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skull has assumed the adult form and structure even
more completely than in the Dogfish. On either side the
flabelliform fin has grown forwards and united with the
cheek, and consequently all the branchial clefts appear
entirely on the ventral surface, while the mandibulo-hyoid
cleft is seen dorsally as the spiracular opening. From
the gill-slits the external branchiae still project as long
filaments. The nasal apertures are on the ventral surface,
and the head is produced in front of them as a flattened
rostrum, very distinct from the cranial convexity. The
eyeballs lie in their sockets on the upper surface, bounded
in front, internally, and behind, by the prominent supra-
orbital ridges.

84. The general character of the metamorphosis is
very similar to that of the Dogfish. The prenasal rostrum



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Ii] THE skate: thibd stage. 31

is mueh larger, and the first labiab lie on either side of
it in fronts parallel with it. The intemasal plate, con-
tinuing the gradual elevation of the anterior part of the
cranial floor, is much broader than in the Do^sh, being
as wide as the cranial floor itself; and instead of giving
rise to distinct nasoseptal wings, it simply rises and
becomes thickened on each side, and is continuous with
the nasal roofs. No fenestra arises in these cartilages nor
is there any development of comua in front. The olfactory
crura pass into the back part of the nasal roofs through
a large fenestra on each side in fix)nt of the fore-bram.
A small antorbital plate is carried on the outside of the
base of each nasal capsule, where it forms the antorbital
boundary.

H5. A large part of the cranial roof is unchondrified
in front of the supraoccipital region. This median fon-
tanelle is elliptic in shape, and limited anteriorly by a
transverse bar of cartilage, and laterally by the supra-
orbital tracts. In front, the cranial cavity is quite un-
closed by cartilage, as in the Dogfish. Instead of the
former continuous parachordal mass in the basicranial
and cervical regions, there is an occipital segmentation;
dividing an occipital cranial tract from a cervical cartila-
ginous tube, which is perforated at the sides by a series of
fissures for the cervical nerves.

86. There is now a complete lunule of cartilage in
the anterior wall of the spiracular opening. It is attached
to the side wall of the cranium, below the antero-extemal
OTsphenotic process of the auditory capsule; and it bears
a comb-like pseudobranchia. Belo.v, the spiracular car-
tilage is attached by ligament to the hindmost part of the
palato-quadrate bar.

87. The upper jaw is directed quite transversely^
instead of being concave behind, as in the Dogfish ; the
two palato-quadrate cartilages are closely in apposition in
the middle line. The hinder jaw is similarly transverse, the
mandibular cartilages being parallel to the palato-quadratej.
The emarginate nasofrontal process of integument leaves



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32 MORPHOLOGY OF THE SKULL. [CHAP.

the anterior jaw largely uncovered in the middle line. The
small valvular nasal openings with the labial cartilages
enclosed in the valves are situated on either side of Sie
nasofrontal process.

88. The hyomandibular cartilage articulates with the
auditory capsule beneath its supero-extemal {pterotic)
ridge ; below, it is entirely detached from the rest of the
hyoid arch, turning forwards behind the spiracle, which it
guards, as the spiracular cartilage does in front. The
hinder end of the palato-quadrate is strongly fastened
by ligament to the lower end of the hyomandibular.

89. The growth of opercular skin-folds between the
branchial arches has left only small sigmoid external open-
ings to the five branchial sacs. They are portions of the
lower parts of the original clefts, and they converge to-
wards the middle line from before backwards. A special
ear-shaped opercular flap covers each of the openings. At
this period the hyoid and branchial arches carry two sets
of branchiae in full function; the external are at their
highest development ; and the internal branchial lamina
are perfect, though small.

90. The epihyal segment of the hyoid arch is attached
by a,n interhyal ligament behind the hyomandibular to the
postero-extemal angle of the skull (pterotic ridge). The
whole hyoid arch is smaller than the succeeding branchial
arches — a great contrast to the Dogfish. The hypohyal is
styliform, and widely separated from its fellow ; indeed it
is attached by ligament to the first hypobranchiaL (Com-
pare Dogfish, § 80, p. 29.)

