forwards to the rostral portion of the skull, and backwards to the
pubic region. Extremely numerous phalangeal elements, longest in the
middle, are supported by the carpals, and form the skeleton of the
lateral expansion of the so-called _disk_ of the Ray’s body, which
thus, in fact, is nothing but the enormously enlarged pectoral fin.
The _pubic_ is represented by a single median transverse cartilage,
with which a tarsal cartilage articulates. The latter supports the
fin-rays. To the end of this cartilage is also attached, in the male
Chondropterygians, a peculiar accessory generative organ or clasper.
The _Holocephali_ differ from the other Chondropterygians in several
important points of the structure of their skeleton, and approach
unmistakably certain Ganoids. That their spinal column is persistently
notochordal has been mentioned already. Their palatal apparatus, with
the suspensorium, coalesces with the skull, the mandible articulating
with a short apophysis of the cranial cartilage. The mandible is
simple, without anterior symphysis. The spine with which the dorsal
fin is armed articulates with a neural apophysis, and is not immovably
attached to it, as in the Sharks. The pubic consists of two lateral
halves, with a short, rounded, tarsal cartilage.
The skeleton of the _Ganoid_ Fishes offers extreme variations with
regard to the degree in which ossifications replace the primordial
cartilage. Whilst some exhibit scarcely any advance beyond the
Plagiostomes with persistent cartilage, others approach, as regards the
development and specialisation of the several parts of their osseous
framework, the Teleosteans so closely that their Ganoid nature can
be demonstrated by, or inferred from, other considerations only. All
Ganoids possess a separate gill-cover.[7]
The diversity in the development of the Ganoid skeleton is well
exemplified by the few representatives of the order in the existing
Fish-fauna. Lowest in the scale (in this respect) are those with a
persistent notochord, and an _autostylic_ skull, that is, a skull
without separate suspensorium--the fishes constituting the suborder
_Dipnoi_, of which the existing representatives are _Lepidosiren_,
_Protopterus_, and _Ceratodus_, and the extinct (as far as demonstrated
at present) _Dipterus_, _Chirodus_ (and _Phaneropleuron_?). In these
fishes the notochord is persistent, passing uninterruptedly into the
cartilaginous base of the skull. Only now and then a distinct vertical
segmentation occurs in the caudal portion of the column, but it does
not extend to the notochord itself, but indicates only the limits
between the superadded apophyseal elements, each neural being confluent
with the opposite hæmal. Some _Dipnoi_ are diphy-, others heterocercal.
Neural and hæmal elements and ribs are well developed. In _Ceratodus_
each neurapophysis consists of a basal cartilaginous portion, forming
an arch over the myelon, and of a superadded second portion. The
latter is separated from the former by a distinct line of demarcation,
and its two branches are more styliform, cartilaginous at the ends
and in the centre, but with an osseous sheath, and coalesced at the
top, forming a gable over an elastic fibrous band which runs along
and parallel to the longitudinal axis of the column (_Ligamentum
longitudinale superius_). To the top of this gable is joined a single
long cylindrical neural spine. From the eleventh apophyseal segment a
distinct interneural spine, of the same structure as the neural, begins
to be developed, and farther on a second interneural is superadded.
Towards the extremity of the column these various pieces are gradually
reduced in size and number, finally only a low cartilaginous band (the
rudiments of the neurapophysis) remaining. The _hæmapophyses_ are in
form, size, and structure, very similar to the neurapophyses; and all
these long bones, including the ribs, have that in common, that they
consist of a solid rod of cartilage enclosed in a bony sheath, which,
after the disappearance or decomposition of the cartilage, appears as
a hollow tube. Such bones are extremely common throughout the order of
Ganoids, and their remains have led to the designation of a family as
_Cœlacanthi_ (κοιλος, hollow; and ἀκανθος, spine).
The primordial _cranium_ of the _Dipnoi_ is cartilaginous, but with
more or less extensive ossifications in its occipital, basal, or
lateral portions, and with large tegumentary bones, the arrangement
of which varies in the different genera. There is no separate
suspensorium for the lower jaw. A strong process descends from the
cranial cartilage, and offers by means of a double condyle (Fig. 35
_s_) attachment to corresponding articulary surfaces of the lower
jaw. Maxillary and intermaxillary elements are not developed, but,
perhaps, represented in _Ceratodus_ by some inconstant rudimentary
labial cartilages situated behind the posterior nasal opening.
Facial cartilages and an infraorbital ring are developed at least
in _Ceratodus_. The presence of a pair of small teeth in front
indicates the vomerine portion (_v_) which remains cartilage, whilst
the posterior pair of teeth are implanted in a pterygo-palatine
ossification (_l_), which sometimes is paired, sometimes continuous.
