with the dorsal and ventral surfaces of the notochord, which, as they form
portions of a system of dorsal and ventral arches, are termed "arcualia"
(Fig. 111). On the dorsal side there are: (i.) a series of paired
_basi-dorsal_ cartilages (neurapophyses or neural arches), the two elements
of each pair contributing to form the side walls of the neural canal in
which the spinal cord is lodged (Fig. 112, A); (ii.) a series of
_inter-dorsal_ cartilages (intercalary neural arches), regularly
alternating with the preceding, and completing the walls of the neural
canal by filling up the intervals between the basi-dorsals; and (iii.) a
series of _supra-dorsal_ elements, typically also in pairs, but in the
Dog-Fish fused to form single median cartilages. Of the latter there are
two sets—one the _supra-basi-dorsals_, or neural spines, are situated over
the basi-dorsals; and the other, _supra-inter-dorsals_, alternating with
the former, lie over the inter-dorsals, the two series forming the
keystones of the dorsal arches, and thus completing the roof of the neural
canal. On the ventral side of the notochord this arrangement is
substantially repeated by a series of ventral arcualia, which, however, are
somewhat differently arranged in the trunk and tail. Thus, in the trunk
there are: (i.) a series of _basi-ventral_ or haemal cartilages,
corresponding with the basi-dorsals above, which grow out laterally into
short processes, the _parapophyses_ or transverse processes, and terminate
in (ii.) short, slender cartilages—the costal elements or _ribs_—which may
perhaps be regarded as the ventral equivalents of supra-basi-dorsals. The
ribs project outwards into the dorsal wall of the coelom and end in the
myocommata separating the myotomes of the body-wall. In the tail the
basi-ventrals lose their ribs and, growing downwards into ventral
prolongations, they unite in pairs beneath the caudal artery and vein, and
so form a series of inverted arches {195}(_haemal arches_) enclosing a
haemal canal (Fig. 112, B). The apex of each arch is prolonged into a
median process or _haemal spine_. Although not recognisable in the
Dog-Fish, paired _inter-ventral_ cartilages, corresponding with the
inter-dorsals above, are present in some Elasmobranchs and alternate with
the basi-ventrals. In the caudal region of others (_e.g._ Skates and Rays)
ventral counterparts of the supra-interdorsals are present, and are termed
_infra-ventral_ cartilages. Much in the same way that their dorsal
equivalents enclose a neural canal, so the ventral arcualia partially
surround the viscera-containing coelom in the trunk; and in the tail, but
more completely, the vestigial coelom of that region or the haemal canal.
[Illustration: FIG. 112.—A, transverse section of a precaudal vertebra; B,
similar section of a caudal vertebra. _h.a_, Haemal arch (basi-ventrals);
_h.c_, haemal canal; _h.s_, haemal spine; _n.c_, neural canal. Other
reference letters as in Fig. 111.]
The different vertebral components are by no means of equal morphological
value. The basi-dorsals and basi-ventrals, and the inter-dorsals and
inter-ventrals, are the primary elements and the most important. The
supra-dorsals are merely cartilages segmented off from the basi-dorsals and
inter-dorsals, while the ribs and the infra-ventrals are similarly derived
from the basi-ventrals and inter-ventrals respectively. As to the vertebral
elements which collectively form a vertebra in the Dog-Fish, it would seem
from evidence afforded by the neuromeres,[175] and more especially by the
facts of development, that each complete skeletal segment or vertebra
consists of a pair of basi-dorsals with the preceding pair of
inter-dorsals, and of a pair of basi-ventrals with the next succeeding pair
of inter-ventrals. It must be emphasised, however, that, considered as a
joint or segment in a flexible back-bone, a vertebra is a physiological
unit, the morphological value of which may differ widely in different
Fishes. Hence, in other Fishes, the grouping of vertebral components to
{196}form individual vertebrae may be quite different to that which takes
place in the Dog-Fish, and may even be accompanied by their more or less
complete fusion.
In the more primitive types of vertebral column, such as are characteristic
of many fossil and not a few existing Fishes, arcualia alone are present,
and remain associated with a persistent notochord which has grown with the
growth of the animal. In the more specialised Fishes, on the contrary, the
need of an axial support for the body, which, while retaining the necessary
flexibility, must possess greater strength, has resulted in the development
of a series of solid cartilaginous, calcified or bony, discoidal joints or
segments, the _centra_, which surround and more or less completely replace
the notochord, and, while supporting, form also a bond of connexion between
the dorsal and ventral arches. Notwithstanding their superficial
resemblance, an important developmental distinction is to be noted in the
mode of formation of centra in different Fishes, which enables one kind to
be distinguished as "_chorda-centra_," and another as "_arch-centra_."[176]
Chorda-centra are centra formed by the conversion of the chordal sheath
into a series of ring-like cartilaginous segments, which subsequently, by a
process of inward thickening, become biconcave, disc-like structures, and
more or less completely replace the notochord, except in the spaces between
them. Arch-centra, on the other hand, owe their formation to the growth of
the bases of the primary arcualia round the notochord, external to the
chordal sheath, and their subsequent fusion to form annular segments,
which, later, become biconcave centra. Of Fishes which possess vertebral
centra the Elasmobranchs alone have chorda-centra; the Holostei and the
Teleostei, and very probably the Crossopterygii also, having arch-centra.
The Dipnoi and the Holocephali, and the Chondrostean Teleostomi are
acentrous—that is, they are devoid of vertebral centra and possess a
persistent notochord. Neither in their embryonic development nor in their
evolution in time are the different vertebral components synchronous in
their appearance. Developmentally, the arcualia are the first to be formed,
and of these those on the dorsal aspect of the notochord appear earlier
than their representatives on the ventral side, while the centra are the
last of all; and in a general way the palaeontological sequence agrees with
the embryological.
{197}The independent evolution of a more specialised vertebral column from
a more primitive one may often be traced within the limits of the same
group of Fishes when the more ancient genera are compared with the more
recent. In the Elasmobranchs and the Crossopterygii, for example, the
oldest known types were acentrous, while the more recent have acquired
calcified or bony centra, and altogether they have reached a more advanced
stage of vertebral evolution. Some Fishes, like the Chondrostei and the
Dipnoi, seem, however, to exhibit comparatively little advance in vertebral
structure, since both the Palaeozoic and the living representatives of
these groups agree in being acentrous.
Some of the more notable features in the structure of the vertebral column
in the Cyclostomata and Fishes will now be briefly considered.
In the Cyclostomata the acentrous vertebral column is more primitive than
in any other Craniates, and in the Lamprey it consists of a persistent
notochord, supporting a series of isolated cartilages on each side of the
spinal cord.[177] As two pairs of these cartilages are included in each
neuromere it is possible that they represent alternating basi-dorsals and
inter-dorsals. There are no ventral arcualia in the trunk and no ribs. In
the Hag-Fish (_Myxine_) the dorsal cartilages are restricted to the tail.
The description of the vertebral column of the Dog-Fish may be taken as
fairly applicable to Elasmobranchs in general, and hence only certain
notable features in some other members of the group need be referred to
here. The most primitive Elasmobranchs, the Palaeozoic genera
_Cladoselache_ and _Pleuracanthus_ were acentrous, although calcified rings
have been observed in a Permian species of the latter genus and scattered
calcifications in others. Some of the earlier Mesozoic genera (e.g.
_Hybodus_) were also devoid of centra, at least in the trunk-region. The
first indication of complete centra occurs in the Lower Lias Cestraciont,
_Palaeospinax_.[178] All the later extinct, as well as all existing forms,
have more or less well-developed centra, hardened by the deposit of lime
salts in their primitively cartilaginous substance, but never in the form
of true bone.
{198}[Illustration: FIG. 113.—Schematic transverse section through the
middle of a Cyclospondylic (A), a Tectospondylic (B), and an
Asterospondylic vertebra (C). _d_, Middle portion of the calcified double
cone; _d′_, additional concentric calcified layers; _d″_, double cone with
radiating calcified layers; _ex.m_, external elastic membrane; _h.a_,
haemal arch; _n.a_, neural arch; _n.c_, notochordal cavity. (From Zittel,
after Hasse.)]
The mode in which the lime is deposited is marked by certain peculiarities
which are characteristic of particular families[179] (Fig. 113). In some
genera, as in the extinct _Palaeospinax_ and the living _Acanthias_ and
_Scymnus_, the calcified portion of each centrum takes the form of a
cylinder constricted across the middle, like two cones joined apex to apex
(cyclospondylic). This condition is probably the most primitive, but it may
be modified in other genera by the further addition of calcic salts in two
different ways. Thus, the deposit may take place by the simple addition of
concentric layers to the original constricted cylinder (tectospondylic), as
in the Skates and Rays; or it may take the form of a series of longitudinal
plates radiating outwards from the cylinder, and giving rise to a star-like
pattern in cross-section (asterospondylic), as in _Scyllium_ and _Lamna_.
In most living Elasmobranchs (e.g. _Scyllium_), but not in such genera as
_Notidanus_, _Heterodontus_, and _Squatina_, the bases of the dorsal and
ventral arches grow round the centra and meet, or even fuse, so that the
latter become surrounded by rings of cartilage which, after a fashion,
suggest incipient arch-centra (Fig. 112, A). The caudal portion of the
vertebral column is often described as "diplospondylic," that is, there are
two centra, two pairs of basi-dorsals, two pairs of inter-dorsals, and two
pairs of basi-ventrals, {199}or in other words, two vertebrae to each
neuromere[180] (Fig. 111, B).
The Holocephali have a vertebral column essentially similar to that of
other Elasmobranchs, but of a more primitive type (Fig. 114). The notochord
is persistent and there are no centra; but ring-like calcifications, four
or five to each neuromere, occur in the chordal sheath in _Chimaera_,
although not in _Callorhynchus_. Ribs are absent. In the whip-like terminal
portion of the tail the arcualia and the notochord become replaced by a
slender continuous filament of cartilage.
[Illustration: FIG. 114.—A, transverse section of the vertebral column of
_Chimaera monstrosa_; B, lateral view, _c.r_, Calcified ring; _h.r_,
basi-ventral; _int_, inter-dorsal; _n.a_, neural arch (basi-dorsal); _nch_,
notochord; _nch.sh_, chordal sheath; _n.sp_, neural spine (supra-dorsal).
(From Parker and Haswell, after Hasse.)]
In the more obvious features of vertebral structure the Dipnoi[181] have
much in common with the Elasmobranchs, especially with certain of the
acentrous Palaeozoic representatives of that group. The notochord is
persistent, centra are wanting, and the different vertebral components
continue to retain their primitive distinctness. On the other hand, the
basi-dorsals are much better developed than the inter-dorsals, which are
either vestigial or absent. The basi-dorsals unite in pairs over the spinal
cord to form complete neural arches, and each arch supports dorsally the
legs of a Λ-shaped, gable-like element or neural spine, which probably
represents a pair of fused supra-basidorsals. Ventrally, there are
basi-ventral cartilages, fused in pairs beneath the notochord, and
supporting well-developed, bone-ensheathed ribs. {200}Inter-ventrals appear
to be absent. Each neuromere corresponds with a pair of basi-ventrals, of
basi-dorsals and of inter-dorsals. The haemal arches and spines are formed
partly by the basi-ventrals, but mainly by the ventral union of the
successive pairs of ribs. As in the Holocephali, the terminal arcualia of
the tail become fused into a straight axial cartilaginous filament,
transversely divided into segments, which replaces the notochord. Each
segment supports a variable number of dorsal and ventral gable-pieces, or
neural and haemal spines. Certain of the vertebral components, such as the
ribs, and the neural and haemal spines, are ensheathed by membrane bone.
[Illustration: FIG. 115.—Side view of the precaudal vertebrae of a Sturgeon
(_Acipenser sturio_). _a.c_, Aortic canal, formed by the median union of
ingrowths from the basi-ventrals and inter-ventrals of opposite sides;
_b.d_, basi-dorsals; _b.v_, basi-ventral; _i.d_, inter-dorsal; _i.v_,
inter-ventral; _n_, notochord; _n.c_, neural canal; _n.sp_, neural spine;
_nt.s_, cuticular sheath of the notochord; _p_, parapophysis; _r_, rib;
_s.n_, aperture for the root of a spinal nerve.]
With certain modifications in details the preceding description will also
apply to the vertebral column of the Chondrostei (Fig. 115). It will be
noted, however, that the inter-dorsals are much better developed than in
the Dipnoi, although when {201}compared with the basi-dorsals they take but
a small share in forming the walls of the neural canal. Well-developed but
somewhat fragmentary inter-ventrals are present. The haemal arches and
spines are formed by the downgrowth and ventral union of the basi-ventrals
as in the Dog-Fish, and apparently without the aid of costal elements. In
_Polyodon_ the ribs are vestigial,[182] but in _Acipenser_ they are well
developed. The neural arches and spines, and their haemal representatives
in the tail, and also the ribs, are partially ossified, or ensheathed by
bone.
[Illustration: FIG. 116.—Diagram to illustrate the grouping of vertebral
elements to form vertebrae, A, in an Elasmobranch, B, in _Amia_, and C, in
_Lepidosteus_. _B.D_, Basi-dorsals; _B.V_, basi-ventrals; _I.D_,
inter-dorsals; _I.V_, inter-ventrals; _in.v.c_, inter-vertebral cartilage
divided by a concavo-convex cleft; _p.c_, precentrum; _pt.c_, postcentrum.
The square blocks represent individual vertebrae, and the oblique lines,
the attachments of the myocommata.]
In the existing Crossopterygii, Holostei, and Teleostei, popularly known as
the "bony Fishes," the vertebral column assumes a more familiar character,
and at the same time we meet with interesting illustrations of the
different methods by which the separate component vertebral elements of the
more primitive types of "backbone" are concentrated together in groups, and
fused to form that complex physiological product, the complete bony
vertebra.[183] In most of these Fishes each vertebra is formed by the
aggregation and fusion of a pair of basi-dorsals and a pair of
basi-ventrals, and includes, in addition, a pair of inter-dorsals, which
may either be the pair in front of the basi-dorsals or the pair behind, and
also a pair of inter-ventrals, which, similarly, may be the {202}pair in
front or behind the basi-ventrals (Fig. 116). The product of this fusion is
a series of bony vertebrae, each consisting of a biconcave arch-centrum,
which includes the fused basal portions of a pair of basi-dorsals and a
pair of basi-ventrals. The distal portions of the basi-dorsals form the
neural arch, while the rib-bearing parapophyses are lateral outgrowths from
the basi-ventrals which otherwise have become merged in the centrum.
Finally, the centrum is completed by its fusion with a pair of
inter-dorsals and a pair of inter-ventrals. Supra-dorsal elements may also
be included as minor contributory factors. The supra-basi-dorsals co-ossify
with their basi-dorsals and then unite to form the ordinary unpaired neural
spine of most bony Fishes, or, as in _Amia_, they remain distinct from each
other, and are obvious as a double spine. In _Lepidosteus_ these elements
co-ossify with the neural arches and form the post-zygapophyses.
Supra-inter-dorsals have been identified in the embryo as distinct
elements, but their eventual fate is not always known. In _Lepidosteus_
they persist as distinct cartilages in the adult (Fig. 118, A).
Well-developed bony ribs are usually present. The haemal arches of the tail
are formed by the downgrowth of the parapophyses and their ribs, or by the
latter alone, and by their ventral union to form haemal spines;
consequently, each arch always includes a pair of costal elements. With
such general features in common there are certain notable variations in
some of these Fishes, to which brief reference may be made.
Little is at present known of the development of the vertebral column in
either of the only two existing genera of Crossopterygii, _Polypterus_[184]
and _Calamichthys_, and hence the precise mode of grouping of their
vertebral components to form vertebrae is unknown. The condition of the
vertebral column in the fossil forms varies greatly in different families,
but in none is it so specialised as in the surviving members of the group.
In the Devonian Holoptychidae, and even in genera so comparatively recent
as the Upper Cretaceous Coelacanth _Macropoma_, the persistence of the
notochord and the absence of centra indicate a very primitive grade of
vertebral evolution. The Devonian and Carboniferous Rhizodontidae (e.g.
_Eusthenopteron_ and _Rhizodus_), on the contrary, seem to have had
well-ossified ring-like vertebrae.
{203}In the caudal region of _Amia_ the basi-dorsals and basi-ventrals, and
the inter-dorsals and inter-ventrals, form separate arch-centra which
remain distinct; hence each vertebra is double, and there is a regular
alternation of arch-bearing "pre-centra" and arch-less "post-centra" (Fig.