91. The branchial arches are almost bent double,
in accordance with the flattened shape of the animal
The pharyngo- and epibranchials are above, the cerato-
branchials external and inferior. As in the Dogfish, theie
is no separate fifth pharyngobranchial. A single series
of cartilaginous rays proceeds externally from each arch;
these are pedate at their outer ends. The first hypo-
branchial is elongated and slender, meeting and coalescing
with its fellow in the middle line. The second, third, and



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II ] THE ADULT DOGFISH. 33

fourth hypobranchials are small The fifth arch has its
hypobranchials fused into a broad flat plate which lies
between the last three branchial arches, having two di-
verging comua in front.

92. There are no extrabranchial cartilages, nor are
there any oral labials. One rostral pair, and three nasal
pairs in the valves of the nasal openings, are the only
representatives of labials found in this Skate ; the internal
labial related to the nostril is the largest, and projects
backwards over the upper jaw,

93. It is of high interest to note the perfection of the
cranial structures here described, when the embryos are still
so small in proportion to their adult size, and also the sim-
plicity of the changes by which they are developed from
the primary elements. The chondrification of mesoblastic
tissue, in continuity with the parts originating earliest,
comprises most of the history of the proper cranial and
capsular skeleton. The facial arches nave attained the
special arrangements found in this group by a method of
segmentation, essentially resembling that which will be
seen to occur in every type examined. The prenasal and
the antorbital pieces are to be carefully borne in mind, for
they throw light upon many facts to be afterwards noticed.
The labial and extrabranchial cartilages and the branchial
rays are structures which are more specially characteristic
of the Elasmobranchs, but they are charged with lessons
for the student who accurately notices their relations.

Skull of the aduU Dogfish.

94. The changes which have to be described are not
morphologically very important. The olfactory capsules
have become relatively larger, and more closely ap-
proximated. The incurved comua projecting from the
nasoseptal laminae have disappeared. The first labials,
instead of being wholly anterior to the nasal region,
extend backwards over the nasal roofs, and are pitteS by
numerous slime-glands. The second labials have partly

B.M. 3



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34 MORPHOLOGY OF THE SKULL. [CHaP.

coalesced with the anterior edges of the nasal domes, and
with the adjacent antero-extemal angles of the intemasal
cartilage. There is a considerable space between the
superficial extrabranchials and the deeper branchial arches.
Between the corresponding bars of each series, numerous
cartilaginous rods are developed, situated in the septa
between the successive gill-pouches.

95. The adult skull may now be described in detail.
It is entirely fibrous and cartilaginous, excepting that
there are closely-set superficial calcifications in tessergp on
most of the cartilages. There are no investing or " splint "
bones; but as a cartilaginous skull this is the most perfect
and complete of its kind.

96. The chondrocranium is very flat both above and
below; perforated behind by the foramen magnum, and
open in front between the hinder boundaries of the nasal
capsules. The latter are large domes attached to the
antero-extemal angles of the cranium. Thus the olfactory
region of the skull is much wider than the interorbital. A
strong concave supraorbital ridge partly roofs each of the
large eyeballs ; it forms a distinct antorbital plate, running
down towards the antorbital process of the cranial floor
behind the olfactory capsule, and at its opposite extremity
coalesces with the front region of the auditory cartilage,
where a blunt sphenotic process projects. Internal to each
supraorbital ridge is a considerable groove, in the front
part of which the supraorbital foramen opens. Mesially
the cranial roof or tegmen cranii is convex.

97. The surface of the otic mass exhibits three well-
marked prominences for the semicircular canals, anterior,
posterior, and external ; and mesiad of the elevation for the
anterior canal is a small opening, which is the remnant
of the primitive involution of the ear-sac. There is also a
strong external pterotic ridge, horizontally placed {pt, o.).