The base of the skull is constantly covered by a large basal
ossification (_o_).
[Illustration: Fig. 35.--Palatal view of Skull of Ceratodus.]
The _hyoid_ is well developed, sometimes reduced to a pair of
ceratohyals, sometimes with a basihyal and glossyhyal. The skeleton of
the _branchial apparatus_ approaches the Teleosteous type, less so in
_Lepidosiren_ than in _Ceratodus_, in which five branchial arches are
developed, but with the lateral and mesial pieces reduced in number.
A large operculum, and a smaller sub- or interoperculum are present.
The _scapular_ arch consists of a single median transverse cartilage,
and a pair of lateral cartilages which bear the articular condyle for
the pectoral limb. The latter cartilages form the base of a large
membrane-bone, and the whole arch is suspended from the skull by means
of an osseous supraclavicle.
The fore-limb of the _Dipnoi_ (Fig. 36) differs externally greatly
from the pectoral fin of other Ganoid fishes. It is covered with
small scales along the middle, from the root to its extremity, and
surrounded by a rayed fringe similar to the vertical fin. A muscle
split into numerous fascicles extends all the length of the fin, which
is flexible in every part and in every direction. The cartilaginous
framework supporting it is joined to the scapular arch by an oblong
cartilage, followed by a broad basal cartilage (_a_), generally single,
sometimes showing traces of a triple division. Along the middle of the
fin runs a jointed axis (_b_), the joints gradually becoming smaller
and thinner towards the extremity; each joint bears on each side a
three, two, or one-jointed branch (_c, d_). This _axial_ arrangement of
the pectoral skeleton, which evidently represents one of its first and
lowest conditions, has been termed _Archipterygium_ by Gegenbaur. It is
found in _Ceratodus_ and other genera, but in _Lepidosiren_ the jointed
axis only has been preserved, with the addition of rudimentary rays in
_Protopterus_.
[Illustration: Fig. 36.--Fore-limb of Ceratodus.]
The _pubic_ consists of a single flattened subquadrangular cartilage,
produced into a long single anterior process. Posteriorly it terminates
on each side in a condyle, to which the basal cartilage of the ventral
paddle is joined. The endoskeleton of the paddle is almost identical
with that of the pectoral.
The Ganoid fishes with persistent notochord, but with a _hyostylic_
skull (that is, a skull with a separate suspensorium) consist of the
suborder _Chondrostei_, of which the existing representatives are the
Sturgeons (_Acipenser_, _Scaphirhynchus_, _Polyodon_), and the extinct
the _Chondrosteidæ_, _Palæoniscidæ_, and (according to Traquair)
_Platysomidæ_.
Their spinal column does not differ essentially from that of the
Dipnoi. Segmentation is represented only as far as the neural and
hæmal elements are concerned. All are eminently heterocercal. Ribs are
present in most, but replaced by ligaments in _Polyodon_.
[Illustration: Fig. 37.--Skull of Polyodon (after Traquair).
_n_, Nasal cavity; _sq_, squamosal; _mh_, hyomandibular;
_sy_, symplectic; _pa_, palato-pterygoid; _m_, Meckelian
cartilage; _mx_, maxillary; _d_, dentary; _h_, hyoid; _op_,
opercle; _br_, branchiostegal; _s.cl_, supra-clavicular;
_p.cl_, post-clavicular; _cl_, clavicle; _i.cl_,
infra-clavicular.]
The primordial cranium of the Sturgeons consists of persistent
cartilage without ossifications in its substance, but superficial bones
are still more developed and specialised than in the Dipnoi; so it is,
at least, in the true Sturgeons, but less so in _Polyodon_ (Fig. 37).
The upper and lateral parts of the skull are covered by well-developed
_membrane bones_, which, from this suborder, upwards in the series,
will be found to exist throughout the remaining forms of fishes. They
are bones, the origin of which is not in cartilage but in membranous
connective tissue. The lower surface of the skull is covered by an
extremely large basal bone, which extends from the vomerine region
on to the anterior part of the spinal column. The nasal excavation
in the skull is rather lateral than inferior. The ethmoidal region
is generally much produced, forming the base of the long projecting
snout. The suspensorium is movably attached to the side of the skull,
and consists of two pieces, a hyomandibular and a symplectic, which
now appears for the first time as a separate piece, and to which the
hyoid is attached. The palato-maxillary apparatus is more complex
than in the Sharks and Dipnoi; a palato-pterygoid consists of two
mesially-connected rami in _Polyodon_, and of a complex cartilaginous
disk in _Acipenser_, being articulated in both to the Meckelian
cartilage. In addition, the Sturgeons possess one or two pairs of
osseous rods, which, in _Polyodon_ at least, represent the maxillary,
and therefore must be the representatives of the labial cartilages
of the Sharks. The Meckelian cartilage is more or less covered by
tegumentary bones.