117, D). In the trunk-region the pre- and post-centra have fused, and in
this region the vertebrae are single.
[Illustration: FIG. 117.—A, precaudal vertebrae of _Caturus furcatus_; B,
similar vertebrae of _Eurycormus speciosus_; C, caudal vertebrae of the
latter species; D, caudal vertebrae of _Amia calva_. _h.a_, Haemal arch;
_h.sp_, haemal spine; _hy.c_, hypo-centrum; _n.a_, neural arch; _n.sp_,
neural spine; _p_, parapophysis; _p.c_, pre-centrum; _pl.c_,
pleuro-centrum; _pt.c_, post-centrum; _r_, rib. (After Zittel.)]
A very primitive type of vertebral column occurs in some of the Jurassic
allies of _Amia_, in which certain of the vertebral components, confluent
in the adult _Amia_, retain some measure of their primitive
distinctness.[185] Thus, in the precaudal region of _Eurycormus_ (Fig. 117,
B) there is a series of alternating dorsal and ventral half-rings of bone,
which between them completely invest the persistent notochord. Each ventral
half-ring or "hypocentrum" represents a pair of fused and ossified
basi-ventrals, and possibly also a pair of included inter-ventrals, and
supports dorsally a pair of basi-dorsals, forming a neural arch, and
laterally a pair of ribs. The dorsal semi-rings, or "pleuro-centra,"
similarly represent fused and ossified pairs of inter-dorsals. In the tail,
modifications approximating to what is seen in the caudal region of _Amia_
are to be noticed (C). By the upgrowth of the ventral arch-bearing
semi-rings, and their conversion into complete rings encircling the
notochord, incipient pre-centra are formed, and by a similar modification
of the {204}down-growing, archless, dorsal half-rings, structures
comparable to post-centra are produced. In brief, _Eurycormus_, as well as
such other extinct Amioid genera as _Caturus_ (Fig. 117, A), _Callopterus_,
and _Euthynotus_, retain in the adult a stage of vertebral evolution which
is closely paralleled by transitory stages in the embryonic and young forms
of _Amia_.
[Illustration: FIG. 118.—A, two vertebrae from the trunk-region of
_Lepidosteus_; B, anterior face of a vertebra. _c.n_, Anterior convex face
of the centrum; _c.n′_, posterior concave face; _h.a_, parapophysis, with
its articular facet for a rib; _i.c_, median cartilage, representing a pair
of fused supra-interdorsals; _i.s_, radial element of the dorsal fin;
_l.l_, superior longitudinal ligament; _n.a_, neural arch. (From
Wiedersheim, after Balfour and W. N. Parker.)]
_Lepidosteus_[186] is unique amongst existing Fishes in having
opisthocoelous vertebrae; that is, the centra are convex in front and
concave behind, and therefore articulate with one another by
ball-and-socket joints (Fig. 118). This condition is due to the development
of a series of intervertebral rings of cartilage round the notochord. The
subsequent inward growth of each of these rings leads to the constriction,
and ultimately to the complete obliteration, of the notochord, much in the
same way as by the growth of ordinary centra. Later, this solid mass of
cartilage becomes transversely divided by a cleft which is convex
anteriorly and concave behind (Fig. 116, C), and of the two portions one
fuses and co-ossifies with the centrum of the vertebra in front, and the
other with the one pertaining to the vertebra behind. Reference to Fig. 116
will show that the grouping of the vertebral elements to form the
individual vertebrae is not the same as in _Amia_.
{205}In the dominant group of existing Fishes, the Teleostei, the centra
are almost invariably biconcave, although in the Eels they may be flat or
even slightly convex in front. Ribs are absent in the Syngnathidae and in
the Plectognathi. In addition to the usual articulation between the centra,
the vertebrae often articulate with one another by means of paired
processes arising from the anterior margin of each neural arch, or from the
centrum at the base of the arch (pre-zygapophyses), and meeting similar
processes which project either from the hinder margin of the arch of the
vertebra in front, or from the adjacent portion of its centrum
(post-zygapophyses). The haemal arches may have similar processes (Fig.
119). One, two, or in some Teleosts, three pairs of slender intermuscular
bones radiate outwards from the centra into the myocommata (epicentrals),
or from the neural arch (epineurals), or from the ribs (epipleurals).
[Illustration: FIG. 119.—A, side view of precaudal vertebrae of a Cod
(_Gadus morrhua_) without the ribs; B, similar view of caudal vertebrae of
the same Teleost. _c_, Centrum; _h.a_, haemal arch; _h.sp_, haemal spine;
_n.a_, neural arch; _n.sp_, neural spine; _p_, parapophysis; _p.z_,
pre-zygapophysis; _pt.z_, post-zygapophysis.]
THE RIBS.—It is doubtful if the structures termed "ribs" are homologous in
the different groups of Fishes. There appear to be two kinds,
distinguishable as dorsal and ventral ribs (Fig. 156). Dorsal ribs are
situated in the fibrous tissue separating the epiaxial from the hypaxial
muscles of the body wall, and they take no part in forming the haemal
arches of the caudal region. Ventral ribs, on the other hand, always lie
internal to the hypaxial muscles, and directly external to the peritoneal
lining of the coelom, and they usually contribute to the formation of
{206}the haemal arches. To the former belong the ribs of the Elasmobranchs,
and to the latter the ribs of the Teleostomi and Dipnoi. _Polypterus_ alone
has both kinds of ribs.
THE SKULL.
The skull is a highly complex structure, the various components of which
are as different physiologically as they are morphologically. It consists
(i.) of the _cranium_, for the enclosure and protection of the brain; (ii.)
of _sense capsules_, which fulfil a like function for the auditory, visual,
and olfactory organs; (iii.) of certain _vertebrae_ or vertebral elements
fused with the hinder part of the cranium; (iv.) of a series of _visceral
arches_; and (v.) of a series of paired or median cartilages developed in
relation with the mouth and nostrils, which may be collectively spoken of
as "_labial_" cartilages.
The cranium is formed in the embryo from two pairs of cartilaginous rods or
plates, developed in the mesoblast of the head. Of these the posterior
pair, or _parachordals_, underlie the hinder part of the brain, and are
situated one on each side of the cranial portion of the notochord. The
anterior pair or _trabeculae_ are pre-notochordal, and lie beneath the
anterior portion of the brain.[187] Between their hinder extremities, and
in front of the anterior termination of the notochord, is the pituitary
body. As development proceeds the parachordals blend with each other and
with the trabeculae, while the latter fuse in front to form a median
plate—the mesethmoid cartilage. The hinder portions of the two trabeculae
remain distinct for some time, and enclose between them the pituitary
fontanelle, but later they fuse beneath the pituitary body, leaving,
however, a pit for its reception—the pituitary fossa. Cartilaginous
capsules are formed round the cranial sense organs. The _auditory or
periotic capsules_ fuse on each side with the parachordals. The _optic
capsules_, either fibrous or cartilaginous, remain free, and do not fuse
with the adjacent trabecular region. The olfactory capsules alone are not
developed independently, but are formed as lateral {207}outgrowths from the
mesethmoid plate. Later, the parachordals and trabeculae grow upwards on
each side round the brain, and to a greater or less extent they meet and
fuse on its dorsal surface, thus enclosing the latter organ in a cranial
cavity, leaving, nevertheless, a large foramen behind (foramen magnum)
through which the brain is continuous with the spinal cord. In this
condition the primitive cartilaginous cranium, with its included
sense-capsules, has reached a stage which is permanently retained in such
Fishes as the Elasmobranchs.
The _visceral arches_ consist of a number of pairs of curved rods of
cartilage, at first simple, but subsequently segmented, and developed in
the splanchnic mesoblastic walls of the oral cavity and pharynx. Each rod
is connected with its fellow by a median cartilage in the floor of the
pharynx, so that the whole form a series of dorsally incomplete hoops
encircling the anterior portion of the alimentary canal. No doubt all the
visceral arches were originally branchial arches, and were so disposed
between the successive gill-clefts as to support their walls and the
vascular folds or gill-lamellae to which they gave rise. In Fishes most of
the arches still retain their primitive gill-supporting function, but the
first or mandibular arch has become modified to form upper and lower jaws,
although in the Sharks and Dog-Fishes it may lie in front of a gill-cleft
and still be associated with vestigial gills. The second or hyoid arch is
less removed from the condition of a branchial arch, and generally supports
either a functional or a vestigial gill, but in most Fishes it has acquired
the secondary function of forming a suspensorium for the attachment of the
jaws to the cranium.
The skull of the common Dog-Fish, _Scyllium canicula_ (Fig. 120),[188] may
be studied as a type which in the adult remains cartilaginous, and has no
secondary addition of cartilage- or membrane-bones. In this Fish the
chondrocranium, or primary cartilaginous cranium, presents the appearance
of a somewhat depressed oblong box, which has a complete roof, side-walls,
and floor, but is open in front (_anterior cranial fontanelle_) and also
behind (_foramen magnum_). The hinder, or parachordal portion of the
cranium surrounds the foramen magnum, and there forms the _occipital
region_. At the ventral margin of the foramen there are two prominences, or
_occipital condyles_, for articulation with the first vertebra, and between
them the remains of the notochord are traceable into the cranial floor.
{208}[Illustration: FIG. 120.—Side view of the skull of the common Dog-Fish
(_Scyllium canicula_). _aud.cp_, Auditory capsule; _br.a_ 1, 5, branchial
arches; _br.r_, _br.r′_, cartilaginous rays attached to the hyoid arch and
the first four branchial arches; _Cr_, cranium; _ex.br_, extra-branchial
cartilages; _hy.cn_, cerato-hyal; _hy.m_, hyomandibular; _lb_, labial
cartilages; _lg_, ligaments passing from the jaws to the cranium and to the
distal end of the hyomandibular; _lg′_, ethmo-palatine ligament; _l.j_,
lower jaw or Meckel's cartilage; _Nv_. 2, optic foramen; _Nv_. 5, foramen
for the Vth and part of the VIIth cranial nerves; _olf.cp_, olfactory
capsule; _or_, orbit; _up.j_, upper jaw or palato-quadrate cartilage. (From
Wiedersheim, after W. K. Parker.)]
In front of the occipital region two lateral bulgings indicate the
_periotic capsules_, and more anteriorly still, in the trabecular region,
the sides of the cranium are modified to form two spacious lateral
recesses, the _orbits_, each of which is bounded above and below by
supra-orbital and infra-orbital ridges respectively, behind by an outgrowth
from the periotic capsule (_post-orbital process_), and in front by a
similar projection from the hinder wall of the olfactory capsule (_lateral
ethmoidal process_). In front of the cranial cavity and the orbits may be
seen the laterally-placed dome-like _olfactory capsules_, which are open
below, where the nasal sacs communicate with the exterior. Between the two
capsules an anterior extension of the cranial floor forms a flattened
_mesethmoidal plate_, behind which is the large, membrane-closed, _anterior
cranial fontanelle_. The lateral walls of the cranium are perforated by
numerous apertures, some of which serve for the entrance or exit of
blood-vessels, and others, mostly pertaining to the inner walls of the
orbits, for the transmission of the different cranial nerves from the brain
to {209}various parts of the head. In many Elasmobranchs the roots of
certain of the anterior spinal nerves perforate the side-walls of the
occipital region, and indicate the fusion of vertebral components with the
cranium. In the cranial roof between the two periotic capsules there are
two small apertures at the bottom of a common median depression: through
each aperture the ductus endolymphaticus (_aqueductus vestibuli_) passes
from the vestibular part of the auditory organ to the exterior of the
skull.
Three cartilaginous rods, one from the roof of each olfactory capsule, and
one, the _prenasal_ or _rostral_ process, from the ethmoid cartilage,
converge and meet, or nearly meet, in front to form the rostrum or support
for the preoral or "cut-water" portion of the head.
The visceral arches are seven in number. The first or _mandibular arch_
consists on each side of an upper portion, the _palato-pterygo-quadrate_ or
_palato-quadrate_ cartilage, which passes forwards in the side-wall of the
oral cavity, along the upper margin of the mouth, its anterior or palatine
part curving inwards to a ligamentous connexion with its fellow beneath the
cranial floor. Each cartilage has an upwardly directed process
(_ethmo-palatine process_) which is connected by a suspensory
_ethmo-palatine ligament_ with the lateral wall of the cranium behind the
lateral ethmoid process. The lower or ventral half of the mandibular arch
(_Meckel's cartilage_) is similar in shape to the upper; it articulates
behind with the quadrate portion of the latter by a movable joint, and is
thence prolonged forwards and downwards in relation with the lower margin
of the mouth to a median ligamentous union with its fellow of the opposite
side. The palato-pterygo-quadrate and Meckel's cartilages together form the
primitive upper and lower jaws, and support the teeth. The _hyoid arch_
also consists of a dorsal and a ventral half on each side. The dorsal half
or _hyomandibular_ element articulates above with the periotic capsule. The
ventral portion, or _cerato-hyal_, passes downwards and is connected with
its fellow by a median copula or _basi-hyal_ cartilage situated in the
floor of the oral cavity. A series of simple cartilaginous rays (_branchial
rays_) are attached to the hinder margins of the hyomandibular and
cerato-hyal elements. The distal end of the hyomandibular is connected by
strong ligaments with the hinder portions of both the
palato-pterygo-quadrate cartilage and Meckel's cartilage; in fact, {210}the
hyomandibular is the effective suspensorium by which the upper and lower
jaws are connected with the skull, and all Fishes in which this arrangement
exists are said to be _hyostylic_.[189] Behind the hyoid arch follow five
_branchial arches_. Each of these is segmented into a dorsal or
_pharyngo-branchial_ element, followed by an _epi-_, a _cerato-_, and a
_hypo-branchial_ piece, but the later element is absent in the fifth arch.
The lateral halves of the last three arches are connected ventrally by a
large median basi-branchial cartilage, but in the first and second arches
by the median apposition of their respective hypo-branchial elements. Like
the hyomandibular and cerato-hyal segments of the hyoid arch, the epi- and
cerato-branchial elements of all the branchial arches except the fifth are
fringed along their outer convex margins by a series of _branchial rays_,
and, in addition, there are three pairs of slender, curved, cartilaginous
rods, or _extra-branchials_, in relation with the distal extremities of the
branchial rays of the second, third, and fourth branchial arches. The
function of the branchial arches, and their branchial rays, and
extra-branchial cartilages, is to support the inter-branchial septa which
separate the gill-clefts and carry the vascular gill lamellae. All the
arches lie near the inner margins of the septa, close to the hypoblastic
epithelium of the pharynx, while the outer portions of the septa are
supported by the branchial rays and the extra-branchials, the latter lying
directly beneath the external skin. The segments of the arches are movably
connected with one another by ligaments; and by the contraction of the
branchial muscles the arches may be separated or approximated so as to
enlarge or diminish the size of the intervening clefts.
The _labial cartilages_ are represented by a pair of slender rods in
relation with the outer surfaces of the palato-pterygo-quadrate cartilages,
and a similar pair in connexion with the Meckelian cartilages. There is
also a pair of small cartilages in relation with the nostrils. It is
probable that the rods which constitute the lateral elements of the rostrum
belong to the same category.
In the Cyclostomes and the Elasmobranchs the skull is entirely
cartilaginous, although it may often be superficially calcified in
Elasmobranchs, and although there may even be definitely and symmetrically
arranged calcified plates in _Pleuracanthus_, true bone is never present.
In many Fishes, and notably in the Teleostomi, {211}the embryonic
cartilaginous cranium becomes complicated by the addition of an extensive
series of investing membrane bones, formed by the ossification of the
connective tissue external to the cartilage, so that a secondary bony
cranium is formed external to the primary cranium much in the same way that
a secondary pectoral girdle is formed in connexion with the primary girdle.
Such bones probably owe their primary origin to the fusion and insinking of
exoskeletal structures (scales or dermal spines). To these investing bones
there may also be added a series of bones formed by the actual conversion
of the cranial cartilage into osseous tissue (cartilage bones), which to a
greater or less extent in different Fishes replaces the original cartilage.
The bones of the skull may conveniently be classified as follows:—(i.)
_Dermal or membrane bones_. Under this head are included—(_a_) the ordinary
investing bones of the skull. (_b_) _Tooth-bones_, that is, bones formed by
the fusion of the bases of teeth and developed in relation with the walls
of the oral cavity. Probably all tooth-bearing bones are of this nature.
