This order contains the Sharks,
the Rays, and the Chimoera
The integument may be naked, and it never possesses scales
like those of ordinary fishes; but, very commonly, it is developed
into papillae, which become calcified, and give rise to
toothlike structures: these, when they are very small and close-set, constitute what is called shagreen
. When larger and more
scattered, they form dermal plates or tubercles; and when, as
in many cases, they take the form of spines, these are called dermal defences
, and, in a fossil state, ichthyodorulites
these constitute what has been called a "placoid exoskeleton;
and, in minute structure, they precisely resemble teeth, as has
been already explained. The protruded surfaces of the dermal
defences are frequently ornamented with an elegant sculpturing,
which ceases upon that part of the defence which is imbedded
in the skin. The dermal defences are usually implanted
in front of the dorsal fins, but may be attached to the tail, or,
in rare cases, lie in front of the paired fins.
The spinal column exhibits a great diversity of structure:
from a persistent notochord exhibiting little advance upon that
of the Marsipobranchii
, or having mere osseous rings developed
in its walls, to complete vertebrae, with deep conical anterior
and posterior concavities in their centra, and having the
primitive cartilage more or less completely replaced by concentric,
or radiating, lamallae of bone. In the Rays, indeed, the
ossification goes so far as to convert the anterior part of the
vertebral column into one continuous bony mass.
|Fig. 32. - Vertical anil longitudinal section of the anterior part of the
body of a Lamprey
(Petromyzon marinus): A, the cranium with its
contiained brain; a, section of the
edge of the cartilage marked a, in
Fig. 30;Olf, entrance into the
which is prolonged
caecal pouch, o; Ph, the
pharynx; Pr, the branchial channel,
inner apertures of the branchial sacs; M, the cavity of the mouth, with
horny teeth; 2, the cartilage
which supports the tongize; 3, the oral ring.
The neural arches are sometimes twice as numerous as the
centra of the vertebrae, in which case the added arches are
The terminal part of the notochord is never enclosed within
a continuous bony sheath, or urostyle
. The extremity of
the vertebral column is generally bent up, and the median finrays
which lie below it are, usually, much longer than those
which lie above it, causing the lower lobe of the tail to be
much larger than the upper. Elasmobranclis
with tails of
this conformation are truly heterocercal
, while those in which
the fin-rays of the tail are equally divided by the spinal column,
or nearly so, are diphycercal
. The Monkfish
) and many other Elasmobranchii
are more diphycercal
The ribs are always small, and may be quite rudimentary.
The skull is composed of cartilage, in which superficial
pavement-like deposits of osseous tissue may take place, but
it is always devoid of membrane bone. When movable
upon the spinal column, it articulates therewith by two condyles.
In its general form and structure, the cartilaginous skull
of an Elasmobranch
corresponds with the skull of the vertebrate
foetus in its cartilaginous state, and there are usually
more or less extensive membranous fontanelles in its upper
walls. The ethmoidal region sends horizontal plates over the
nasal sacs, the apertures of which retain their embryonic situation
upon the under-surface of the skull.
Neither premaxillae nor maxillae are present, the "jaws"
of an Elasmobranch consisting, exclusively, of cartilaginous
representatives of the primary palato-quadrate arch and of
The former of these, the so-called upper jaw, may either
be represented, as in the Chimaera
(Fig. 33), by the anterior
portion (B, D
) of a triangular cartilaginous lamella, which
stretches out from the sides of the base of the skull, and is
continuous with the representative of the hyomandibular suspensoriura;
or there may be, on each side, a cartilaginous bar
movably articulated in front with the fore-part of the skull;
and, posteriorly, furnishing a condyle, with which the ramus
of the lower jaw, representing Meckel's cartilage, articulates.
|Fig. 33. - Vertical section of the skull of Chimaera monstrosa,
without the labial and nasal
cartilages: A, the basi-occipital regrion; P, the pituitary fossa; Na, partition between
the olfactory sacs;B, alveolus for the anterior upper-jaw tooth; C, D, the region of the
triangular cartilage which answers to the hyomandibular and
quadrate; D, B, that
which answers to the quadrate, pterygoid, and
palatine; Mn, the mandible; I Or, the
interorbital septum; asc and psc the anterior and posterior semicircular canals; I., II.,
exits of the olfactory, optic, fifth and eighth pairs of nerves.