In the gill-cover, besides the operculum, a sub- and interoperculum may
be distinguished in _Acipenser._
The hyoid consists of three pieces, of which the posterior bears a
broad branchiostegal in _Polyodon._
In the scapulary arch the primordial cartilaginous elements scarcely
differ from those of the _Dipnoi._ The membrane-bones are much
expanded, and offer a continuous series suspended from the skull. Their
division in the median ventral line is complete.
[Illustration: Fig. 38.--Fore-limb of Acipenser.]
The pectoral is supported by a cartilaginous framework (Fig. 38)
similar to that of _Ceratodus_, but much more shortened and reduced in
its periphery, the branches being absent altogether on one side of the
axis. This modification of the fin is analogous to the heterocercal
condition of the end of the spinous column. To the inner corner of a
basal cartilage (_a_) a short axis (_b_) is joined, which on its outer
side bears a few branches (_d_) only, the remaining branches (_c_)
being fixed to the basal cartilage. The dermal fin-rays are opposed to
the extremities of the branches, as in the _Dipnoi_.
The _pubic_ consists of a paired cartilage, to which tarsal pieces
supporting the fin-rays are attached.
The other living Ganoid fishes have the spinous column entirely or
nearly entirely ossified, and have been comprised under the common name
_Holostei_. However, they form three very distinct types; several
attempts have been made to coordinate with them the fossil forms, but
this task is beset with extreme difficulties, and its solution hitherto
has not proved to be satisfactory.
The _Polypteroidei_ have their spinous column formed by distinct
osseous _amphicœlous_ vertebræ, that is, vertebræ with concave anterior
and posterior surfaces. It is nearly diphycercal; a slight degree of
heterocercy obtains, inasmuch as the last vertebra is succeeded by a
very thin cartilaginous filament which penetrates between the halves
of one of the middle rays of the terminal fin. The rays above this
cartilaginous filament are articulated to interneurals, those below
lack interhæmals, and are attached either to the hæmals or vertebral
centres. The neural arches, though ossified, do not coalesce with
the centrum, and form one canal only, for the myelon. There are
no intermediate elements between the neural spines. Interneurals
developed, but simple, articulating with the dermoneurals. The
abdominal vertebræ have parapophyses developed with epipleural
spines. Only the caudal vertebræ have hæmal spines, which, like the
interhæmals, agree in every essential respect with the opposite
neurals. _Ribs_ are inserted, not on the parapophyses, but on the
centre, immediately below the parapophyses.
The _skull_ of _Polypterus_ (Fig. 39) shows a great advance towards
the Teleosteous type, the number of separable bones being greatly
increased. They are arranged much in the same fashion as in Teleostei.
But a great portion of the primordial cranium remains cartilaginous.
The membrane-bones which cover the upper and lower surfaces of the
brain-case are so much developed as to cause the underlying cartilage
to disappear, so that a large vacuity or fontanelle exists in the
substance of the upper as well as lower cartilaginous wall. Of
ossifications belonging to the primordial skull must be noticed the
single occipital with a mastoid on each side. They are separated by
persistent cartilage from the sphenoids and postfrontals; the former,
which are the largest ossification of the primordial cranium, enclose
the anterior half of the brain cavity. Finally, the nasal portion
contains a median ethmoid and a pair of præfrontal bones.
[Illustration: Fig. 39.--Skull of Polypterus. (After Traquair.)
Fig. I. Upper aspect of the Primordial Cranium, with the
membrane-bones removed. Fig. II. Lower aspect of the same. Fig.
III. Side view, with the membrane-bones. Fig. IV. Lower aspect
of the Skull, part of the bones being removed on one side. The
parts shaded with oblique lines are cartilage of the primordial
skull.
_An_, Angular; _ao_, ante-orbital; _Ar_,
articulary; _B_, basal; _D_, dentary; _E_,
ethmoid; _F_, frontal; _Ma_, mastoid; _Mp_,
metapterygoid; _Mx_, maxillary; _N_, nasal; _O_,
operculum; _Oc_, occipital; _Pa_, parietal; _Pl_,
palatine; _Pmx_, præmaxillary; _po_, post-orbital;
_Prf_, prefrontal; _Pt_, post-temporals; _Ptf_,
postfrontal; _Ptr_, pterygoid; _Q_, quadrate;
_S_, suspensorium; _So_, sub-operculum; _Sp_,
sphenoid; _Spl_, splenial; _St_, supratemporals;
_T_, tympanic lamina; _Tu_, turbinal; _v_, vomer;
_x x_, small ossicles; _x’ x’_, spiraculars.]