(_c_) _Sensory canal bones_, that is, tubular bones developed round the
sensory canals of the head. Certain of these bones may secondarily acquire
the shape and character of investing bones while still retaining protective
relations to their sensory canals. (ii.) _Cartilage bones_.
As an easily obtainable example of a skull which has acquired a fairly
complete series of both cartilage- and membrane-bones, while retaining a
well-developed primary cranium, the skull of the Salmon (_Salmo salar_) may
be described.[190] At an early stage of development, even so late as the
second week of hatching, the primary cranium is still entirely
cartilaginous, and in this condition the Salmon's skull is comparable with
that of an adult Dog-Fish. As development proceeds the primary cranium
becomes supplemented by the addition of numerous investing dermal bones
which form the secondary cranium, and later cartilage bones appear and, to
a considerable extent, replace the original cartilage. The Salmon's skull
is interesting in this respect, that the primary cranium grows with the
growth of the Fish, so that in the adult the nasal, ethmoidal, and prenasal
regions are entirely cartilaginous, and in the hinder part of the cranium
cartilage is largely persistent between the cartilage bones.
Dealing first with the cartilage bones of the primary cranium, {212}it may
be stated that there are formed in that part of the parachordal cartilage
surrounding the foramen magnum a median _basioccipital_ below, which is
concave behind where it articulates with the centrum of the first vertebra,
a _supraoccipital_ above, and two laterally-placed _exoccipital_ bones
(Figs. 121, 122). Each periotic capsule is ossified by the formation of
five bones in the primitively cartilaginous mass, the _prootic_,
_sphenotic_, _opisthotic_, _epiotic_, and the _pterotic_. The inner walls
of the capsules have atrophied in the adult, and hence the cavities which
contain the auditory organs appear as open lateral recesses of the cranial
cavity. In front of the periotic capsules there are various bones which are
formed in the cartilage of the trabecular part of the cranium. Thus, in
front of the basi-occipital, and developed in the cartilage of the cranial
floor, there is a median Y-shaped _basisphenoid_, and, at some distance
above it on each side, an _alisphenoid_ helps to form the lateral wall of
the cranial cavity. Between the eyes the side walls of the cranium fuse to
form a vertical inter-orbital septum, and, in consequence, two
_orbito-sphenoid_ bones, which normally form the lateral cranial walls in
this region, become partially confluent in the median line and close the
cranial cavity in front. The only cartilage bones found in the massive
persistent portion of the primary cranium which forms the pre-orbital
region are the projecting _lateral ethmoids_, forming the posterior
boundaries of the recesses for the olfactory organs, and separating the
latter from the orbits.
[Illustration: FIG. 121.—Side view of the cranium of a Salmon (_Salmo
salar_). Most of the membrane bones and the jaws have been removed. The
cartilage is dotted. _al.s_, Alisphenoid; _bo_, basioccipital; _bs_,
basisphenoid; _eo_, exoccipital; _ep_, epiotic; _l.eth_, lateral ethmoid;
_ol_, olfactory capsula; _op_, opisthotic; _o.s_, orbito-sphenoid; _pr.o_,
prootic; _ps_, parasphenoid; _pt.o_, pterotic; _so_, supraoccipital;
_sp.o_, sphenotic; _t.c_, trabecular cornu; _u.l.c_, _u.l.c_^2, first and
second upper labial cartilages; _v_, vomer; II, foramen for the optic
nerve. (From W. K. Parker.)]
The roof and floor of the primary cranium is completed by {213}certain
investing dermal bones (Fig. 123, A). A pair of large _frontal_ bones form
the cranial roof, and also help to roof in the orbital cavities. Behind the
frontals, and separated from each other by the supraoccipital, there is a
pair of small _parietals_, and anterior to the frontals a median _dermal
mesethmoid_. A small _nasal_ bone overlies each olfactory recess.
Ventrally, the base of the cranium, from the basi-occipital to the prenasal
region, is strengthened by a large _parasphenoid_ behind, and a much
smaller _vomer_ in front, both of which lie in the roof of the mouth. The
vomer is a tooth-bone, and probably the parasphenoid also.
[Illustration: FIG. 122.—Vertical and longitudinal section of the cranium
of _Salmo salar_, showing the right half of the cranial cavity. Cartilage
is dotted. _f_, Frontal; _v′_, fat-containing cavity in the mesethmoid
cartilage; V, VII, IX, X, foramina for the fifth, seventh, ninth, and tenth
cranial nerves. Remaining reference-letters as in Fig. 121. (From W. K.
Parker.)]
The mandibular arch (Fig. 123, B) is more modified than that of the
Dog-Fish. The palato-pterygo-quadrate bars, or primitive upper jaw, no
longer meet in front beneath the cranial floor, but each separately
articulates in front with the lateral ethmoid of its side. Although still
partly cartilaginous each bar is largely replaced either by cartilage
bones, or by bones which begin as membrane bones or as tooth-bones and
complete their growth by invading the cartilage and becoming in part
cartilage bones. Its anterior portion is formed by a _palatine_ bone which
articulates with the lateral ethmoid, and the middle portion by a
_pterygoid_ and a _mesopterygoid_ bone, while the hinder part is ossified
above as a _metapterygoid_ and below as a _quadrate_. The latter
articulates with the lower jaw. Functionally, however, the primitive upper
jaw is now replaced by a secondary upper jaw, formed on each side by a
series of tooth-bones, situated external to the former, and meeting in
front of the prenasal region of the primary cranium (Fig. 123, A). The
series includes a dentigerous _premaxilla_ and _maxilla_, and a small
toothless, scale-like _jugal_ bone. Each half of the lower jaw (Fig. 123,
A, B) consists of a rod-like Meckel's cartilage or primary lower jaw.
{214}[Illustration: FIG. 123.—A, view of the left side of the skull of a
Salmon; B, the left half of the primary upper and lower jaws, and the hyoid
arch. The cartilage is dotted. _an_, Angular; _ar_, articular; _b.hy_,
basi-hyal; _br.r_, branchiostegal rays; _c_, cranium; _c.h_, cerato-hyal;
_c.or_, circum-orbital bones; _d_, dentary; _d.eth_, dermal mesethmoid;
_ep.h_, epihyal; _ep.o_, epiotic; _eth.p_, ethmo-palatine process; _f_,
frontal; _h.hy_, hypo-hyal; _hym_, hyomandibular; _i.op_, inter-operculum;
_j_, jugal; _mks_, Meckel's cartilage; _mpg_, mesopterygoid; _mt.pg_,
metapterygoid; _mx_, maxilla; _n_, nasal; _op_, operculum; _op′_, condyle
on the hyomandibular for the operculum; _orb_, orbit; _p_, parietal; _pa_,
palatine; _p.mx_, premaxilla; _p.op_, pre-operculum; _pt_, pterygoid;
_pt.o_, pterotic; _q_, quadrate; _so_, supra-occipital; _s.op_,
suboperculum; _sp.o_, sphenotic; _s.t_, supra-temporal (or squamosal);
_st.hy_, stylo-hyal; _sy_, symplectic; _u.l.c_, _u.l.c′_, upper labial
cartilages; _u.l.c^2_, second upper labial. (From W. K. Parker.)]
The hinder part of this is ossified to form the _articular_, which has a
deeply concave surface for articulation with the quadrate; and below this
there is a small membrane bone, the _angular_. The rest of the cartilage is
partially {215}ensheathed on its outer side by a large tooth-bone, the
dentigerous _dentary_. The hyoid arch is similar to that of the Dog-Fish,
except that its primitively cartilaginous segments are almost completely
ossified (Fig. 123, B). The large upper segment or _hyomandibular_ bone
articulates mainly with the pterotic, but partly also with the sphenotic
element of the periotic capsule; below it is connected with a slender
_symplectic_ bone, and from the cartilage connecting the two depends the
rest of the hyoid arch, consisting in succession of _stylo-hyal_,
_epi-hyal_, _cerato-hyal_, and _hypo-hyal_ bones, with a median
teeth-bearing _basi-hyal_. The palato-pterygo-quadrate bar has no direct
connexion with the skull, except anteriorly where its palatine element
articulates with the lateral ethmoid. The real suspensorium is formed by
the hyomandibular and symplectic bones, to which the hinder margins of the
quadrate and metapterygoid bones are rigidly attached by suture, hence, as
in the Dog-Fish, the skull is hyostylic. Behind the hyoid arch there are
five branchial arches, which generally resemble those of the Dog-Fish,
except that their component segments are ossified as cartilage bones.
Connected with the hyomandibular and cerato-hyal elements of the hyoid arch
there is, on each side, a series of membrane bones for the support of the
movable operculum or gill-cover. These consist of an _operculum_ above,
which articulates with a backwardly projecting process from the
hyomandibular, followed in succession below by a _sub-operculum_ and an
_inter-operculum_, the latter being connected by ligament with the angle of
the lower jaw. The series is completed by ten sabre-shaped _branchio-stegal
rays_, which are attached to the cerato-hyal and support the lower margin
of the gill-cover.
Sensory canal bones are represented in the Salmon by a ring of small bony
plates which encircle the orbit (Fig. 123, A), and by one or two small
bones situated above and on the outer side of each periotic capsule
(_squamosals_). To these may be added the _pre-operculum_ situated external
to the hinder margins of the hyomandibular and quadrate bones, firmly
clamping these bones together, and also the _post-temporals_, by which the
secondary pectoral girdle is attached to the skull. The _nasal_ bones may
also be regarded as pertaining to the same series.
In other Fishes with a more or less complete bony skull there are certain
additional cartilage- and membrane-bones which are not {216}present in the
Salmon. There is usually a median ossification of the ethmoid cartilage,
the _mesethmoid_. An _entopterygoid_ is sometimes added to the
palato-pterygo-quadrate series of bones. An ossification of the anterior
extremity of each Meckelian cartilage may form a _mento-Meckelian_ bone.
Certain additional membrane bones are sometimes developed in relation with
the lower jaw, such as _splenial_ and _coronary_ bones on the inner side,
and a _supra-angular_ bone at the angle of the jaw, above the angular
element. To these there may be added the singular series of
_infra-dentaries_, which in some fossil Crossopterygii (e.g. _Rhizodopsis_)
fringe the outer margin of the jaw beneath the true dentary (Fig. 274, A).
A system of _jugular plates_ may also form a characteristic armature for
the throat between the lateral halves of the lower jaw (Fig. 274, C).
Besides those already mentioned, additional sensory canal bones are present
in some Fishes. A transverse row of plates (_supra-temporals_) sometimes
crosses the occipital region behind the parietals. There are also other
canal-ossicles which lose their identity by fusing with certain cranial or
periotic bones. Thus, each of the pterotic and sphenotic bones often
includes a superficial dermal bone transmitting a section of a sensory
canal, which has fused with it; and as the frontal bone is often similarly
perforated, it may be taken that it also includes a canal-ossicle; and the
same can often be said of the articular and dentary bones of the lower
jaw.[191]
Having now considered the general structure of a primitive cartilaginous
type of skull, and the nature, disposition, and terminology of the various
membrane- and cartilage-bones which may be added to, or more or less
completely replace the former, reference will now be made to the more
important features in the structure of the skull in the Cyclostomata and
the Fishes.
In the Cyclostomata the skull presents a remarkable combination of
characters, in some of which it is more primitive than in any other
Craniates, while in others it has evidently attained a very high degree of
specialisation on lines peculiar to the group, but differing in the two
subdivisions.
{217}[Illustration: FIG. 124.—Skull, with branchial basket and anterior
part of the vertebral column, of _Petromyzon marinus_. _a.d.c_, Anterior
dorsal cartilage; _a.lat.c_, anterior lateral cartilage; _an.c_, annular
cartilage; _au.c_, auditory capsule; _br.b.1-9_, vertical bars of the
branchial basket; _br.cl.1-7_, external branchial clefts; _cn.c_, cornual
cartilage; _cr.r_, cranial roof; _l.c.1-4_, longitudinal bars of branchial
basket; _lg.c_, lingual cartilage; _m.v.c_, median ventral cartilage;
_n.a_, neural arches; _na.ap_, nasal aperture; _n.ch_, notochord; _Nv^2_,
foramen for optic nerve; _olf.c_, olfactory capsule; _pc.c_, cartilage
surrounding pericardial cavity; _p.d.c_, posterior dorsal cartilage;
_p.lat.c_, posterior lateral cartilage; _sb.oc.a_, subocular arch; _st.p_,
styloid process; _sty.c_, styliform cartilage; _t_, teeth. (From Parker and
Haswell, after W. K. Parker.)]
In the Lamprey[192] (Fig. 124) the paired parachordals and trabeculae
together form a trough-like chondrocranium, which has only a fibrous roof,
except where a slender _synotic_ band of cartilage extends between the two
periotic capsules. The floor is also incomplete, a large pituitary
fontanelle remaining to indicate the original separation of the trabeculae
while transmitting the hypophysial or pituitary caecum. The notochord
traverses the floor of the parachordal portion of the cranium as far as the
pituitary fontanelle, and from the sides of the synotic ring the auditory
capsules project in the shape of conspicuous lateral prominences. In front
the otherwise open end of the cranial cavity is closed by the
dorsally-placed and unpaired olfactory capsule, which is perforated behind
by two apertures for the olfactory nerves, and has only a fibrous connexion
with the cranial walls. Anteriorly to the olfactory capsule the cranial
floor is prolonged forwards over the roof of the mouth as a large
laterally-expanded plate, formed by the united anterior portions of the
trabeculae, and no doubt representing the _mesethmoid_ cartilage of the
Dog-Fish. So far the cranium presents no special difficulty, and in its
general features may be readily compared with that of an embryonic
Elasmobranch. As for the rest of the skull, it is obvious that it has been
greatly modified, partly to form and to support the skeletal framework of
the remarkable suctorial buccal funnel, and partly to form the singular
rasping lingual apparatus. Hence it is always difficult and sometimes
impossible to identify with {218}certainty the component parts as being
represented in other Craniates. On each side of the cranium, beneath the
eye, there is a characteristic V-shaped _subocular arch_. Of its two legs
the hinder one is continuous above with the periotic region of the cranium,
and the other with the anterior trabecular region, while the pointed apex
is directed obliquely downward and forward. From the hinder margin of the
posterior limb a slender _styloid process_ passes downward in the side wall
of the pharynx, and terminates below in a forwardly directed _cornual_
cartilage. A velum, fringed along its free margin with a series of
tentacles, projects forwards into the oral cavity from between the oral
apertures of the oesophagus and the branchial canal, and probably serves to
prevent the entrance of foreign particles to the gill-sacs. This valve-like
velum is supported by a _velar skeleton_, consisting of two lateral
cartilages which are prolonged into the tentacles, and extend transversely
between the inner surfaces of the two styloid processes. The apex of each
subocular arch is connected with a small and somewhat triangular cartilage
(_postero-lateral cartilage_), which is directed upward and forward, and
lies in the side wall of the oral cavity. With some degree of probability
the subocular arch may be compared to the palato-quadrate cartilage of a
skull which has become "autostylic" in order to form a rigid support for
the skeleton of the buccal funnel; the styloid processes and cornual
cartilages to the hyoid arch; while the relations of the posterior lateral
cartilages to the subocular arches suggest that they may possibly be
regarded as Meckelian cartilages which have lost their primitive function
of forming biting jaws. In the median line below, and projecting backward
for some distance beneath the branchial canal, there is a long and stout
_lingual_ cartilage, carrying a small median and a still smaller pair of
lateral cartilages at its anterior extremity, where it supports the lingual
teeth and projects into the buccal funnel beneath the mouth. In front of
the lingual cartilage, and connected by fibrous tissue with the inferior
and hinder margin of the _annular_ cartilage, there is a median T-shaped
element, the _median ventral_ cartilage. It has been conjectured that the
lingual cartilage is a free basi-hyal element, and the median ventral
cartilage the equivalent, elsewhere unknown, of the corresponding element
of the mandibular arch.[193]
{219}The remaining anterior skull elements are principally skeletal
supports for the roof and walls of the buccal funnel. The roof is supported
by an extended _anterior dorsal_ cartilage, which is overlapped behind by
the ethmoid cartilage, while the circular margin of the funnel is
strengthened by a large ring-like _annular_ cartilage. On each side of the
latter there is a slender, rod-like, _styloid_ cartilage, and above the
latter a small _anterior lateral_ cartilage. All these cartilages are
usually termed labial cartilages, and it is at least possible that they
possess representatives in the similarly named structures of the Dog-Fish
and the larvae of some of the tailless Amphibia. It must not be forgotten,
however, that the annular cartilage bears some resemblance to the ring of
cartilage which encircles the lips of the buccal cavity in Amphioxus.