In the latter case, which is that met with in the Sharks
and Rays (Figs. 34 and 35), a single cartilaginous rod (g
movably articulated with the skull, in the region of the periotic
capsule, upon each side; and, by its opposite extremity,
is connected by ligamentous fibres both with the palatoquadrate
) and with the mandibular or Meckelian cartilage
). This cartilaginous suspensorium
represents the hyomandibular and the symplectic bones of the Teleostei
gives attachment to the hyoidean apparatus (Hy
). The latter consists of a lateral arch upon each side, united with its
fellow, and with the branchial arches, by the intermediation
of medial basal elements below; and it is succeeded by a
variable number of similar arches, which support the branchial
From the hyoidean and from the branchial arches cartilaginous
filaments pass directly outward, and support the walls
of the branchial sacs. Superficial cartilages, which lie parallel
with the branchial arches, are sometimes superimposed
upon these. There are no opercular bones, though cartilaginous
filaments which take their place (Fig. 34, Op
) may be
connected with the hyomandibular cartilage; and, in the
great majority of the Elasmobranchii
, the apertures of the
gill-sacs are completely exposed. But in one group, the Chimaera
, a great fold of membrane extends back from the
suspensorial apparatus, and hides the external gill-apertures.
Large accessory cartilages, called labial
, are developed at
the sides of the gape in many Elasmobranchii
. (Figs. 34 and 35, i, k, l.
The pectoral arch consists of a single cartilage on each
side. The two become closely united together in the ventral
median line, and are not directly connected with the skull.
The pelvis is also represented by a pair of cartilages, which
may coalesce, and are invariably abdominal in position.
There are always two pairs of lateral fins corresponding
with the anterior and posterior limbs of the higher Vertebrata
The pectoral fins, the structure of which has already been described,
are always the larger, and sometimes attain an enormous
size relatively to the body.
|Fig. 34 and 35. - The skull of Squatina, viewed from the side (Fig. 34.) and from above (Fig. 35.): a, ethmoidal region; b, prefrontal; c, postorbital; d, postauditory processes; e, occipital condyles; f, occipital foramen; g, suspensorium: h, upper dentigerous arch: i, k, l, labial cartilages: Mn, mandile; Au, auditory chamber; Or, orbit, N, nasal chamber; Op, opercular cartilaginous filaments: Er, brancbiostegal rays: Hy, hyoidean arch.
In these fishes, teeth are developed only upon the mucous
membrane which covers the palato-quadrate cartilage and the
mandible. They are never implanted in sockets, and they
vary greatly in form and in number.
In the Sharks they are always numerous, and their crowns
are usually triangular and sharp, with or without serrations
and lateral cusps. As a rule, the anterior teeth on each side
have more acute, the posterior more obtuse crowns. In the
Port Jackson shark (Cestracion
), however, the anterior teeth
are not more acute than the most obtuse teeth of the others,
while the middle teeth acquire broad, nearly flat, ridged
crowns, and the hindermost teeth are similar but smaller.
The Rays usually have somewhat obtusely-pointed teeth, but
, the middle teeth have transversely-elongated,
and the lateral ones hexagonal, flat crowns, and the various
teeth are fitted closely by their edges into a pavement. In Aetobatis
only the middle transversely elongated teeth remain.
In the Sharks and Rays the teeth are developed from papillae,
or ridges, situated at the bottom of a deep fold within the mucous
membrane of the jaw. The teeth come to the edge of
the jaw, and, as they are torn away or worn down by use,
they are replaced by others, developed, in successive rows,
from the bottom of the groove. No such successive development
takes place in the Chimaera
As in other fishes, there are no salivary glands. The wide
oesophagus leads into a stomach which is usually spacious and
sac-like, but sometimes, as in Chimaera
, may be hardly distinct
from the rest of the alimentary canal. No diverticulum filled
with air, and constituting a swimming-bladder, as in Ganoid
and many Teleostean fishes, is connected with either the oesophagus,
or the stomach, though a rudiment of this structure has
lately been discovered in some Elasmobranchs.