Only a very small portion of the bones described are visible
externally, nearly the whole of the primordial cranium being covered
by the membrane-bones. Of these are seen on the upper surface a pair
of parietals, frontals, “nasals,” and turbinals; on the lower surface
a large cross-shaped basal, anteriorly bordered on each side by a
pterygoid, parallel to a palatine which forms a suture with the double
vomer. The suspensorium has in front a metapterygoid and quadrate bone,
and an operculum and sub-operculum are attached to it behind.
Præmaxillaries and maxillaries are now fully developed, but immovably
attached to the skull. The lower jaw is ossified, and consists of an
articulary, angular, dentary, and splenial. Of labial cartilages a
rudiment at the angle of the mouth has remained persistent.
The side of the skull, in front of the operculum, is covered by a large
irregularly-shaped bone (_T_) (corresponding to the “tympanic lamina”
of _Ceratodus_, Fig. 35, _q_), held by some to be the præoperculum;
along its upper circumference lies a series of small ossicles, of which
two may be distinguished as spiraculars, as they form a valve for the
protection of the spiracular orifice of these fishes. An infraorbital
ring is represented by a præ- and post-orbital only.
Each _hyoid_ consists of three pieces, none of which bear
branchiostegals, the single median piece being osseous in front and
cartilaginous behind. Four branchial arches are developed, the foremost
consisting of three, the second and third of two, and the last of a
single piece. There is no lower pharyngeal. Between the rami of the
lower jaw the throat is protected by a pair of large osseous laminæ
(_gular plates_), which have been considered to represent the urohyal
of osseous fishes.
The scapulary arch is almost entirely formed by the well-developed
membrane-bones, which in the ventral line are suturally united.
The pectoral fin is supported by three bones, pro-, meso-, and
metapterygium, of which the dilated middle one alone bears rays, and is
excluded from the articulation with the shoulder-girdle.
The pubic consists of paired bone, to which tarsal bones supporting the
fin-rays are attached.
In the _Lepidosteoidei_ the vertebræ are completely ossified, and
_opisthocœlous_, having a convexity in front and a concavity behind,
as in some Amphibians. Though the end of the body externally appears
nearly diphycercal, the termination of the vertebral column is,
in fact, distinctly heterocercal (Fig. 40). Its extremity remains
cartilaginous, is turned upwards, and lies immediately below the scutes
which cover the upper margin of the caudal fin. It is preceded by a
few rudimentary vertebræ which gradually pass into the fully developed
normal vertebræ. The caudal fin is suspended from hæmapophyses only,
and does not extend to the neural side of the vertebral column. The
neural arches coalesce with the centrum; interneurals simple. The
abdominal vertebræ have parapophyses, to which the ribs are attached.
Only the caudal vertebræ have hæmal spines.
[Illustration: Fig. 40.--Heterocercal Tail of Lepidosteus.
_n_, Vertebral column; _h_, hæmal spines; _dn_, fulcra; _dh_,
lower fulcra.]
In the skull of _Lepidosteus_ the cartilage of the endocranium is
still more replaced by ossifications than in _Polypterus_; those
ossifications, moreover, being represented by a greater number of
discrete bones; especially the membrane-bones are greatly multiplied:
the occipital, for instance, consists of three pieces; the vomer is
double as in _Polypterus_; the maxillary consists of a series of
pieces firmly united by suture. The symplectic reaches the lower jaw,
so that the articulary is provided with a double joint, viz. for the
symplectic and quadrate; the component parts of the lower jaw are as
numerous as in reptiles, a dentary, splenial, articulary, angular,
supra-angular, and coronary being distinct. The sides of the head
are covered with numerous bones, and a præoperculum is developed in
front of the gill-cover which, again, consists of an operculum and
sub-operculum.
Each hyoid consists of three pieces, of which the middle is the
longest, the upper bearing the largest of the three branchiostegals
which _Lepidosteus_ possesses; a long and large glossohyal is
intercalated between the lower ends of the hyoids. There are five
branchial arches, the hindmost of which is modified into a lower
pharyngeal; upper pharyngeals are likewise present as in the majority
of Teleosteous fishes. No gular plate.
Of the scapulary arch the two halves are separated by a suture in the
median line; the membrane-bones are well developed, only a remnant of
the primordial cartilage remaining; the supraclavicle is very similar
to that of Teleosteous fishes, less so the post-temporal. The base to
which the limb is attached is a single osseous plate, supporting on its
posterior margin semi-ossified rods in small number, which bear the
pectoral rays.