The complex supporting skeleton of the gill-sacs forms a basket-like
structure. It consists on each side of nine unsegmented, irregularly
curved, and slightly branched cartilaginous rods, situated in the outer
margins of the inter-branchial septa, directly internal to the skin. The
first lies directly behind the styloid process (hyoid arch), the second and
third in front of and behind the first gill-sac, and of the remainder one
lies just behind each of the six succeeding gill-openings; above and below
each gill-aperture the rods are connected by longitudinal bars, and also in
the median ventral line by a pair of similar partially united bars. The
dorsal ends of the rods are also connected on each side by another
longitudinal bar, which runs alongside the notochord and in front blends
with the chondrocranium. The rods forming the last pair are continuous with
a cup-like cartilage, supporting the lateral and hinder walls of the
pericardium.
This singular branchial basket undoubtedly bears a superficial resemblance
to the branchial arches of Fishes, but in any comparison of the two
structures it is well to bear in mind that the branchial rods of the
Lamprey are situated along the outer edges of the inter-branchial septa,
and are therefore external to the gill-sacs and branchial arteries, and
further, that they are developed in the somatic mesoblast of the embryonic
protovertebrae, whereas true branchial arches are situated at the inner
margins of the septa, internal to the gill-clefts and branchial arteries,
and have their origin from the splanchnic layer of the mesoblast. So far as
their position is concerned, the rods agree rather with the
{220}extra-branchial cartilages of an Elasmobranch than with the more
deeply-seated branchial arches.
[Illustration: FIG. 125.—Side view of the skull of _Bdellostoma_; the
gill-apertures and their cartilages have been omitted. A, Auditory capsule;
B, B′, B″, the anterior, middle, and posterior segments of the lingual bar;
_b^1_, cartilage connecting the hyoid arch with the second branchial arch;
_br^1_, _br^2_, first and second branchial arches; _c.c_, coronal
cartilage; _Cr_, cranium; _D_, dental plate; _dt_, median dorsal tooth;
_Ex.n.c_, external part of the naso-pituitary canal; _Hp_, hypophysial
plate; _Hy_, hyoid arch; _N_, subnasal cartilage; _nc_, neural canal; _Nt_,
notochord; _OC_, olfactory capsule; _PL_, palatine portion of the
palato-quadrate cartilage _PQ_; _S_, supra-pharyngeal plate supporting the
velum; _t_, tendon of the retractor mandibuli muscle; _t^1_, _t^2_, _t^3_,
tentacular cartilages; _t^4_, cartilage supporting mouth lobe; _tr_,
trabecula; _V^1_, rod connecting _S_ with the inner surface of the hyoid
arch of its side; _V_, outer lateral rod which joins _V^1_; 1, 2, 3,
fenestrae. (Modified from Ayers and Jackson.)]
While the skull of the Myxinoid Cyclostomes[194] is constructed on the same
general lines as that of the Lamprey, it is in some respects more
primitive. It is also clear that in other features the skull has undergone
marked specialisation on lines of its own, and in some points again it
seems to deviate less from the more normal Craniate type. Of the more
obvious differences, as illustrated by the skull of _Bdellostoma_ (Figs.
125-127), it will be sufficient here to mention the following: (i.) The
more primitive condition of the chondrocranium, the roof and side walls of
the cranial cavity being entirely membranous. (ii.) The non-development of
a suctorial buccal funnel and the presence of oral tentacles, associated
with the absence of the complex system of labial cartilages and the
substitution of a special tentacular skeleton. (iii.) The special
modifications induced by the length and physiological importance of the
naso-pituitary canal and by its communication with the pharynx after
perforating the pituitary fontanelle in the cranial floor.
{221}[Illustration: FIG. 126.—View of the upper surface of the dental plate
of _Bdellostoma_. _t_, Tendon of retractor muscle. (From Ayers and
Jackson.)]
[Illustration: FIG. 127.—Dorsal view of the skull of _Bdellostoma_.
Reference letters as in Fig. 125. (After Ayers and Jackson.)]
Under this head may be included the depression of the mesethmoid or
hypophysial plate for the support of the naso-pituitary canal, the forward
prolongation and median union of the palato-quadrate cartilages of opposite
sides beneath the external portion of the canal, apparently for the support
of the latter, and the encircling of the canal by supporting annular rings
of cartilage. (iv.) The presence of two branchial arches, connected, as in
Fishes, with a median basi-branchial segment which forms the middle one of
the three divisions of the lingual apparatus. (v.) The reduction of the
complicated extra-branchial basket to small isolated cartilages in relation
with the gill-apertures and the œsophago-cutaneous duct. (vi.) The
extraordinary development of the lingual apparatus, of which it has
{222}been remarked that it "dominates the whole body, everything else
yields to it." Meckel's cartilages are represented either by the cornual
cartilages, as seems most probable, or by the dental plate (Fig. 125,
_c.c._ and D).
[Illustration: FIG. 128.—Lateral view of the skull of _Notidanus_
(_Heptanchus_) _cinereus_; _mck_, Meckel's cartilage, or primitive lower
jaw; _pal.qu_, palato-quadrate cartilage or primitive upper jaw; _pt.orb_,
post-orbital process of the cranium with which the post-orbital process of
the palato-quadrate articulates. (From Parker and Haswell, after
Gegenbaur.)]
In the generality of Elasmobranchs the skull resembles that of the Dog-Fish
in essential structure. The more important modifications within the limits
of the group relate to differences in the mode of attachment of the
primitive upper jaw to the skull, and the number of branchial arches. In
most Elasmobranchs the skull is hyostylic, as in _Scyllium_, but there are
two genera which, in different ways, are exceptions to this rule. In
_Notidanus_ the hinder part of each palato-quadrate cartilage grows upwards
into a strong post-orbital process, which articulates with the suitably
modified post-orbital process of the periotic capsule (Fig. 128); hence the
primitive upper jaw acquires a direct dorsal connexion with the cranium,
and, as the hyoid arch is now relieved from taking any part in its support,
the hyomandibular is reduced to the condition of a relatively slender rod
of cartilage. By this arrangement both the mandibular and hyoid arches have
their own separate and independent connexions with the cranium, and the
skull is said to be _amphistylic_.[195] The Port Jackson Shark
{223}(_Heterodontus_) exhibits another and quite different modification. In
this Fish the dorsal border of each palato-quadrate cartilage fits into a
deep groove along the infero-lateral surface of the cranium, and is firmly
attached thereto by ligament. Thus the first step is taken towards that
more complete fusion of the two structures which is so characteristic a
feature in the more typically _autostylic_ Fishes like the Holocephali and
the Dipnoi. Autostylism, whether incipient, as in _Heterodontus_, or
complete, is to be regarded as a secondary modification, which may be
independently acquired in widely different groups of Fishes, and is usually
associated with the need of a firm and rigid support for an exceptionally
massive dentition.[196]
[Illustration: FIG. 129.—Lateral view of skull of _Chimaera monstrosa_.
_a.s.c_, Position of anterior semicircular canal; _c.hy_, cerato-hyal;
_e.hy_, epi-hyal; _fr.cl_, frontal clasper; _h.s.c_, position of horizontal
semicircular canal; _i.o.s_, inter-orbital septum; _lb.1_, _lb.2_, _lb.3_,
labial cartilages; _Mck.C_, mandible; _Nv.2_, optic foramen; _Nv.10_, vagus
foramen; _olf.cp_, olfactory capsule; _op.r_, opercular rays; _pal.qu_,
palato-quadrate; _ph.hy_, pharyngo-hyal, _or_ hyomandibular; _p.s.c_,
position of posterior semicircular canal; _qu_, quadrate region; _r_,
rostrum. (From Parker and Haswell, after Hubrecht.)]
In the Holocephali (_e.g._ _Chimaera_[197]) the cranium retains its
primitively cartilaginous condition, and assumes a somewhat peculiar
appearance owing to the lateral compression and vertical growth of its
inter-orbital and nasal regions (Fig. 129). There is a complicated series
of labial cartilages in relation with the ventrally-placed nostrils and the
upper and lower jaws. In the males of _Chimaera_ and _Callorhynchus_, but
not in _Harriotta_, a movable cartilage is attached to the cranial roof,
and supports the frontal clasper. The skull is typically autostylic. Along
{224}the whole length of its dorsal border the palato-quadrate cartilage is
fused with the inferior lateral margin of the cranium from the periotic to
the olfactory region, thus forming a triangular plate of cartilage, the
base of which is continuous with the cranium, while the downwardly directed
apex provides an articular surface for the lower jaw. The hyoid arch is
little better developed than the succeeding branchial arches, and includes
a vestigial hyomandibular, an epi-hyal, and a cerato-hyal. As in other
autostylic skulls the hyomandibular element is attached by ligament to the
hinder margin of the palato-quadrate, instead of being directly connected
with the periotic capsule, and obviously takes no part in supporting the
jaws. Branchial rays for the support of the operculum are attached to the
cerato-hyal, and some of them have their bases fused together. The five
branchial arches resemble those of the Dog-Fish, except that they tend to
become concentrated beneath the skull.
[Illustration: FIG. 130.—Side view of the skull of a Sturgeon, with the
investing membrane bones removed. _a_, Pharyngo-branchial; _AF_, antorbital
or lateral ethmoid cartilage; _AR_, articular; _b_, epi-branchial; _c_,
cerato-branchial; _C_, notochord; _Cop_, basi-branchials; _d_,
hypo-branchial; _De_, dentary; _GK_, auditory capsule; _Hm_, hyomandibular;
_hy_, cerato-hyal; _Ih_, inter-hyal; _Md_, lower jaw; _Na_, nasal capsule;
_Ob_, neural arches; _Orb_, Orbit; _PF_, post-orbital process; _PQ_,
palato-quadrate; _Ps_, _Ps′_, _Ps″_, parasphenoid; _Psp_, neural spines;
_Qu_, quadrate; _R_, rostrum; _Ri_, ribs; _Sp.N_, foramina for spinal
nerves; _Sy_, symplectic; _WS_, vertebral column; _x_, foramen for the
vagus nerve; I-V, branchial arches; II-V, foramina for the optic and the
fifth cranial nerves. (From Parker and Haswell, after Wiedersheim.)]
The existing Chondrostei,[198] and especially the Sturgeon, are remarkable
for the persistence and continuous growth of the chondrocranium, and the
absence of true cartilage bones.
{225}[Illustration: FIG. 131.—Lateral view of the primary and secondary
upper and lower jaws of _Polyodon_. _b.br′_, First basi-branchial; _ch_,
cerato-hyal; _d_, dentary; _hy.h_, hypo-hyal; _hy.m_, hyomandibular;
_i.hy_, inter-hyal; _i.op_, inter-operculum; _lgs_, ligaments connecting
the palato-quadrate cartilage with the hyomandibular; _mk.c_, Meckel's
cartilage; _mx_, maxilla; _op_, operculum; _pa_, palatine; _pa.q_,
palato-quadrate; _ps.l_, pre-spiracular ligament; _q_, quadrate cartilage;
_sym_, symplectic. (From Bridge.)]
Numerous dermal bones invest the dorsal surface of the chondrocranium, and
only to a limited extent correspond with the less numerous membrane bones
of the Salmon. To these are added a series of circum-orbital bones and a
large parasphenoid. Undoubtedly the most striking feature in these Fishes
is the primitive character of the upper jaw. In _Polyodon_ (Fig. 131) the
palato-quadrates are wholly cartilaginous, and, as in the Dog-Fish, they
meet in front beneath the basis cranii, where the two are connected by
ligament. The secondary upper jaw is but feebly developed, and is
represented on each side by a thin splint-like maxilla in relation with the
outer surface of each palato-quadrate cartilage, which meets its fellow in
front. There are no premaxillae. The lower jaw is also very primitive.
Meckel's cartilages are persistent, and except for a mento-Meckelian bone
on each side, they are unossified, although membrane bones representing
dentary and splenial elements are present. The skull is hyostylic. The
hyoid and branchial arches are only partially ossified. Each opercular fold
is supported by an operculum and an interoperculum, and both of these
retain somewhat the shape of the cartilaginous hyoidean rays which they
have replaced. In the Sturgeon (Fig. 130) the upper jaw is greatly modified
in relation with the singular mouth of this Fish. The palato-quadrate
cartilages meet not only in front, but also along their dorsal margins,
and, with the help of the similarly opposed and somewhat fragmentary
metapterygoid {226}cartilages, they form a complete concave roof for the
protrusible spout-like mouth. Palatine, mesopterygoid, and pterygoid bones
invest, and in some measure replace these cartilages. In brief, the skull
of the Chondrostei occupies an interesting intermediate position between
the purely cartilaginous and mainly bony types. While retaining a
well-developed and unossified primary cranium, it has acquired a complete
secondary cranium of dermal bones. Equally notable is the condition of the
jaws. Unique among the Teleostomi in possessing the typical Elasmobranch
union of the palato-quadrate cartilages beneath the basis cranii, the
Chondrostei are so far specialised that they have acquired certain of the
membrane bones which constitute the secondary jaws of the more typical bony
Fishes.
As regards the general structure of the skull and the nature and
disposition of its cartilage- and membrane-bones, the remaining living
Teleostomi have much in common with the Salmon. In all the skull is
hyostylic, and, unlike the Chondrostei, each half of the primitive upper
jaw remains distinct from its fellow, and is separately articulated in
front with the lateral ethmoid of the same side by its palatine element.
The palato-quadrate cartilage is always more or less completely replaced by
bones similar to those of the Salmon, and although they often carry teeth,
as a rule they do little more than constitute a rigid buttress for the
fixation of the quadrate condyle for the lower jaw. The secondary upper jaw
is nearly always well developed, and includes a premaxilla as well as a
maxilla on each side. There are, however, certain features in each of the
minor groups which are either distinctive or highly characteristic.
In the surviving Crossopterygii (e.g. _Polypterus_[199]) the
chondro-cranium is complete in the ethmoidal and post-orbital regions,
except where it has been partially replaced by cartilage bones, but in the
inter-orbital region the continuity of the roof is interrupted by a large
fontanelle, which is only closed by the investing frontal bones (Fig. 132,
C). There is also a large basi-cranial fontanelle in the sphenethmoid,
closed, however, by the underlying parasphenoid. A large "occipital" bone
continuously ossifies in the occipital cartilage and completely surrounds
the foramen magnum.
{227}[Illustration: FIG. 132.—A, side view of the skull of _Polypterus_; B,
dorsal view, showing the chief dermal bones; C, similar view of the
chondro-cranium after the removal of the dermal bones. _An_, Angular; _Ar_,
articular; _D_, dentary; _E_, mesethmoid; _f.m_, foramen magnum; _Fr_,
frontal; _l.e_, lateral ethmoid; _Mx_, maxilla; _Na_, _Na′_, nasal and
accessory nasal bones; _occ_, occipital; _ol_, nasal aperture; _Op_,
operculum; _op.o_, opisthotic; _O.t_, _os_ terminate; _Pa_, parietal;
_Pm.x_, premaxilla; _P.t_, post-temporal; _Ptf_, post-frontal; _Qu_,
quadrate; _S.b_, _S.b′_, circum-orbital ossicles; _S.Op_, sub-operculum;
_Sp_, splenial; _sp.eth_, sphenethmoid; _sp.o_, sphenotic; _Spr_,
spiracular ossicles, between which is the spiracle; _S.t_, supra-temporals;
_Y_, cheek-plate (pre-operculum); _Y′_, _Y″_, smaller cheek-plates; _z_,
_z_, _z_, _z_, post-spiracular ossicles; _z′_, _z′_, prespiracular
ossicles. In C the cartilage is dotted. (From Traquair.)]
Prootics and pterotics are absent, and the opisthotics seem to be confluent
with their respective epiotics. The floor and side walls of the
inter-orbital section of the cranium are formed by a remarkable
"sphenethmoid" bone which occupies the position of the paired ali- and
orbito-sphenoids in other bony Fishes; and in one species, _P.
lapradei_,[200] it forms in front distinct tubular investments round the
olfactory nerves. In many respects this bone is singularly like the
sphenethmoid bone of the Frog and other tailless Amphibia. A median ethmoid
as well as lateral ethmoids are present. In addition to the ordinary dermal
bones which invest the cranial roof there is a transverse row of
supra-temporal plates crossing the cranial roof behind the paired parietals
(Fig. 132, A). Fringing the outer margins of the frontals and parietals a
row of pre- and post-spiracular ossicles extends nearly to the orbits, and
between two of them, which form a {228}valve, is the spiracular aperture
itself. There is a dentigerous splenial on the inner surface of the lower
jaw. The hyoid arch has no separate symplectic bone. An operculum and a
suboperculum are present, but no inter-operculum; and unless the hinder
part of the large cheek-plate, which is traversed by the mandibulo-hyoid
sensory canal, represents a pre-operculum, the latter is wanting.