The intestine is short, and usually commences by a dilatation
separated from the stomach by a pyloric valve. This
duodenal segment of the intestine is usually known as the Bursa Entiana
. It receives the hepatic and pancreatic ducts,
and, in the foetus, the vitelline duct. Beyond this part, the
absorptive area of the mucous membrane of the small intestines
is increased by the production of that membrane into a
fold, the so-called spiral valve
, the fixed edge of which usually
runs spirally along the wall of the intestine. In some sharks
) the fixed edge of the fold runs
straight and parallel with the axis of the intestine, and the
fold is rolled up upon itself into a cylindrical spiral.
|Fig. 35.-The aortic bulb of a Shark (Lamna), laid open to show the three rove of valves, v, v, v, and the thick muscular wall, m.
The short rectum terminates in the front part of a cloaca,
which is common to it and the ducts of the renal and the reproductive
organs. The peritoneal cavity communicates with
that of the pericardium in front, and, behind, opens externally
by two abdominal pores
. The heart presents a single auricle,
receiving the venous blood of the body from a sinus venosus
There is a single ventricle, and the walls of the aortic bulb contain
striped muscular fibres, and are rhythmically contractile,
pulsating as regularly as those of the auricle and ventricle.
The interior of the bulb exhibits not merely a single row
of valves at the ventriculo-bulbous aperture, but several other
transverse rows of semilunar valves, which are attached to the
walls of the bulb itself, and at its junction with the aorta.
These valves must be of great importance in giving full effect
to the propulsive force exerted by the muscular wall of the
In a good many Elasmobranchii
there is a spiracle
aperture leading into the cavity of the mouth, on the upper
side of the head, in front of the suspensorium. From this
aperture (which, according to the observations of Prof. Wyman, is the
remains of the first visceral cleft of the embryo,
as well as from the proper branchial clefts, long branchial filaments
protrude, in the foetal state. These disappear in the
adult, the respiratory organs of which are flattened pouches,
with traversely-plaited walls, from five to seven in number.
They open by external clefts upon the sides (Sharks and Chimaera
or under-surface (Rays), of the neck, and, by internal
apertures, into the pharynx.
The anterior wall of the anterior sac is supported by the
hyoidean arch. Between the posterior wall of the first, and
the anterior wall of the second sac, and between the adjacent
walls of the other sacs, a branchial arch with its radiating cartilages
is interposed. Hence the hyoidean arch supports one
series of branchial plates or laminae; while the succeeding
branchial arches, except the last, bear two series, separated by
a septum, consisting of the adjacent walls of two sacs with the
interposed branchial skeleton.
The cardiac aorta, a trunk which is the continuation of the
bulb of the aorta, distributes the blood to the vessels of these
sacs; and it is there aerated by the water which is taken in at
the mouth and forced through the pharyngeal apertures, outward.
The kidneys of the Elasmobranchii
do not extend so far
forward as those of most other fishes. The ureters generally
become dilated near their terminations, and open by a common
urinary canal into the cloaca behind the rectum.
|Fig. 37. - The brain of the Skate (Raia batis). A. From above; B. A portion of the ventral
aspect enlarged; s, the olfactory bulbs; a, the cerebral hemispheres which are united In the middle line; b, the thalamencephalon; c, the mesencephalon; d, the cerebellum; a a, the plaited bands formed by the corpora restiformia; I, II., IV., V., the cerebral
nerves or the corresponding pairs; f, the medulla eblongata: w, a blood-vessel. In B; ch, the chiasma of the optic nerves; h, the pituitary body; n and v. vessels connected with It; k,the saccus vasculosus; B, the pyramids of the medulla oblongata.