The pubic consists of paired bone, the anterior ends of which overlap
each other, the extremity of the right pubis being dorsad to that of
the left. The elements representing a tarsus are quite rudimentary and
reduced in number (two or three).
The vertebral column of the _Amioidei_ shows unmistakable characters
of the Palæichthyic type. The arrangement of its component parts is
extremely simple. The centra of the amphicœlous vertebræ are well
ossified, but the neural and hæmal arches do not coalesce with the
centra, from which they are separated by a thin layer of cartilage.
Singularly, not every vertebra has apophyses: in the caudal portion of
_Amia_ the vertebræ are alternately provided with them and lack them.
The heterocercal condition of the spinous column is well marked: as in
the other Holostei the hindmost vertebræ are turned upwards, become
smaller and smaller in size, and lose their neural arches, the hæmals
remaining developed to the end. Finally, the column terminates in a
thin cartilaginous band, which is received between the lateral halves
of the fifth or sixth upper caudal ray. Interneurals and interhæmals
simple. Only the abdominal vertebræ have parapophyses, with which the
ribs are articulated.
The configuration of the skull, and the development and arrangement
of its component parts, approaches so much the Teleosteous type that,
perhaps, there are greater differences in skulls of truly Teleosteous
fishes than between the skulls of _Amia_ and many _Physostomi_.
Externally the cranium is entirely ossified; and the remains of the
cartilaginous primordial cranium (which, however, has no vacuity in its
roof) can only be seen in a section, and are of much less extent than
in many Physostomous fishes. The immovable intermaxillary, the double
vomer, the plurality of ossifications representing the articulary,
the double articulary cavity of the mandible for junction with the
quadrate and symplectic bones, remind us still of similar conditions
in the skull of _Lepidosteus_, but the mobility and formation of the
maxillary, the arrangement of the gill-covers, the development of the
opercles, the suspensorium, the palate, the insertion of a number of
branchiostegals on the long middle hyoid piece, the composition of the
branchial framework (with upper and lower pharyngeals), are as in the
Teleosteous type. A gular plate replaces the urohyal.
The scapular arch is composed entirely of the membrane-bones found in
the _Teleostei_, and the two sides are loosely united by ligament.
The base to which the limb is attached is cartilaginous; short
semi-ossified rods are arranged along its hinder margin and bear the
pectoral rays.
The skeleton of the hind-limb agrees entirely with that of
_Lepidosteus_.
[T. W. Bridge, The Cranial Osteology of _Amia calva_; in
Journ. Anat. and Physiol. vol. xi.]
In the _Teleosteous_ fishes the spinous column consists of completely
ossified amphicœlous vertebræ; its termination is _homocercal_--that
is, the caudal fin appears to be more or less symmetrical, the last
vertebra occupying a central position in the base of the fin, and being
coalesced with a flat osseous lamella, the _hypural_ (Fig. 23, 70), on
the hind margin of which the fin-rays are fixed. The hypural is but a
union of modified hæmapophyses which are directed backwards, and the
actual termination of the notochord is bent upwards, and lies along the
upper edge of the hypural, hidden below the last rudimentary neural
elements. In some Teleosteans, as the _Salmonidæ_, the last vertebræ
are conspicuously bent upwards: in fact, strictly speaking, this
homocercal condition is but one of the various degrees of heterocercy,
different from that of many Ganoids in this respect only, that the
caudal fin itself has assumed a higher degree of symmetry.
The neural and hæmal arches generally coalesce with the centrum, but
there are many exceptions, inasmuch as some portion of the arches of a
species, or all of them, show the original division.
The vertebræ are generally united with one another by zygapophyses, and
frequently similar additional articulations exist at the lower parts
of the centra. Parapophyses and ribs are very general, but the latter
are inserted on the centra and the base of the processes, and never on
their extremities. The point of insertion of the rib, more especially
on the anterior vertebræ, may be still higher--viz. at the base of the
neural arch, as in _Cotylis_ and allied genera, and even on the top of
the neurapophysis, as in _Batrachus_.
There is a great amount of variation as regards the degree in which
the primordial cranium persists; it is always more or less replaced by
bone; frequently it disappears entirely, but in some fishes, like the
_Salmonidæ_ or _Esocidæ_, the cartilage persists to the same or even
to a greater extent than in the Ganoidei holostei. Added to the bones
preformed in cartilage are a great number of _membrane-bones_. The
different kinds of these membrane-bones occur with greater or less
constancy throughout this sub-class; they often coalesce with, and are
no more separable from, the neighbouring or underlying cartilage-bones.