Branchiostegal rays are absent, but there is a single pair of large jugular
plates.
Very little is certainly known about the cranial cartilage-bones in the
fossil members of the group, but the investing dermal bones, which bear a
general resemblance to those of _Polypterus_, are often somewhat more
numerous, and they form a very complete dermal armature for the entire
head. There is a very complete ring of circum-orbital bones, and very often
a ring of sclerotic plates. Two large cheek-plates are often present.
Nothing comparable to pre- and post-spiracular ossicles is known, but
squamosal and supra-temporals can often be identified. To the ordinary
bones of the lower jaw there may be added a series of infra-dentary plates,
and besides the paired principal jugular plates there may also be present a
small anterior median plate and a series of small lateral jugular plates on
each side, as in the Carboniferous _Rhizodopsis_ (Fig. 274). Most of the
superficial dermal bones, both in the living and extinct Crossopterygii,
are invested externally by a granulated or rugose layer of enamel-like
ganoin.
In the Holostei, and especially in _Amia_, the skull approximates more
closely to the normal Teleostean type as represented by the Salmon's skull.
In _Amia_[201] all the occipital cartilage-bones are present—a
basi-occipital, two exoccipitals, and a supra-occipital; and, except for
the absence of a pterotic, the periotic series of bones is also complete.
Paired ali- and orbito-sphenoids form the lateral walls of the
inter-orbital portion of the cranial cavity. Above, the complete
cartilaginous roof of the cranial cavity is invested by a shield of
suturally united and ganoin-covered dermal plates. The hyomandibular
element has a symplectic bone at its distal extremity. There is a complete
series of opercular bones, and the branchiostegal rays are numerous. A
single median jugular plate is present. The lower jaw has on each side five
dentigerous splenial bones in addition to dentary and angular bones, while
cartilage-bones are {229}represented by articular and mento-Meckelian
elements. In its essential structure the skull of _Lepidosteus_[202]
resembles that of _Amia_, but it has obviously undergone much
specialisation. In some species (_e.g._ _L. osseus_) its appearance is
greatly modified by the exceptional length and tapering shape of the beak,
due to the elongation of that part of the skull which lies between the
orbital and nasal regions; but in _L. platycephalus_ the reduced length and
greater width of the beak, combined with its somewhat flattened condition,
impart an almost Crocodilian aspect to the head. Amongst other points of
difference it may be mentioned that in _Lepidosteus_ the continuity of the
chondro-cranial roof is interrupted by a large superior fontanelle. There
is no supra-occipital, and there are no lateral ethmoids, at all events in
the usual position. The inter-orbital portion of the cranial cavity is
largely obliterated by the formation of an inter-orbital septum, consisting
of a thin vertical plate of bone, which either represents a pair of fused
orbito-sphenoids or a pair of similarly modified lateral ethmoids. In
addition to the ordinary investing dermal bones, including circum-orbitals,
squamosal, and supra-temporals, there are numerous scale-like ossicles
which take the place of the cheek-plates of _Polypterus_. The maxillae are
segmented into numerous dentigerous bones fringing the margins of the upper
jaw. The lower jaw has no mento-Meckelian bones, but there is a very
complete series of dermal elements, including dentary, coronary, splenial,
angular, and supra-angular bones in addition to an articular
cartilage-bone. One of the most remarkable features in the skull of
_Lepidosteus_ is the existence of a secondary articulation between the
metapterygoid bones and a pair of transversely elongated condyles formed on
each side by a lateral outgrowth from the parasphenoid and alisphenoid
bones. By a horizontal sliding movement of the former on the latter,
provision is made for the lateral expansion and contraction of the walls of
the oral cavity and the separation and approximation of the lateral halves
of the upper jaw.[203]
The generality of Teleosts[204] more or less closely agree with _Amia_ in
the main features of their cranial structure. There are, however, certain
minor features which are characteristic if not {230}always distinctive of
the group. As a rule, to which, nevertheless, there are notable exceptions,
there is little of the primary cartilaginous cranium in the adult, nearly
the whole of it having become absorbed or converted into cartilage-bones. A
supraoccipital is invariably present, and usually a mesethmoid and a
basisphenoid. An additional bone is added to the periotic series, viz. a
pterotic. Supra-temporal bones and jugular plates are always absent, and it
may be doubted if mento-Meckelian bones and dentigerous splenials are ever
developed in the lower jaw. Within the group itself the skull exhibits many
notable modifications, of which only a few can here be mentioned. The
shape, size, and character of the mouth and jaws, the extent to which they
can be protruded and retracted, and the nature of the dentition, are the
source of many characteristic modifications in the structure and appearance
of the fore-part of the skull, and these again largely depend upon
differences of habit and food. A protrusible mouth, or a mouth which is
projected forwards, is usually associated with a suspensorium
(hyomandibular) of considerable length, and so greatly inclined forwards as
to make a more or less acute angle with the forepart of the cranium.
The presence or absence of an inter-orbital septum is also a feature in
which considerable variation occurs. In some Teleosts there is no septum,
and the cranial cavity is prolonged forwards between the orbits, where its
lateral walls are formed by well-developed, paired ali- and orbito-sphenoid
bones, as, for example, in the Carp and other Cyprinidae. In others the
fusion of the cranial walls is accompanied by the median union of the
orbito-sphenoids, so that a partly bony and partly cartilaginous
inter-orbital septum is found, and the cranial cavity becomes largely
obliterated in this region, as in the Salmon; or the orbito-sphenoids may
be non-existent, the cartilage may undergo absorption, and the
inter-orbital septum may become reduced to a vertical fibrous sheath
extending between the frontals above and the parasphenoid below, as is the
case in the Cod (_Gadus_).
An interesting modification of certain of the bones of the primary and
secondary upper jaw occurs in the Siluridae. In these Fishes the maxillae
are very small and edentulous, and serve no other purpose than forming
basal supports for the maxillary barbels, while the rod-like palatine bone,
losing its connexion with the pterygoid portion of the primitive upper jaw,
{231}but retaining its articulation with the lateral ethmoid, serves to
support the maxilla, and at the same time receives the insertion of the
muscles by which the barbel is moved in various directions.
In the Plectognathi the premaxillae are co-ossified with the maxillae. Many
other interesting cranial modifications occur in Teleosts, and to some of
them reference is made in subsequent chapters.
In some respects the skull of Dipnoi[205] is remarkably like that of the
Holocephali, especially in its typical autostylism; but in possessing both
cartilage- and membrane-bones it in some measure approaches the Teleostome
skull. The investing dermal bones are not always easy to identify with
those of other Fishes. In _Neoceratodus_ an anterior median membrane-bone
or dermal mesethmoid covers the ethmo-nasal region, and, on each side of
it, forming the anterior boundary of the orbit, there is situated a
pre-orbital or dermal lateral ethmoid. Behind the mesethmoid there is a
much larger posterior median bone, and on each side a singular backward
prolongation of the dermal lateral ethmoid separates it from a squamosal
element. The latter bone descends on the outer surface of the quadrate
portion of the palato-quadrate cartilage as far as the condyle for the
lower jaw. Collectively, these bones form a fairly complete investment to
the upper surface of the cranium, but the posterior median bone and the
adjacent portions of the dermal lateral ethmoid and the squamosal are
widely separated from the underlying chondrocranium by the powerful jaw
muscles, and in this respect they differ from the ordinary roofing bones of
other Fishes.
In _Protopterus_ (Fig. 133) and _Lepidosiren_ (Fig. 134) the posterior
median bone is non-existent, and its place is taken by a large
fronto-parietal, which forms the greater part of the cranial roof, internal
to the jaw muscles, and is much larger in the latter Dipnoid than in the
former. Circum-orbital bones are present only in _Neoceratodus_. A large
parasphenoid supports the cranial floor. Vomers are absent, although there
are two small vomerine teeth.
{232}[Illustration: FIG. 133.—Side view of the skull of _Protopterus_, with
the pectoral girdle and fin. _an_, Angular; _an.c_, antorbital cartilage;
_c.c_, coracoid cartilage (epi-coracoid); _c.hy_, cerato-hyal; _cl_,
clavicle; _c.r_, cranial rib; _c.sc_, coraco-scapular cartilage; _d.e_,
dermal ethmoid; _d.l.e_, dermal lateral ethmoid; _e.g.f_, external gills;
_eo_, exoccipital; _f.p_, fronto-parietal; _mk.c_, Meckel's cartilage;
_n.a_, neural arches; _ol.c_, fenestrated roof of the olfactory capsule;
_p.f_, skeleton of the pectoral fin; _p.pt_, palato-pterygoid bone; _p.q_,
palato-quadrate cartilage; _s.cl_, supra-clavicle; _sp_, splenial; _sq_,
squamosal; 1-6, the branchial arches; the segmentation of the second and
third arches is not shown. (From Wiedersheim.)]
Relatively small opercular and inter-opercular bones are present, and on
the inner surface of each may be found vestigial remains of cartilaginous
hyoidean rays. The chondrocranium is complete in _Neoceratodus_, but in the
remaining genera it has undergone considerable absorption in the
inter-orbital region, so that the roof and floor, and, in part, even the
side walls of the cranial cavity, are formed by the fronto-parietal and
parasphenoid bones. Two exoccipitals are present in all Dipnoi. There are
small labial cartilages in relation with the ventrally-placed nostrils, and
large lateral outgrowths from the ethmoid cartilage furnish the olfactory
organs with conspicuous lattice-like roofs. A pair of strong
palato-pterygoid bones fringe the lower margins of the palato-quadrate
cartilage, and meeting in front beneath the ethmoid region their symphysial
extremities support the large palatal teeth. The Meckelian cartilages are
persistent in all Dipnoi. In _Neoceratodus_ each is flanked by a dentary
and an angular externally, and internally by a splenial; but in
_Protopterus_ and _Lepidosiren_ distinct dentary bones are wanting. The
hyoid arch is best {233}developed in _Neoceratodus_,[206] and includes a
small hyomandibular cartilage, a partially bony cerato-hyal and
cartilaginous hypo-hyal and basi-hyal element. In the other genera (Fig.
133) only a cerato-hyal is retained. The branchial arches are but feebly
developed in the Dipnoi. _Neoceratodus_ has five, of which the first four
are divided into epi-branchial and cerato-branchial segments, while the
fifth is undivided. _Protopterus_ has six, but only the second and third
are segmented as in _Neoceratodus_.[207] In _Lepidosiren_ all the arches
are simple undivided rods.
In all three genera the skull conforms to the same general type of
structure, but it is much more primitive in _Neoceratodus_ than in the
other two genera.
[Illustration: FIG. 134.—Dorsal view of the skull of _Lepidosiren_. _an.c_,
Condyle on the quadrate cartilage for the lower jaw; _n.sp_, neural spine;
_op_, operculum. For other reference letters see Fig. 133. (From Bridge.)]
With reference to the fossil Dipnoi, it may be stated that, so far as they
are known, the cranial roofing bones are more numerous than in the existing
genera, and they cannot readily be compared with those of the latter, or
with the numerically reduced and more definitely arranged bones of most
Teleostomi. There is also evidence that in some fossil Dipnoi (e.g.
_Dipterus_) the chondrocranium and the mandibular suspensorium
(palato-quadrate) must have been replaced by cartilage bones to an extent
which has no parallel in any of the surviving types.[208] Jugular bones
were present in _Dipterus_ and _Phaneropleuron_.
{234}MEDIAN FINS AND APPENDICULAR SKELETON
[Illustration: FIG. 135.—The cartilaginous radialia of the first dorsal fin
of _Mustelus antarcticus_. (From Mivart.)]
THE MEDIAN FINS.—Whether existing in the form of a continuous fin, or as
discontinuous isolated fins, the median fins are provided with skeletal
supports, and also with muscles, primitively formed from intrusive clusters
of cells derived from a variable number of the neighbouring myotomes, for
their varied movements. The skeletal structures of the dorsal and anal fins
consist of a series of bony or cartilaginous, rod-like, and typically
tri-segmented radial elements or pterygiophores,[209] supporting distally a
series of dermal structures in the shape of numerous slender horny fibres
or ceratotrichia, as in the Elasmobranchii and Holocephali, or a smaller
number of bony dermal fin-rays, which are probably modified scales or
lepidotrichia,[210] as in the Teleostomi. The typical tri-segmented
character of the radialia is often retained in many existing Elasmobranchs
(Fig. 135) and in _Pleuracanthus_, in _Neoceratodus_ amongst the Dipnoi, in
the Chondrostei, in existing Holostei (Fig. 136), and to a greater or less
extent in several families of Teleosts (_e.g._ Salmonidae, Esocidae,
Cyprinidae, and some Acanthopterygii); but in the latter group the radialia
are greatly prone to reduction, and hence they are more generally
bi-segmented, and sometimes consist of a single proximal segment only (e.g.
_Gymnotus_). In all these Fishes the proximal segments are the longest and
the most persistent, and when reduction occurs it is at the expense of the
middle and distal segments.
{235}[Illustration: FIG. 136.—The tri-segmented radialia and the fin-rays
of part of the dorsal fin of _Amia calva_. _p.s_, _m.s_, and _d.s_, The
proximal, middle, and distal segments of a radial; _f.r_, fin-rays. (From
Bridge.)
FIG. 137.—The first four radialia of the dorsal fin of _Mesoprion gembra_,
showing the chain-links for the ring-like bases of the fin-rays. _r.e^1_,
_r.e^4_, First and fourth proximal radialia.]
The cause of this reduction is often, but not always, to be found in the
fact that, whenever the dermal fin-rays take the form of stout spines, as
in the anterior dorsal fin in many Acanthopterygian Teleostei, the
segmentation of their radialia would obviously detract from their value as
skeletal supports, and hence they rarely consist of more than their
proximal segments, although the radialia which in the same Fish support
soft rays may be bi-segmented or tri-segmented. The radialia are, however,
unsegmented, even slightly branched, cartilaginous rods in the
Cyclostomata; short simple rods in the Holocephali; and equally simple bony
rods in the dorsal fin of _Polypterus_, where they support the strong
spines of the numerous finlets; but they are bi-segmented in the soft-rayed
anal fin. As previously mentioned, the proportional share taken by the
radialia and the horny fibres or the dermal fin-rays in the support of the
fins differs greatly in different Fishes. In the Cyclostomata radialia are
the sole, and in Elasmobranchs the main supports, and they may extend
nearly to the free margin of the fin. In the more specialised Fishes, as in
most Teleostomi, the reverse is the case. The radialia sink into the
muscles of the body-wall and leave the strongly developed fin-rays as the
sole support of the visible portions of the fins. In not a few Fishes there
is an obvious segmental correspondence between the radialia and the
vertebral neural or haemal spines, to the extent that the former equal the
latter in number and articulate with their distal extremities, as, for
example, in the caudal region of _Pleuracanthus_ and in existing
{236}Dipnoi. In others again, as in most Teleostomi, there is no such
segmental relation, and the radialia are more numerous than the vertebrae
whenever the two are co-extensive. The exoskeletal fin-supports exhibit
similar relations to their radialia, but in inverse order. Much more
numerous than the radialia in the Elasmobranchs, Holocephali, and the
Dipnoi, the former become gradually reduced in the Teleostomi, until in the
Holostei and Teleostei they correspond in number with the supporting
radialia. Complete numerical correspondence between the neural and haemal
spines and the radialia and fin-rays is very rare, and has only been
observed in the caudal region of certain Crossopterygii (_e.g._ the
Coelacanthidae).[211]
[Illustration: FIG. 138.—The posterior dorsal fin of _Holoptychius
leptopterus_ from the old Red Sandstone of Nairnshire. Traces of dermal
fin-rays may be seen at the distal margin of the fin. (After Smith
Woodward.)]