The brain is well developed. It usually presents a large
cerebellum, overlying the fourth ventricle, the side-walls of
which (corpora restiformia
) are singularly folded (Fig. 37, A
and moderate-sized optic lobes, which are quite distinct
from the conspicuous thalamencephalon, or vesicle of the third
ventricle. The third ventricle itself is a relatively wide and
short cavity, which sends a prolongation forward, on each side,
into a large, single, transversely-elongated mass (Fig. 37, a
which is usually regarded as the result of the coalescence of
the cerebral hemispheres, but is perhaps, more properly, to be
considered as the thickened termination of the primitive encephalon,
in which the lamina terminalis
and the hemispheres
are hardly differentiated. The large olfactory lobes are usually
prolonged into pedicles, which dilate into great ganglionic
masses where they come into contact with the olfactory sacs
(Fig. 37, A., s). The latter always open upon the under-surface
of the head. A cleft, which extends from each nasal aperture
to the margin of the gape, is the remains of the embryonic
separation between the naso-frontal process and the maxillopalatine
process, and represents the naso-palatine passage of
the higher vertebrata
. The optic nerves fuse into a complete
chiasma (Fig. 37, B, ch
), as in the higher Vertebrata
some Sharks, the eye is provided with a third eyelid or nictitating
membrane, moved by a single muscle, or by two muscles,
arranged in a manner somewhat similar to that observed in
birds. In both Sharks and Rays, the posterior surface of the
sclerotic presents an eminence which articulates with the extremity
of a cartilaginous stem proceeding from the bottom of
Except in Chimaera
, the labyrinth is completely enclosed
in cartilage. In the Rays, the anterior and posterior "semicircular"
canals are circular, and open by distinct narrow ducts
into the vestibular sac. In the other Elasmobranchii
arranged in the ordinary way. A passage, leading from the
vestibular sac to the top of the skull, and opening there by a
valvular aperture, represents the canal by which, in the vertebrate
embryo, the auditory involution of the integument is at
first connected with the exterior.
The testes are oval, and are provided with an epididymis
and vas deferens, as in the higher Vertebrata
. The vas deferens
of each side opens into the dilated part of the ureter.
Attached to the ventral fins of the male are peculiar appendages,
The ovaria are rounded, solid organs. There are usually
two, but in some cases, as in the Dogfishes and nictitating
Sharks, the ovary is single and symmetrical. The oviducts
are true Fallopian tubes, which communicate freely with the
abdominal cavity at their proximal ends. Distally, they dilate
into uterine chambers, which unite and open into the cloaca.
The eggs are very large, and comparatively few.
The Dogfishes, the Rays, and the Chimaera, are oviparous,
and lay eggs, enclosed in hard, leathery cases; the others are
viviparous, and, in certain species of Mustelus
) and Carcharias
a rudimentary placenta is formed, the vascular walls
of the umbilical sac becoming plaited, and interdigitating with
similar folds of the wall of the uterus.
The embryos of most Elasmobranchs are, at first, provided
with long external branchial filaments, which proceed from the
periphery of the spiracle, as well as from most of the branchial
arches. These disappear, and are functionally replaced by
internal gills as development advances.
are divided into two groups,
and the Plagiostomi
In the Holocephali
, the palato-quadrate and suspensorial
cartilages are united with one another and with the skull into
a continuous cartilaginous plate; the branchial clefts are covered
by an opercular membrane. The teeth are very few in
number (not more than six, four of which are in the upper,
and two in the lower jaw, in the living species), and differ in
structure from those of the Plagiostomi
. This sub-order contains
the living Chimaera
, the extinct
; and, very probably,
some of the more ancient Elasmobranchs, the teeth of which
are so abundant in the Carboniferous limestones.
In the Plagiostomi
, the palato-quadrate and suspensorial
cartilages are distinct from one another, and are movable upon
the skull. The branchial clefts are not covered by any opercular
membrane. The teeth are usually numerous.
are again subdivided into the Sharks
), with the branchial apertures at the sides
of the body, the anterior ends of the pectoral fins not connected
with the skull by cartilages, and the skull with a median facet
for the first vertebra; and the Rays (Rajae), with the branchial
clefts on the under-surface of the body, the pectoral fins united
by cartilages to the skull, and no median articular facet upon
the occiput for the first vertebra.
are essentially marine in their habits;
though Sharks are said to occur very high up in some of the
great rivers of South America.
Both divisions of the Plagiostomi
occur in the Mesozoic
rocks. In the Palaeozoic epoch, dermal defences and teeth of Elasmobranchii
abound in the Permian and Carboniferous
formations, and are met with in the Upper Silurian rocks.
But, except in the case of Pleuracanthus
(a Selachian), it is
impossible to be certain to what special divisions they belong.