All these bones have been topographically enumerated in Chapter IV.
Many attempts have been made to classify the bones of the Teleosteous
skull, according to their supposed relation to each other, or with
the view to demonstrate the unity of plan on which the skull has been
built; but in all either the one or the other of the following two
principles has been followed:--
A. The “vertebral doctrine” starts from the undeniable fact that the
skull is originally composed of several segments, each of which is
merely the modification of a vertebra. The component parts of such a
cranial segment are considered to be homologous to those of a vertebra.
Three, four, or five cranial vertebræ have been distinguished, all
the various bones of the fully-developed and ossified skull being
referred, without distinction as to their origin, to one or the other
of those vertebral segments. The idea of the typical unity of the
osseous framework of Vertebrates has been worked out with the greatest
originality and knowledge of detail, by _Owen_, who demonstrates that
the fish-skull is composed of _four_ vertebræ.
The bones of the fish-skull are, according to him, primarily
divisible into those of the _neuroskeleton_, _splanchnoskeleton_, and
_dermoskeleton_.
The bones of the _neuro_- or proper _endoskeleton_ are arranged in
a series of four horizontally succeeding segments: the occipital,
parietal, frontal, and nasal vertebræ; each segment consisting of an
upper (neural) and a lower (hæmal) arch, with a common centre, and with
diverging appendages.
The _neural_ arches of the four vertebræ, in their succession from the
occiput towards the snout, are:--
1. _Epencephalic arch_, composed of the occipitals.
2. _Mesencephalic arch_, composed of basisphenoid, alisphenoid,
parietal, and mastoid.
3. _Prosencephalic arch_, composed of presphenoid, orbito-sphenoid,
frontal, and postfrontal.
4. _Rhinencephalic arch_, composed of vomer, prefrontal, and nasal.
The _hæmal_ arches in the same order of succession are:--
1. _Scapular_ or _scapulo-coracoid_ arch, composed of suprascapula,
scapula, and coracoid; its appendage consists of the ulna, radius and
carpal.
2. _Hyoid_ or _stylo-hyoid_ arch, composed of stylohyal, epihyal,
ceratohyal, basihyal, glossohyal, and urohyal; its appendage is the
branchiostegals.
3. _Mandibular_ or _tympano-mandibular_ arch, composed of epi-, meso-,
pre-, and hypo-tympanic, and the bones of the lower jaw; its appendage
consists of the præoperculum and the other opercles.
4. _Maxillary_ or _palato-maxillary_ arch, composed of palatine,
maxillary, and premaxillary; its appendage consists of the pterygoid
and entopterygoid.
Parts of the _splanchnoskeleton_ are held to be the ear-capsule
or petrosal and the otolite, the eye-capsule or sclerotic, the
nose-capsule or “ethmoid” and turbinal; the branchial arches.
The bones of the _dermoskeleton_ are the supratemporals, supraorbitals,
suborbitals, and labials.
B. In the second method of classifying the bones of the skull
prominence is given to the facts of their different origin as
ascertained by a study of their development. The parts developed from
the primordial skull, or the cartilaginous case protecting the nervous
centre are distinguished from those which enclose and support the
commencement of the alimentary canal and the respiratory apparatus, and
which, consisting of several arches, are comprised under the common
name of _visceral skeleton of the skull_. Further, a distinction is
made between the bones preformed in cartilage and those originating in
tegumentary or membranous tissue. It is admitted that the primordial
cranium is a coalition of several segments, the number of which is
determined by that of the visceral arches, these representing the hæmal
arches of the vertebral column; but the membrane-bones are excluded
from a consideration of the vertebral division of the primordial skull,
as elements originally independent of it, although these additions have
entered into special relations to the cartilage-bones.
With these views the bones of the Teleosteous skull are classified
thus:--
1. _Cartilage-bones of the primordial skull_.--The _basi-occipital_
(5 in Figs. 23–26) has retained the form of a vertebral centrum; it
is generally concave behind, the concavity containing remains of the
notochord; rarely a rounded articulary head of the first vertebra fits
into it, as in _Symbranchus_, and still more rarely it is provided
with such an articulary head (_Fistularia_); frequently it shows two
excavations on its inner surface for the reception of the _saccus
vestibuli_. The _exoccipitals_ (10) are situated on the side of the
basi-occipital, and contribute the greater portion of the periphery of
the foramen magnum; frequently they articulate with the first vertebra,
or meet in the upper median line, so as to exclude the supraoccipital
from the foramen magnum. The _supraoccipital_ (8) is intercalated
between the exoccipitals, and forms a most prominent part by the median
crest, which sometimes extends far forwards on the upper side of the
skull, and offers attachment to the dorsal portion of the large lateral
muscle of the trunk. When the interior portions of this bone remain
cartilaginous, some part of the semicircular canals may be lodged in it.