[Illustration: FIG. 139.—A dermal fin-ray and its supporting radial or
pterygiophore in the Trout (_Salmo fario_). _D.F.R_, Dermal fin-ray; PTG.1,
PTG.2, _ptg.3_, the proximal, middle, and distal segments of which the
tri-segmented radial consists; _ptg.3_ is cartilaginous; the other two are
bony. (From Parker and Haswell.)]
In not a few Fishes the radialia of the median fins undergo modifications
which offer an interesting parallel to an early stage in the evolution of
the paired fins from primitively continuous lateral fins. The concentration
of radialia which occurs in isolated median fins often results, through
growth pressure, in the complete fusion of the proximal segments of more or
fewer of the radialia into two or three basal supports, or even into a
single basal piece. Examples of such basal fusion are frequent in the
dorsal fins of Elasmobranchs, and the same modification may also be seen in
the anal fin of _Pleuracanthus_, and especially in the {237}dorsal fin of
the Devonian Crossopterygian, _Holoptychius_[212] (Fig. 138), where several
radialia, which are free distally, have their bases united into a single
basal piece, or basipterygium. In most Teleostomi elevator and depressor
muscles arise from the radialia, and are inserted into different points on
the bases of the fin-rays, and by their contraction the latter may either
be elevated into an erect position, or folded back like a fan along the
middle line of the body, where, as in some Teleosts, there is a groove for
their reception. When fin-rays are only capable of simple elevation or
depression, the connexion between a radial element and its fin-ray is
usually by some form of a hinge-joint, the cleft base of the ray clipping
the distal segment of the radial (Fig. 139). In some Teleosts the
articulation of the two is by means of a kind of chain-link (Fig. 137). In
those Fishes in which the median fins are capable of lateral undulatory
movements the articulation is of a more mobile character.
[Illustration: FIG. 140.—Caudal end of the vertebral column of a Trout
(_Salmo fario_). CN, Centrum; _D.F.R_, dermal fin-rays; H.SP, haemal spine;
H.ZYG, haemal zygapophysis; N.SP, neural spine; N.ZYG, neural zygapophysis;
UST, the up-tilted, partly ossified, and unsegmented terminal portion of
the notochord, or urostyle. (From Parker and Haswell.)]
In the different types of caudal fin, diphycercal, heterocercal, and
homocercal, the supporting elements of the ventral lobe are formed by the
haemal spines of the terminal caudal vertebrae which are inclined
backwards, and are often greatly expanded for the purpose (Fig. 140). The
dorsal lobe may be supported either by the adjacent neural spines, or by
radialia, or by both.
THE APPENDICULAR SKELETON.[213]—It is probable that the skeleton of the
paired fins and the pectoral and pelvic girdles have been formed from the
supporting radialia of the isolated and enlarged anterior and posterior
portions of primitively continuous lateral {238}fins, by a sequence of
structural modifications in the same direction as in the median fins. The
initial stage was probably marked by the fusion of the proximal portions of
the radialia to form a basal support or basipterygium for the free distal
portions. Subsequently, it may be, a rudiment of the future limb-girdle
became segmented off from the inner extremity of the basipterygium, and by
its dorsal and ventral growth in the body-wall the lateral half of a girdle
was developed. The subsequent union of the two halves across the
mid-ventral line resulted in the evolution of the dorsally incomplete hoop
of cartilage which is the primary form of the complete limb-girdle in
Craniates. The primitive fin skeleton or "archipterygium" was formed from
the residue of the basipterygium in conjunction with the free distal
radialia which it carried. The precise structure of the archipterygium is
purely hypothetical. Possibly it was a biserial fin of the _Pleuracanthus_
or _Neoceratodus_ type, consisting of a cartilaginous segmented axis,
fringed along its anterior and posterior, or pre-axial and post-axial
margins, by a series of slender, simple, or jointed radialia (Fig. 147); or
it may have been a uniserial structure, somewhat resembling the pelvic fin
of _Pleuracanthus_, or the pectoral and pelvic fins of existing
Elasmobranchs (Figs. 250, 141), in which an axis formed by the residue of
the basipterygium or metapterygium had a fringe of radialia on its anterior
or preaxial side only. If the archipterygium was biserial then the
uniserial fin was probably derived from it by the subsequent suppression of
all the post-axial radialia; or, if uniserial, the biserial fin was evolved
by a later extension of radialia on to the post-axial margin. The evidence
of comparative anatomy is not conclusive as to the nature of the
archipterygium, and palaeontology seems to support either view with
puzzling impartiality.[214] It may be admitted that the lateral fin theory
offers the best solution of the problem of the origin of the paired fins,
but it must be borne in mind that no Fish, living or fossil, is known to
possess fins of this nature, unless the singular lateral lobes of some
Ostracodermi (_e.g._ the Coelolepidae) are kindred organs[215]; neither do
continuous lateral fins ever exist as vestiges, unless, indeed, the
bilateral series of spines, which extend between the pectoral and pelvic
{239}fins, in some of the Lower Devonian Acanthodei (e.g. _Climatius_), may
be regarded in that light.
_The Pectoral and Pelvic Girdles_.—The pectoral girdle is more primitive in
_Cladoselache_ and _Pleuracanthus_ than in any other Elasmobranch. In the
former (Fig. 145, A) it may be doubted if the girdle has passed beyond the
basipterygial stage, and although a definite girdle is present in the
latter genus (Fig. 250) its lateral halves retain their primitive
distinctness. Existing Elasmobranchs, including the Holocephali, have a
pectoral girdle in the form of a dorsally incomplete hoop of cartilage
imbedded in the muscles of the body-wall, close behind the last branchial
arch (Fig. 141). The upper or dorsal portion of each half is the scapula,
and the ventral is the coracoid. Between these two portions of the girdle,
and defining their limits, there are articular surfaces for the basal
cartilages of the pectoral fin.
[Illustration: FIG. 141.—The right half of the pectoral girdle and the fin
of an Elasmobranch (_Chiloscyllium_). _d.r_, Dermal horny fibres; _meso_,
mesopterygium; _meta_, metapterygium; _pect_, pectoral girdle; _pro_,
propterygium. (From Parker and Haswell.)]
_Cladoselache_ (Fig. 145, B) had no pelvic girdle, nor does it appear that
this primitive Elasmobranch had acquired even a basipterygium.
_Pleuracanthus_, on the contrary, had a pair of pelvic rudiments distinct
from well-developed basipterygia. In other Elasmobranchs there is a
distinct girdle, formed by the median union of primitively distinct lateral
rudiments, consisting of a simple transverse bar of cartilage, imbedded in
the ventral abdominal wall, just in front of the cloacal aperture, and
having articulated to each of its outer extremities the basal cartilage
(metapterygium) of the pelvic fin.
{240}[Illustration: FIG. 142.—The left half of the pelvic girdle and the
right pelvic fin of _Chiloscyllum_. _meta_, Metapterygium; _pelv_, pelvic
girdle. (From Parker and Haswell.)]
[Illustration: FIG. 143.—Left half of the pectoral girdle of a Trout
(_Salmo fario_), seen from the inner surface. _CL_, Clavicle (cleithrum);
COR, coracoid; _D.F.R_, dermal fin-rays; MS.COR, meso-coracoid; _P.CL_,
_P.CL′_, post-clavicles; PTG.1, proximal; _ptg.2_, distal pterygiophores;
_P.TM_, post-temporal; _S.CL_, supra-clavicle; SCP, scapula. (From Parker
and Haswell.)]
Sometimes there is a rudiment of a dorsally-directed "iliac" process at
each extremity of the girdle, but in no Fish do these processes ever
acquire a dorsal connexion with the vertebral column. In the Holocephali
the iliac processes are better developed than in any other Fishes, but
ventrally the lateral halves of the girdle are united by ligament alone. In
the Teleostomi important differences are observable in both girdles. The
primary cartilaginous pectoral girdle now consists of distinct lateral
halves which have no ventral connexion with each other. In addition, there
is developed on the outer surface of each half a series of membrane bones,
which form a secondary girdle (Fig. 143). From above downward the series
includes a supraclavicle and a cleithrum (clavicle of Teleosts) which are
always present, and to these may be added in the {241}Crossopterygii and
Chondrostei an infraclavicle or clavicle proper, while one or two
"post-clavicles" may be present in relation with the hinder margin of the
cleithrum. The infraclavicles, or in their absence the cleithra (_e.g._
Holostei and most Teleostei), usually meet in a median ventral symphysis,
so that the secondary girdle tends to acquire the characteristic hoop-like
arrangement of its parts which has been lost in the primary girdle. With
the development of a bony secondary girdle, the primary girdle (scapula and
coracoid) becomes much reduced, and, as a rule, does little more than
connect the fins with the cleithra. The secondary girdle acquires a dorsal
connexion with the skull on each side by means of the post-temporal bone,
which is attached below to the supra-clavicle and above to the periotic
capsule. In the Chondrostei and the Dipnoi the primary girdle retains its
primitive cartilaginous condition, but in the Crossopterygii, Holostei, and
in all Teleosts it is ossified as distinct scapulae and coracoids. To these
may be added in some Teleosts a mesocoracoid formed by a separate
ossification of the coracoid cartilage (Fig. 143).[216]
[Illustration: FIG. 144.—Ventral view of the pelvic girdle of
_Protopterus_. _a_, Prepubic process; _b_, lateral process for the fin;
_c_, epipubic process; _Gr_, ridge for the origin of the fin muscles; _HE_,
skeleton of the fin; _M_, myotomes; _M′_, myocommata. (From Wiedersheim.)]
With the possible exception of small paired or median cartilages inserted
between the inner extremities of the basipterygia in _Polypterus_ and a few
other Teleostomi, the pelvic girdle is absent in all the existing members
of this group, having either become completely suppressed, or remaining
unseparated from the basipterygia of the pelvic fins.[217] In the Dipnoi
(Fig. 144) there is a true pelvic girdle which has some points of
resemblance to that of certain of the caudate Amphibia. It is represented
by a median, lozenge-shape, cartilaginous {242}plate, produced in front
into a long tapering epipubic process, and on each side of this into a
forwardly inclined prepubic process. The hinder part of the plate bears two
short processes for the basal cartilages of the pelvic fins. There is no
trace, however, of iliac processes.
THE PECTORAL FINS.—The skeleton of the pectoral fins exhibits remarkable
structural variations in different Elasmobranchs. In the existing members
of the group two large basal cartilages, the propterygium and the
mesopterygium, are formed by the concentration and fusion of the proximal
portions of certain of the preaxial radialia, and they, with the
metapterygium, articulate with the pectoral girdle; hence the fin is
tribasal as well as uniserial (Figs. 141 and 146, A, B). In striking
contrast to all other Elasmobranchs the pectoral fin of _Cladoselache_
(Fig. 145, A) is far more primitive than in any other Fish. Each fin is
supported by a distal series of slender, more or less parallel, unjointed,
cartilaginous radialia, and basally by a similar series of shorter,
stouter, and less numerous cartilages, which apparently were imbedded in
the body-wall, the entire fin skeleton presenting a striking resemblance to
an isolated median fin in which the supporting radialia have concentrated
by growth pressure, and their proximal portions have been reduced in number
by partial fusion.[218] _Pleuracanthus_, on the other hand, had a biserial
fin, the preaxial and postaxial radialia supporting fan-like clusters of
horny fibres at their distal ends (Fig. 250).
[Illustration: FIG. 145.—A, Pectoral fin, and B, pelvic fin of
_Cladoselache_. (From Bashford Dean.)]
{243}[Illustration: FIG. 146.—Pectoral fins of various Fishes. A,
_Acanthias vulgaris_; B, _Raia sp_.; C, _Chimaera monstrosa_; D, _Acipenser
rhynchaeus_; E, _Amia calva_; F, _Lepidosteus platyrhynchus_; G,
_Polypterus bichir_; H, _Salmo salvelinus_. The preaxial side of each fin
is to the left and the postaxial to the right. _f.r_, Dermal fin-ray; _ms_,
mesopterygium; _mt_, metapterygium; _p_, propterygium; _r_, free radialia;
1, 5, the preaxial and postaxial basal elements in a Teleost, which may be
mesopterygial and metapterygial pieces respectively, the three remaining
basal pieces probably being intrusive metapterygial radialia directly
articulating with the pectoral girdle. In B, D, E, and F, similar intrusive
radialia are shown. (From Gegenbaur.)]
The broadly lobate pectoral fin of the existing Crossopterygii (Fig. 146,
G) is uniserial, closely resembling that of the more typical
Elasmobranchs.[219] There are three basal elements, a propterygium, a
mesopterygium, and a metapterygium, each of which supports a series of
partially ossified radialia. Little is known of the endoskeletal elements
of the broadly or acutely lobate fins of the fossil Crossopterygii, but it
seems probable that their disposition was uniserial and abbreviate in
obtusely lobate fins and biserial in acutely lobate fins. In the remaining
Teleostomi (Actinopterygii) the endoskeletal elements become {244}gradually
reduced in number and importance, their place as fin-supports being usurped
by the dermal fin-rays. In addition, more than three, usually several,
basal elements articulate directly with the pectoral girdle, and hence the
fins become multi-basal. In the Chondrostei and the Holostei a
metapterygium is always recognisable, supporting several radialia along its
preaxial border, as in _Acipenser_ (Fig. 146, D) and _Amia_ (Fig. 146, E),
or only a single one, as in _Lepidosteus_ (Fig. 146, F). The anterior part
of the fin is supported by a variable number of cartilaginous or bony
radialia, which, with the metapterygium, articulate with the limb-girdle.
In Teleosts the process of reduction reaches its maximum. Usually there is
but a single row of short, hour-glass-shaped ossicles, of which the
postaxial one may represent a vestigial metapterygium, and sometimes there
is also a distal row of small cartilages or ossicles, partially hidden in
the cleft bases of the dermal fin-rays (Fig. 146, H). In all these Fishes
the fin is a much reduced uniserial fin, in which more or fewer of the
preaxial radialia have acquired a direct secondary connexion with the
pectoral girdle. Of living Dipnoids _Neoceratodus_ has a nearly typical
biserial fin, but, as seems to be the case in all fins of this type at
present known, there is a marked absence of symmetry in the number and
disposition of the radialia on the two sides of the axis. There is also
much individual variation. No two fins are precisely alike, and the
radialia may sometimes divide. In the very acutely lobate fins of the
remaining Dipnoids it is evident that great reduction has taken place.
_Protopterus_ has lost all trace of postaxial radialia, and in
_Lepidosiren_ even the preaxial have atrophied, leaving only the long
jointed axis to represent the originally biserial fin.
[Illustration: FIG. 147.—The left pectoral fin of _Neoceratodus_. _a_, _b_,
First two segments of the axis; _FS_, preaxial horny fibres; †, †, pre- and
post-axial radialia. (After Wiedersheim.)]
{245}THE PELVIC FINS.—In the simplicity of their endoskeletal supports the
pelvic fins of _Cladoselache_ are the most primitive type of paired fins at
present known (Fig. 145, B). In general structure they resemble the
pectorals, but the radialia are fewer in number, less modified by
concentration, and exhibit little, if any, trace of basal fusion. Add to
such features as these the apparent absence of any trace of pelvic
rudiments, or of basipterygia, and it will be obvious that the pelvic fins
differ but little from the median fins of the same Fish except that they
are paired. In _Pleuracanthus_ the pelvic fins differ from the
corresponding pectorals in being uniserial instead of biserial (Fig. 250).
All other Elasmobranchs, including the Holocephali, have uniserial fins,
which consist of a large metapterygium, supporting a preaxial fringe of
segmented radialia. A propterygium is sometimes present, notably in some of
the Skates and Rays, and, like the metapterygium, it is directly connected
with the pelvic girdle.
[Illustration: FIG. 148.—Skeleton of a pelvic fin of _Polyodon folium_,
ventral view, with the anterior margin of the fin to the right; to show the
partial fusion of the proximal portions of primitively distinct radialia to
form a basipterygium. _b_, Inner or mesial extremity of the basipterygium;
_d.p_, dorsally directed, rudimentary iliac process; _n_, foramen for
nerves. (After Rautenfeld.)]
The skeleton of the pelvic fins of the Teleostomi is often extremely
degenerate. It is perhaps best developed in the Chondrostei,[220] where
each fin is supported by numerous segmented radialia, more or fewer of
which fuse towards the base of the fin, and those form a large and slightly
ossified basipterygium (Fig. 148). In the living Crossopterygii, Holostei,
and Teleostei, the pelvic fins are similar in essential structure, but are
very degenerate. The basipterygium is usually well developed and is always
bony (Fig. 149), and in many Teleosts it acquires so extensive a sutural
connexion with its fellow that, physiologically, it supplies the place of a
true pelvic girdle. At its distal end there may be a single row of small
cartilaginous or bony nodules, representing vestigial radialia, as in the
Crossopterygii, Holostei, and {246}Teleostei, but even these may be absent,
and the dermal fin-rays then articulate directly with the basipterygium.
Little is known of the skeleton of the pelvic fins in the fossil
Crossopterygii, but there is evidence of the existence of a higher grade of
structure than in their surviving allies. In _Eusthenopteron_,[221] for
example, the fin is supported by an axis of at least three bony segments,
with at least three ossified preaxial radialia; hence, it has obviously
undergone less degeneration than in _Polypterus_, where the fin-skeleton is
essentially Teleostean. In the Dipnoi the pelvic fins are similar to the
corresponding pectoral fins, but individual variation is more marked and
even the central axis may divide.[222] In the males of all existing
Elasmobranchs, including the Holocephali, certain of the more distally
situated metapterygial radialia become modified to form a supporting
skeleton for the copulatory organs, the claspers, or mixipterygia. In the
latter group the anterior claspers are also provided with cartilaginous
supports articulating with the pelvic girdle directly in front of the
pelvic fins.
[Illustration: FIG. 149.—Skeleton of the left pelvic fin of a Trout (_Salmo
fario_), seen from the dorsal surface. B.PTG, Basipterygium; _D.F.R_,
dermal fin rays; PTG, distal radialia. (From Parker and Haswell.)]
{247}CHAPTER IX
THE DENTITION, ALIMENTARY CANAL, AND DIGESTIVE GLANDS
The alimentary canal is a muscular tube with an epithelial lining, formed
for the reception and the digestion of the food. It begins with a mouth,
and from thence it extends backwards through the coelom, finally
communicating with the exterior either by a cloacal or by an anal orifice.
The oral or buccal cavity into which the mouth leads is a stomodaeum, and
is lined by inpushed epidermis, while the hinder portion of the cloaca and
the anus are lined by a somewhat similar inpushing of the epidermis which
forms the proctodaeum. The rest of the alimentary canal, consisting in
succession of a pharynx, an oesophagus, a stomach, and an intestine,
constitutes the mesenteron, and is lined by endoderm. Teeth are developed
from the walls of the stomodaeum, and glands for the secretion of digestive
fluids from the endoderm of the mesenteron.
DENTITION.
In the Lampreys among the Cyclostomata teeth are developed in the form of
yellow conical structures on the inner surface of the buccal funnel, and on
the extremity of the rasping "tongue" (Fig. 91, A). Each tooth consists of
an axial papilla of the dermis, sometimes enclosing a pulp-cavity, and
invested by the epidermis, and also by a stratified horny cone which forms
the projecting hard part of the tooth. The dermal papilla with its
ectodermal investment bears a superficial resemblance to the germ of a true
calcified tooth, but no odontoblasts are formed, nor any calcic deposit,
the laminated horny teeth being formed by the gradual conversion of the
successive strata of the {248}epidermic cells into horny layers.[223] The
old teeth are vertically replaced by new teeth developed beneath the
functional teeth. With the exception of a median tooth above the oral
aperture, _Myxine_ and its allies have only lingual teeth. These are
comb-like, and they are formed by the basal fusion of primitively distinct
tooth-germs. The structure and development of the teeth of the Cyclostomes
lend no support to the view that the teeth are degenerate calcified
structures. With greater probability they represent a stage in the
evolution of teeth and dermal spines, which has been succeeded by a later
stage in which calcification superseded cornification as a method of
hardening.
[Illustration: FIG. 150.—Vertical section of developing tooth in
_Petromyzon marinus_, showing a successional tooth, which is just beginning
to cornify at its apex beneath the functional tooth. _d_, Dermis; _d.p_,
dermal papillae; _ep_, epidermis lining buccal funnel; _ep^1_, epidermis
which has formed the horny functional tooth _ht_; _ep^2_, epidermis forming
the horny cone of the successional tooth _ht^1_. (From Warren.)]
True calcified teeth first make their appearance in Fishes, where they
assume the form of modifications of exoskeletal structures.[224] The teeth
of Elasmobranchs are identical in essential structure, as well as in the
manner of their development, with the ordinary dermal spines of the skin,
and in the embryo the {249}dermal spines form a continuous series with
those which invest the jaws and eventually become teeth (Fig. 151). It is
only later, when lips become apparent, that the continuity of the teeth and
dermal spines is interrupted, and the two structures assume their
distinctive characters.
The tissues of which the teeth of Fishes are composed are (1) _dentine_,
which is a non-vascular, calcified tissue, traversed by numerous radiating,
branched, dentinal tubuli, into which extend protoplasmic prolongations
from the cells (scleroblasts) by which the dentine is secreted. Dentine
forms the greater part of the body of a tooth. (2) _vasodentine_ and (3)
_osteodentine_ are modifications of ordinary dentine, the former containing
blood-vessels ramifying in its substance but no dentinal tubules, and the
latter more closely resembling bone. (4) _enamel_, an exceptionally dense,
non-vascular, non-tubular tissue, which may or may not exhibit traces of
the prismatic structure so characteristic of this tissue in the higher
Vertebrates, forms the outer investment of the teeth.
[Illustration: FIG. 151.—Transverse section through the lower jaw of an
embryo _Scyllium_, to show the gradual transition from dermal spines (_d_,
_d_, _d_) on the outer surface of the jaw to teeth (_t_, _t_, _t_) on the
oral surface. _c_, Cartilage of the lower jaw. (From Gegenbaur.)]
As regards their fixation, the more primitive forms of teeth, such as those
of Elasmobranchs, are simply embedded in the gums, and are only connected
with the jaws by fibrous tissue; but in some of the older fossil Sharks the
fixation of the teeth is effected by the mutual articulation of the basal
plates of the teeth with one another. The Chondrostean _Polyodon_, so
{250}shark-like in many other respects, also has teeth implanted basally in
the gums, and quite free from any special connexion with the jaw-bones. In
some Teleosts with movable teeth, the latter are merely attached to the
jaws by fibrous, and often elastic, ligaments, as in the Pike (_Esox_) and
the Angler-Fish (_Lophius_). As a rule, however, the teeth are directly
ankylosed to the bones developed in relation with the jaws. Very rarely,
as, for example, in some Characinidae, are the teeth implanted in sockets.
Nearly all Fishes are polyphyodont, that is, the old teeth are constantly
replaced by new teeth as fast as they become worn down or fall out. In the
Sharks and Dog-Fishes, for example, where the teeth are arranged in rows
parallel to the axis of each jaw, the functional teeth along the upper edge
of the jaw are usually erect, while those in the rows more internally
situated point inwards towards the oral cavity; and behind these again
there are rows of developing teeth in different stages of growth, and
partially hidden beneath a projecting fold of the oral mucous membrane
(Fig. 152). As the teeth in use become lost they are successively replaced
by the inner rows, which, with the mucous membrane in which they are
embedded, move forwards to the edge of the jaw, where they become erect and
functional. The teeth of the Holocephali and of the Dipnoi are not shed,
but the loss which they sustain through wear and tear is made good by
persistent growth at their bases. In the Teleostomi the succession is less
regular, new teeth being formed between or at the bases of the old teeth.
In the case of socketed teeth the succession is usually vertical, the new
teeth being formed at the sides of the old ones; and by the absorption of
the bases of the latter, the former come to lie directly below them, and
eventually they occupy the same sockets.
[Illustration: FIG. 152.—Transverse section through the jaw of a Shark
(_Carcharias_), showing how the teeth are replaced. _c_, Cartilage of the
jaw; _t_, functional tooth; _t′_, its immediate successor; _t"_, _t"_,
still younger teeth, covered by the fold of mucous membrane, _m. m_. (From
Ridewood.)]
As might be expected from the remarkable diversity in the {251}habits and
in the food of different Fishes, the teeth exhibit an equally striking
diversity in form, size, and structure. The most primitive type of tooth
resembles an ordinary dermal spine, and is little more than a simple
pointed cone. A few Elasmobranchs and many Teleostomi possess teeth of this
kind. By the flattening of the cone parallel to the axis of the jaw, the
tooth becomes triangular, and then the margins may either remain smooth and
trenchant, or they may become complicated by the formation of marginal
serrations or of accessory basal cusps, and by such modifications the
characteristic teeth of most Elasmobranchs are formed. The simple cone may
also be modified to form crushing teeth—short, blunt, more or less
hemispherical teeth—or even transformed into a mosaic of hexagonal plates,
as in the Myliobatidae amongst Elasmobranchs. Massive, flattened,
scroll-like crushing teeth are also formed by the fusion of adjacent teeth,
or of several successional teeth, and of such composite teeth we have
examples in the Heterodontidae and in the Palaeozoic Cochliodontidae. By a
somewhat similar process of concrescence the anomalous composite teeth of
such Teleosts as the Diodons and Tetrodons, and of the Parrot-Fish
(_Scarus_), have been evolved. The singular dental structures of the
Holocephali are probably composite teeth, and it is certain that the highly
characteristic teeth of the Dipnoi have resulted from the basal fusion of
primitively distinct simple conical denticles. The dentition is often
heterodont. In _Heterodontus_ (_Cestracion_), for example, the anterior
teeth in each jaw are pointed and prehensile, while the hinder ones are
scroll-like and crushing. Prehensile and crushing molar-like teeth are also
present in such Teleosts as many of the Sparidae, and in the Wolf-Fish
(_Anarrhichas_). The existence of sexual differences in the dentition is
illustrated in the Skates and Rays (_Raia_), where teeth which are simple
and pointed in the male become flattened and plate-like in the female. A
few Teleosts, like the Syngnathidae, Cyprinidae, and some Siluridae, are
entirely devoid of jaw-teeth.
In addition to jaw-teeth, many Teleosts possess pharyngeal or gill-teeth,
developed in connexion with the inner margins of the branchial arches, to
which they are usually firmly ankylosed (Figs. 352, 412 and 413). As a rule
"the pharyngeal dentition is inversely proportional to the extent of tooth
development {252}on the jaws."[225] Pharyngeal teeth differ greatly in size
and structure in different Teleosts, and, like the jaw-teeth, they are
capable of replacement by vertical succession. The teeth are sometimes
restricted to the inferior pharyngeal bones (cerato-branchials of the last
branchial arch), and then, as in the Carp (_Cyprinus_), they may bite
against a callous pad on the under surface of the basioccipital bone; or,
as in some of the Wrasses (_Labrus_), the inferior teeth are opposed to
superior teeth on the upper pharyngeal bones (pharyngo-branchials of more
or fewer of the branchial arches). When pharyngeal teeth are present it is
probable that they are the principal masticatory organs, the jaw-teeth
being used for seizing or holding the prey.
ALIMENTARY CANAL.
A protrusible tongue is never developed in Fishes. A rudiment of that organ
is present in the Elasmobranchs (Fig. 153) and Dipnoi, and also in the
Crossopterygii, and usually consists of an elevated area of mucous membrane
provided with free lateral edges and a forwardly projecting apex; it is
supported by the basi-hyal element of the hyoid arch. In the Crossopterygii
(e.g. _Polypterus_) the tongue contains muscle fibres, and in the Dipnoi,
where the organ is better developed than in any other Fishes, special
lingual muscles are present.
The pharynx succeeds the oral cavity, and is perforated on each side by the
branchial clefts (Figs. 153, 154). The rest of the alimentary canal differs
considerably in various Fishes in the degree of distinctness of its several
regions, and in the extent to which it is convoluted. As a rule the pharynx
is followed in succession by an oesophagus, a stomach, and an intestine
(Fig. 153), the latter terminating in a portion usually termed the
"rectum." The boundaries of these regions are not always very obvious, but
are indicated by variations in calibre, by changes in the character of the
lining epithelium, by special valves or sphincter muscles, or by the
entrance of the ducts of certain glands like the pancreas and liver.
{253}[Illustration: FIG. 153.—Dissection of a male Dog-Fish (_Scyllium_).
The left side of the body is cut away to the median plane so as to expose
the abdominal and pericardial cavities and the neural canal in their whole
length. The alimentary canal and the liver have been drawn downwards, and
the oral cavity, the pharynx, part of the intestine, and the cloaca have
been opened. The cartilaginous parts of the skeleton are dotted, and the
calcified portions of the vertebral centra are black. _abd.cav_, Abdominal
cavity; _au_, auricle; _b.br_, basi-branchial; _b.hy_, basi-hyal; _c.art_,
conus arteriosus; _cd.a_, caudal artery; _cd.st_, cardiac part of the
stomach; _cd.v_, caudal vein; _cl_, cloaca; _cn_, centrum; _cr_, cranium;
_crb_, cerebellum; _d.ao_, dorsal aorta; _dien_, thalamencephalon; _epid_,
epididymis; _fon_, fontanelle; _gul_, oesophagus; _h.a_, haemal arch;
_i.br.a^1_-_i.br.a^5_, internal gill-clefts; _int_, intestine; _kd_,
kidney; _l.j_, lower jaw; _l.lr_, left lobe of liver; _med.obl_, medulla
oblongata; _mes_, mesentery; _n.a_, neural arch; _n.cav_, neural canal;
_olf.l_, olfactory lobes; _opt.l_, optic lobes; _pan_, pancreas; _pcd.cav_,
pericardial cavity; _pct.a_, pectoral arch; _ph_, pharynx; _pin_, pineal
body; _p.n.d_, vestigial Müllerian duct; _prs_, prosencephalon; _pty_,
pituitary body; _pv.a_, pelvic arch; _pyl.st_, pyloric portion of the
stomach; _r_, rostrum; _r.lr_, right lobe of liver; _rct.gl_, rectal gland;
_sp_, spiracle; _sp.cd_, spinal cord; _spl_, spleen; _sp.s_, sperm sac;
_sp.vl_, spiral valve; _s.v_, sinus venosus; _tng_, tongue; _ts_, testis;
_u.g.s_, urino-genital sinus; _u.j_, upper jaw; _ur_, metanephric duct;
_v_, ventricle; _v.ao_, ventral aorta; _v.def_, vas deferens or mesonephric
duct; _vs.sem_, vesicula seminalis. (From Wiedersheim, after T. J.
Parker.)]
The oesophagus is occasionally separated from the stomach by a slight
constriction, but more frequently the replacement of the squamous
epithelium of the oesophagus by the columnar epithelium of the stomach and
the appearance of gastric glands in the wall of the latter cavity afford
the only distinction between the two regions. The {254}commencement of the
intestine is usually indicated by a pyloric "valve" (Fig. 155, A, B), in
the form of a ring-like, inwardly projecting thickening of the
circularly-disposed muscle fibres of the terminal extremity of the stomach,
and usually also by the entrance of the distinct or united ducts of the
liver and pancreas; sometimes, as in certain Elasmobranchs and in the
Dipnoi, by a special dilatation or "Bursa Entiana" (Fig. 155, A). The
rectum, or terminal portion of the intestine, is distinguished from the
rest of the gut by its straight course to the cloacal aperture or the anus,
and sometimes by an increase in calibre. In _Box vulgaris_ and a few other
Teleosts[226] a caecal diverticulum indicates the commencement of the
rectum, while in a few cases the pre-rectal portion of the intestine
communicates with the enlarged rectal segment by a much constricted
valvular orifice which is suggestive of the ileo-colic valve of the higher
Vertebrates,[227] as in the Teleosts _Amiurus catus_,[228] _Trigla
gurnardus_, and _Cyclopterus lumpus_.
The relation of the regional divisions of the intestine in Fishes to those
of other Vertebrates are somewhat difficult to determine. If we may regard
the "rectal" gland of Elasmobranchs and the intestinal caecum of certain
Teleosts as homologous with each other, and with the caecum coli of the
higher Vertebrates, then it would seem that by far the greater part of the
intestine of Fishes, including that portion in which a spiral valve may be
developed, is homologous with the pre-caecal segment of the gut or small
intestine in other Vertebrates, and that the post-caecal section, or large
intestine, of the latter is represented in Fishes only by that relatively
short portion of the gut which lies posterior to the rectal gland or its
homologue in Teleosts, the equivalent of the colon of Mammalia being, as in
Amphibia, Reptiles, and Birds, practically undifferentiated.[229]
In the Cyclostomata the alimentary canal retains much of its primitive
simplicity. It pursues a straight course from mouth to anus, and the usual
regions are very obscurely indicated. The same remarks apply also to the
Holocephali and a few Teleosts, although in these Fishes the limits of the
different regions are somewhat more clearly defined. In the Dipnoi (Fig.