The region of the skull which succeeds the bones described encloses at
least the greater portion of the labyrinth, and its component parts
have been named with reference to it by some anatomists.[8] The
_alisphenoids_ (11) (_Prooticum_) form sutures posteriorly with the
basi- and exoccipitals, and meet each other in the median line at the
bottom of the cerebral cavity; they contribute to the formation of a
hollow in which the hypophysis cerebri and the saccus vasculosus are
received; in conjunction with the exoccipital it forms another hollow
for the reception of the vestibulum; generally it is perforated by the
Trigeminal and Facial nerves. The _paroccipitals_ (9) (_Epioticum_)
lodge a portion of the posterior vertical semicircular canal, and form
a projection of the skull on each side of the occipital crest, to which
a terminal branch of the scapular arch is attached. The _Mastoid_ (12
+ 13) (_Opisthoticum_) occupies the postero-external projection of
the head; it encloses a part of the external semicircular canal; is
generally coalesced with a membrane-bone, the superficial _squamosal_,
which emits a process for the suspension of the scapular arch, and is
frequently, as in the Perch, divided into two separate bones.
The anterior portion of the skull varies greatly as regards form, which
is chiefly dependent on the extent of the cerebral cavity; if the
latter is advanced far forwards, the lateral walls of the primordial
cranium are protected by more developed ossifications than if the
cerebral cavity is shortened by the presence of a wide and deep orbit.
In the latter case parts which normally form the side of the skull are
situated in front of the brain-case, between it and the orbit, and
generally reduced in extent, often replaced by membranes; especially
the interorbital septum may be reduced to membrane. The most constant
ossifications of this part of the skull are the _orbitosphenoids_
(14), which join the upper anterior margin of the alisphenoids. They
vary much with regard to their development--they are small in Gadoids;
larger in the Perch, Pike, Salmonoids, Macrodon, and the Clupeoids;
and very large in Cyprinoids and Siluroids, in which they contribute
to the formation of the side of the brain-case. The single Y-shaped
_Sphenoideum anterius_ (15) is as frequently absent as present; it
forms the anterior margin of the fossa for the hypophysis. Finally, the
_postfrontal_ (4) belongs also to this group of cartilage-bones.
The centre of the foremost part of the skull is occupied by the
_ethmoid_ (3), which shows great variations as regards its extent
and the degree of ossification; it may extend backwards into the
interorbital septum, and reach the orbitosphenoids, or may be confined
to the extremity of the skull; it may remain entirely cartilaginous,
or ossify into a lamina which separates the two orbits and encloses
an anterior prolongation of the brain-case, along which the olfactory
nerves pass: modifications occurring again in higher vertebrates. A
paired ossification attached to the fore-part of the ethmoid is the
_pre-frontals_ (2), which form the base of the nasal fossa.
2. _Membrane-bones attached to the primordial skull_.--To this group
belong the _parietals_ (7) and _frontals_ (1). The _squamosal_ (12) has
been mentioned above in connection with the mastoid. The _supraorbital_
is always small, and frequently absent. The lower surface of the skull
is protected by the _basisphenoid_ (parasphenoid) (6) and the _vomer_
(16), both of which, especially the latter, may be armed with teeth.[9]
3. _Cartilage bones of the alimentary portion of the visceral skeleton
of the skull_.--The suspensorium consists of three cartilage-bones,
and affords a base for the opercular apparatus as well as a point of
attachment to the hyoid, whilst in front it is connected with the
palato-pterygo-palatine arch. They are the _hyomandibular_ (23),
_symplectic_ (31), and _quadrate_ (26), connected by means of the
_metapterygoid_ (27) with the _ecto-_ (24) and _ento-pterygoid_ (25),
the foremost bone of the arch being the _palatine_ (22). All these
bones have been sufficiently described above (p. 55), and it remains
only to be mentioned that the bones of the palatine arch are but rarely
absent, as for instance in _Murænophis_; and that the symplectic
does not extend to the articulary of the mandible, as in _Amia_
and _Lepidosteus_, though its suspensory relation to the Meckelian
cartilage is still indicated by a ligament which connects the two
pieces. Of the mandibulary bones the _articulary_ (35) is distinctly
part of Meckel’s cartilage. Frequently another portion of cartilage
below the articulary remains persistent, or is replaced by a separate
membrane-bone, the angular.
4. _Membrane-bones of the alimentary portion of the visceral skeleton
of the skull_.--The suspensorium has one tegumentary bone attached
to it, viz. the _præoperculum_ (30); it is but rarely absent, for
instance in _Murænophis_. The _premaxillary_ (17) and _maxillary_ (18)
of the Teleostei appear to be also membrane-bones, although they are
clearly analogous to the upper labial cartilages of the Sharks. The
premaxillaries sometimes coalesce into a single piece (as in _Diodon_,
_Mormyrus_), or they are firmly united with the maxillaries (as in all
_Gymnodonts_, _Serrasalmo_, etc.) The relative position and connection
of these two bones differs much, and is a valuable character in the
discrimination of the various families. In some, the front margin of
the jaw is formed by the premaxillary only, the two bones having a
parallel position, as it has been described in the Perch (p. 53); in
others, the premaxillary is shortened, allowing the maxillary to enter,
and to complete, the margin of the upper jaw; and finally, in many no
part of the maxillary is situated behind the premaxillary, but the
entire bone is attached to the end of the premaxillary, forming its
continuation. In the last case the maxillary may be quite abortive.
The mobility of the upper jaw is greatest in those fishes in which the
premaxillary alone forms its margin. The form of the premaxillary is
subject to great variation: the beak of _Belone_, _Xiphias_ is formed
by the prolonged and coalesced premaxillaries. The maxillary consists
sometimes of one piece, sometimes of two or three. The principal
membrane-bone of the mandible is the _dentary_ (34), to which is added
the _angular_ (36) and rarely a smaller one, the _splenial_ or os
_operculare_, which is situated at the inside of the articulary.
5. _Cartilage-bones of the respiratory portion of the visceral skeleton
of the skull._--With few exceptions all the ossifications of the hyoid
and branchial arches, as described above (p. 58), belong to this group.
6. _Membrane-bones of the respiratory portion of the visceral skeleton
of the skull._--They are the following: the opercular pieces, viz.
_operculum_ (28), _sub-operculum_ (32), and _interoperculum_ (33). The
last of these is the least constant; it may be entirely absent, and
represented by a ligament extending from the mandible to the hyoid.
The _urohyal_ (42) which separates the musculi sternohyoidei, and
serves for an increased surface of their insertion; and finally the
_branchiostegals_ (43), which vary greatly in number, but are always
fixed to the cerato- and epi-hyals.
7. _Dermal bones of the skull._--To this category are referred some
bones which are ossifications of, and belong to, the cutis. They are
the _turbinals_ (20), the _suborbitals_ (19), and the _supratemporals_.
They vary much with regard to the degree in which they are developed,
and are rarely entirely absent. Nearly always they are wholly or partly
transformed into tubes or hollows, in which the muciferous canals with
their numerous nerves are lodged. Those in the temporal and scapulary
regions are not always developed; on the other hand, the series of
those ossicles may be continued on to the trunk, accompanying the
lateral line. In many fishes those of the infraorbital ring are much
dilated, protecting the entire space between the orbit and the rim of
the præoperculum; in others, especially those which have the angle of
the præoperculum armed with a powerful spine, the infraorbital ring
emits a process towards the spine, which thus serves as a stay or
support of this weapon (_Scorpænidæ_, _Cottidæ_).
The _pectoral_ arch of the Teleosteous fishes exhibits but a remnant of
a primordial cartilage, which is replaced by two ossifications,[10] the
_coracoid_ (51) and _scapula_ (52); they offer posteriorly attachment
to two series of short rods, of which the proximal are nearly always
ossified, whilst the distal frequently remain small cartilaginous
nodules hidden in the base of the pectoral rays. The bones, by
which this portion is connected with the skull, are membrane-bones,
viz. the _clavicle_ (49), with the _postclavicle_ (49 + 50), the
_supraclavicle_ (47), and _post-temporal_ (46). The order of their
arrangement in the Perch has been described above (p. 59). However,
many Teleosteous fish lack pectoral fins, and in them the pectoral arch
is frequently more or less reduced or rudimentary, as in many species
of _Murænidæ_. In others the membrane-bones are exceedingly strong,
contributing to the outer protective armour of the fish, and then the
clavicles are generally suturally connected in the median line. The
postclavicula and the supraclavicula may be absent. Only exceptionally
the shoulder-girdle is not suspended from the skull, but from the
anterior portion of the spinous column (_Symbranchidæ_, _Murænidæ_,
_Notacanthidæ_). The number of basal elements of each of the two series
never exceeds five, but may be less; and the distal series is absent in
Siluroids.
The _pubic_ bones of the Teleosteous fishes undergo many modifications
of form in the various families, but they are essentially of the same
simple type as in the Perch.