155, A), a contracted sigmoid curve between the somewhat dilated stomach
and the spacious intestine is the only departure from the straight course
of the preceding groups.
{255}[Illustration: FIG. 154.—Dissection of a male Teleost (_Salmo fario_)
from the left side. _a.bl_, Air-bladder opened; _an_, anus; _au_, auricle;
_b.a_, bulbus aortae; B.HY, basi-hyal; B.OC, basioccipital; _cd.a_, caudal
artery; _cd.v_, caudal vein; CN, centrum; _crb_, cerebellum; _d.f.1_, first
dorsal fin; _D.F.R_, dermal fin-rays; _du_, duodenum or anterior segment of
the intestine; _FR_, frontal; _g.bl_, gall-bladder; _gul_, oesophagus or
gullet; H.SP, haemal spine; _int_, intestine; _kd_, kidney; _kd′_,
"head-kidney"; _lg_, tongue; _lr_, liver; N.SP, neural spine; _opt.l_,
optic lobes; _PA.SPH_, parasphenoid; _ph_, pharynx; _pn.b_, pineal body;
_pn.d_, bristle passed into ductus pneumaticus; _prsen_, prosencephalon;
_pty.b_, pituitary body; PTG, pterygiophores, or radial elements of dorsal
and ventral fins; _pv.f_, pelvic fin; _py.c_, pyloric caeca; _S.ETH_,
supra-ethmoid; S.OC, supra-occipital; _spl_, spleen; _st_, stomach; _ts_,
testis; _u.bl_, urinary bladder; _u.g.s_, urino-genital sinus and its
external aperture; _ur_, ureter or kidney-duct; _v_, ventricle; _v.ao_,
ventral aorta; _v.df_, vas deferens; _v.f_, ventral fin; _VO_, vomer. (From
Parker and Haswell.)]
{256}In the remaining Fishes the degree of convolution varies within rather
wide limits. The oesophagus is usually straight and wide, but in
_Lutodeira_, among Teleosts, it is long and even convoluted, and in the
Plectognath Teleosts it gives off a large sac-like outgrowth ("air-sac"),
which extends anteriorly as far as the head, and posteriorly to the
beginning of the tail, and communicates with the oesophagus by two
apertures. The stomach may be U-shaped with the concavity directed
forwards, and consisting of a right limb passing backwards from the
oesophagus, and a left limb curving forwards to its junction with the
intestine (Fig. 153). In such instances as these the stomach and the
adjacent section of the intestine describe a characteristic siphonal curve.
In certain other Fishes (Fig. 160), the oesophageal portion of the stomach
terminates behind in a tubular or sac-like dilatation at some distance
posterior to the laterally situated pylorus, which indicates the origin of
the intestine. The intestine is straight, or nearly so, in Elasmobranchs,
Crossopterygii, and Dipnoi, and also in a few Teleosts; but sometimes, and
very generally in Teleosts, it is more or less convoluted, notably in some
of the Mugilidae, and in the Loricariidae, where, as in _Plecostomus_, it
is disposed in numerous spiral coils like a watch-spring. The terminal
portion of the intestine or rectum either opens into a cloaca, which also
receives the urinary and genital ducts, as in Elasmobranchs (Fig. 153), and
Dipnoi (Fig. 155, A), or opens externally by an anus, situated in front of
the separate or united urinogenital ducts, as is the case with all the
remaining groups of Fishes (Fig. 154). The cloacal aperture is invariably
situated near the junction of the caudal and trunk regions, and as a rule
is median in position, rarely, as in the Dipnoi, displaced to the right or
left of the middle line; but the anus differs greatly in position,
sometimes retaining its primitive position at the hinder end of the trunk,
as in the Holocephali, Chondrostei, Crossopterygii, Holostei, and many
Teleosts, or occupying almost any position between that point and, as in
the "Electric Eels" (Gymnotidae), the ventral surface of the throat (Fig.
351.)
{257}[Illustration: FIG. 155.—A, alimentary canal and liver of a female
_Protopterus_, from the left side. Part of the left wall of the stomach and
intestine, and the peritoneal investment of the spleen have been removed.
_a.p_, Abdominal pore; _b.d_, bile-duct; _b.ent_, Bursa Entiana; _cl_,
cloaca; _cl.ap_, cloacal aperture; _cl.c_, caecum cloacae; _c.m.a_,
coeliaco-mesenteric artery; _cy.d_, bile duct; _k.d_, kidney duct; _m.a_,
mesenteric arteries; _od_, oviduct; _pt.c_, post-caval vein or inferior
vena cava; _p.v_, portal vein; the other reference letters as in B. (From
Newton Parker.) B, viscera of an adult female _Lepidosteus_, ventral view.
The oesophagus, the commencement of the intestine and the rectum have been
laid open. _ab_, air-bladder; _an_, anus; _b.d_, intestinal aperture of the
bile-duct; _g.b_, gall-bladder; _gl_, oesophageal aperture of the
air-bladder; _h.d_, hepatic duct; _l_, liver; _oes_, oesophagus; _py_,
pylorus; _py.c_; pyloric caeca; _py.c′_, the four intestinal orifices of
the pyloric caeca; _r_, rectum; _s_, spleen; _sp.v_, spiral valve; _st_,
stomach. (From Balfour and Newton Parker.)]
{258}[Illustration: FIG. 156.—Transverse section of a Fish, diagrammatic.
_cn_, Centrum; _coel_, coelome; _d.a_, dorsal aorta; _d.f_, dorsal fin;
_d.m_, dorsal muscles; _d.ms_, dorsal mesentery; _f.r_, fin ray; _gon_,
gonad; _int_, intestine; _l.v_, lateral vein; _msn_, mesonephros; _msn.d_,
mesonephric duct; _n.a_, neural arch; _p_, parietal layer of the
peritoneum; _p′_, visceral layer; _p.c.v_, posterior cardinal vein; _pn.d_,
Müllerian duct; _r_, ventral rib; _r′_, dorsal rib; _sp.c_, spinal cord;
_t.p_, transverse process; _v.m_, ventral muscles; _v.ms_, ventral
mesentery. (Modified, after Parker and Haswell.)]
The whole length of the alimentary canal from the oesophagus to the rectum
is invested externally by the visceral layer of the peritoneum (Fig. 156),
which histologically consists of a stratum of connective tissue, supporting
on its free surface an epithelial stratum (coelomic epithelium). Primarily,
the investing peritoneum is continued both dorsally and ventrally into
bilaminar suspensory folds, the dorsal and ventral mesenteries (_d.ms_,
_v.ms_), which extend to the mid-dorsal or mid-ventral line of the
abdominal cavity. The two layers then separate and become continuous with
the parietal layer of the peritoneum lining the whole of the inner surface
of the body-wall. Embryologically, the two mesenteries owe their formation
to the fusion above and below the mesenteron of the contiguous walls of two
laterally situated and primitively distinct coelomic cavities. The dorsal
mesentery in the adult is occasionally complete, as in the Myxinoid
Cyclostomata and in the Elasmobranch _Hypnos subnigrum_,[230] and also in
some Dipnoi and in a few Teleosts, but much more frequently it is reduced
by absorption to anterior and posterior remnants, or to a series of
isolated bands, or even, as in the Lamprey (_Petromyzon_), to a few
filaments accompanying the intestinal blood-vessels. The ventral mesentery,
on the contrary, is rarely present, and if present is never complete. In
_Lepidosteus_[231] a ventral mesentery {259}is said to be present in
connexion with that part of the intestine which contains the spiral valve.
In _Protopterus_,[232] and also in _Neoceratodus_,[233] there is a
well-developed ventral mesentery in relation with the greater part of the
length of the intestine, although in the former Dipnoid its continuity is
interrupted by one or two vacuities, and in the latter the mesentery is
incomplete posteriorly. A ventral mesentery is also present in the
intestinal region of some of the Muraenidae among Teleosts.[234]
[Illustration: FIG. 157.—Transverse section through a portion of the wall
of the intestine, combined from the condition seen in both the higher and
the lower Vertebrata. Semi-diagrammatic. _a.c_, Epithelial cells in the
amoeboid state; _b.v_, blood-vessels; _c.m_, circular muscular layer; _g_,
one of Lieberkühn's glands in the higher Vertebrates; _i.ep_, intestinal
epithelium; _l_, leucocytes; _l′_, leucocytes in the intestinal epithelium;
_l.f_, lymph follicles; _l.m_, longitudinal muscular layer; _lym_,
lymphatic vessels; _p_, visceral layer of the peritoneum; _sm_, the
submucosa; _v_, villi of the higher Vertebrates. (From Wiedersheim.)]
Internal to its peritoneal investment the wall of the alimentary canal
consists in succession from without inwards of (1), a {260}muscular coat,
(2) the submucosa, and (3) an epithelial stratum or mucous membrane, the
first two of these layers, with the addition of the peritoneum, being
derivatives of the inner or splanchnic portion of the embryonic
mesoblast.[235]
Excluding the oesophagus, where the muscular coat is mainly composed of
striated fibres, the musculature of the alimentary canal usually consists
solely of non-striated, spindle-shaped fibres disposed in two layers, an
external stratum of longitudinally arranged fibres, and an inner stratum of
circularly disposed fibres (Fig. 157), with the addition, in the stomach,
of an oblique layer between the two. In the oesophagus the reverse
arrangement may exist, the circular layer being external and the
longitudinal internal. The muscular coat varies considerably in thickness
in different regions and in different Fishes, and in the Cyclostomata, the
Holocephali, some Teleosts, and the Dipnoi may be very feebly developed, or
even entirely absent, as in the intestine of the Hag-Fish (_Myxine_). In
the Gillaroo Trout (_Salmo stomachicus_),[236] on the contrary, the distal
section of the siphonal stomach has its musculature unusually thickened, so
as to form an incipient gizzard for the crushing of the shells of the
freshwater Molluscs on which the Fish feeds. In some of the Mullets
(Mugilidae),[237] a true gizzard is developed by the enormous thickening of
the muscular coat of the caecal stomach, the cavity of which, in
consequence, is reduced to a mere vertical fissure, and is lined by an
exceptionally thick, horny epithelium.
There are a few exceptions to the rule that the muscular fibres are of the
non-striated variety. Thus in some Teleosts, as in the Tench (_Tinca
vulgaris_), striated fibres are continued from the oesophagus into the
walls of the stomach and intestine, and there form an outer longitudinal
and an inner circular layer, situated externally to the corresponding
layers of the non-striated stratum. {261}In the Siluroid, _Amiurus_, the
striated fibres of the outer circular layer of the oesophagus are
continued, although but sparsely, into the inner circular layer of the
stomach.
The submucosa (Fig. 157) lies between the muscular layer externally and the
epithelial lining internally, and is characteristically developed in the
stomach, and even more so in the intestine. Histologically, it consists of
a framework of connective tissue, enclosing in its meshes masses of
leucocytes (lymphoid tissue), some of which are amoeboid and migratory, and
may even be found between the cells of the intestinal epithelium (including
in some instances the cloacal epithelium), probably actively participating
in the transmission of food material from the alimentary canal to the
lymphatics and blood-vessels; while other and somewhat similar, but larger,
leucocytes (phagocytes), are concerned with the elimination of waste
substances or noxious micro-organisms. In addition to the diffused lymphoid
tissue of the submucosa, special rounded or oval, and sometimes encapsuled,
masses of this tissue (lymph follicles) are common in the intestinal wall
(Fig. 157) of _Acipenser_, the Dipnoi and some Elasmobranchs, and are
perhaps the only representatives in Fishes of the solitary follicles or
"Peyer's patches" of the higher Vertebrates. A mass of lymphoid tissue
exists in the axis of the spiral valve of _Acipenser_, which has been
compared with a similarly situated structure in _Lepidosiren_.[238] In some
Elasmobranchs a large lymphoid organ is imbedded in the submucosa of the
oesophageal wall, while a local thickening of the tissue is met with in the
pyloric sphincter. _Protopterus_ is remarkable among Vertebrates for the
extraordinary development of lymphoid tissue,[239] which, apart from its
distribution in the submucosa, is abundantly present between the
longitudinal and circular muscle layers, and the peritoneal and muscular
coats of the intestine.
In addition to the lymphoid tissue the submucosa contains non-striated
muscle cells and plexuses of capillary blood-vessels, which in certain
Loaches (e.g. _Misgurnus_), where intestinal respiration occurs, extend
between the cells of the intestinal epithelium. A network of lymphatic
spaces or vessels surrounds the blood-vessels. In some Elasmobranchs the
small arteries of the submucosa of the stomach are provided with singular
sphincter muscles, which {262}occasionally encircle both the artery and the
corresponding vein.[240]
The lining epithelium differs considerably in character in different
portions of the alimentary canal. The epithelium of the mouth, pharynx, and
anterior section of the oesophagus is often squamous and is succeeded in
the hinder part of the oesophagus, and in the stomach and intestine, by a
columnar epithelium. As a rule the epithelium of the rectum is also
columnar, but in Elasmobranchs it may become squamous. Goblet cells are of
very frequent occurrence throughout the whole length of the alimentary
canal, from the mouth to the rectum inclusive, interspersed between the
superficial epithelial cells; in the same position in the intestine
migratory leucocytes have been found. The primitive ciliation of the
Vertebrate alimentary canal is retained to a greater or less extent in many
Fishes, and is sometimes, but not always, associated with a feeble
development of the musculature. In the larval form of _Petromyzon_
(_Ammocoetes_), the whole canal is ciliated except the pharynx and rectum;
but in the adult ciliation is retained only in places which gradually
become fewer as the rectum is approached. In the Myxinoids, however, cilia
are said to be absent.
In the Dipnoi (e.g. _Protopterus_) the epithelium of the stomach and
intestine is largely ciliated, but in Elasmobranchs, ciliation is usually
restricted to the posterior portion of the oesophagus and the edge of the
spiral valve. Among the more generalised Teleostomi (e.g. _Acipenser_,
_Lepidosteus_, _Amia_), the oesophagus, stomach, and intestine may be
ciliated, but to an extent which varies in different genera. The pyloric
appendages, when present, are also more or less extensively ciliated. In
Teleosts, however, the recorded instances of ciliation are relatively rare.
Nevertheless, ciliated epithelium has been found in the intestine of a few
species (e.g. _Rhombus aculeatus_ and _Syngnathus acus_), and also in the
pyloric appendages; in the stomach (e.g. _Perca_ and _Esox_), and even in
the oesophagus (e.g. _Perca_).
The mucous membrane, including the submucosa, is frequently developed into
variously arranged ingrowths projecting into the lumen of the alimentary
canal; these are generally of the nature of longitudinal or transverse
ridges, or a combination of the two, giving rise to retiform structures.
The simple longitudinal {263}folds, which are sometimes found in the
oesophagus, stomach, and rectum, often disappear on distension, and
probably merely provide for the enlargement of these cavities during the
deglutition of relatively large prey, or for the accumulation of faeces. On
the other hand, the permanent and often complicated folds of the intestinal
mucous membrane are probably related to an increase in the secretive or
absorptive area of this portion of the alimentary canal. In the stomach the
mucous membrane is usually smooth, rarely, as in the "Electric Eel"
(_Gymnotus_), reticulate. In the intestine the folds assume a highly
characteristic and often complicated disposition.[241] In the Cyclostomata
the folds are simple and longitudinally arranged. In Elasmobranchs (Fig.
158, A), obliquely transverse folds are present in addition, and, uniting
with the longitudinal ridges, bound linear depressions.
[Illustration: FIG. 158.—The intestinal mucous membrane of different
Fishes, to show the transition from simple longitudinal and transverse
folds to crypts. A, Of an Elasmobranch; B, C, and D, of various Teleosts.
(After Wiedersheim.)]
In various Teleostomi (Fig. 158, B, C, D), the union of the two series of
folds becomes more or less retiform, and the network of intersecting ridges
bounds a series of deep tubular crypts which appear to penetrate to a
considerable distance into the intestinal wall, and possibly foreshadow the
characteristic Lieberkühn's glands of Mammalia. Crypts may also be found in
the stomach, where they receive the apertures of the gastric glands, as in
_Amiurus_, but more usually they are restricted to the intestine. In the
Dipnoi (e.g. _Protopterus_) the mucous membrane of the {264}stomach,
and—excluding the Bursa Entiana where a number of oblique folds are
present—of the intestine also, is, on the contrary, perfectly smooth.