98. Viewing the skull from behind, the foramen
magnum is seen, somewhat triangular in shape, bounded



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n.] THE ADULT DOGFISH. 35

at the basal angles by the well-developed occipital con-
dyles. Between these is a slight elevation, marking the
original situation of the notdchord. The lateral regions of
the cranial •cartilage extend widely outwards, the upper
edge, as seen from behind, being longer than the lower,
which is constituted by the basilar plate. The ^eater
portion of each lateral mass of cartilage is periotic ; but
the foramen magnum is completely encirclea by an occi-
pital ring not due to the ear-capsule. Laterally, the
basilar plate furnishes the lower part of the facet for the
hyomandibular.

99. The inferior surface of the cranium is an oblong
tract, of which the hinder half is the basilar plate, extend-
ing under the ear-capsules, and the fore part is due to
the trabeculae. A notch under the front of the auditory
region marks the separation between the basilar plate and
the hinder angle of the trabecula. Farther forwards the
trabecular plate is narrower ; but behind the nasal region
it sends out an antorbital process on either side, to which
the ethmopalatine ligament is attached. Between the
olfactory sacs the trabecular cartilage is much narrowed,
but it grows upwards and forwards to coalesce with the
adjacent part of each nasal roof. On each side of this
ethmoidal region the olfactory fibres pass through a large
membranous fenestra down into the nasal sacs. These
have their folds of membrane pinnately arranged, and con-
taining no cartilage. The prenasal rostrum (p, n.) projects
forwards and upwards for a short distance beyond the
intemasal region. The inner wall of each olfactory cap-
sule presents a large unchondrified space or fenestra ; it is
in the region originally formed by the nasoseptal wing.

100. The long first labials (I. 1) meet in the middle
line anteriorly to any other skeletal element, and curve
backwards over the outer part of the olfactory capsules.
The second labials (Z. 2) are partly confluent with the
front of the capsules, and with the anterior angles of the
intemasal plate at the base of the rostrum. A third labial

3—2

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36



MORPHOLOGY OF THE SKULL.



[chap.



(1. 3) lies in the skin-fold protecting the outer edge of each
nasal aperture.

101. Laterally, the skull presents continuous cartilage,
embodying olfactory capsule, supraorbital bar, antorbital
and postorbital (sphenotic) processes, auditory invest-
ment, and basilar plate. The hinder part of the skull
is very little higher than the front region. The rounded
olfactory boss and the smaller antorbital prominence are
succeeded by the concave orbital wall ; while, behind, the
ear-cartilage with its sphenotic process {sp, o.) and pter-
otic ridge (pt o.) forms the outstanding buttress of the
cranial edifice.

102. In this skull, having no bones or distinct car-
tilages, the nerve foramina must be carefully studied as

Fig. 7.



I ^Or y 5p.O Aa




ex-'P^^



SkoU of adult Dogfish, side view. 0,e, occipital condyle ; au, peri-
otic capsule; pUo, pterotio ridge; sp.o. sphenotic process; 8,or, supra-
orbital ridge; na, nasal capsule; p,n, prenasal cartilage; II. optic
foramen; K, trigeminal foramen; pLpLygu, palato-quadrate arcade;
m. pt. metapterygoid ligament (includmg a smaU cartilage) ; pi. tr,
ethmo-palatine or palato-trabecular ligament ; mck. lower jaw ; sp. spi-
racle; h,fn. hyomandibular ; c.Ay. ceratohyal; 9n.A.2. mandibulo-hyoid
ligament; ph.br. pharyngobranchial ; e.br. epibranchial ; c.hr, cerate-
branchial; h.hr. hypobranchial ; b.br. basibranchial ; ex.br. extra-
branchial; 21,2,3,4,5, labial curtilages; the dotted lines within mck
indicate the basihyal.



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II.] THE ADULT DOGFISH. 37

regional landmarks. On the upper surface there is a
snail foramen for the supraorbital nerve in a groove be-
tween the fore part of the supraorbital ridge (s. or.) and
the frontal part of the cranial roof. The olfactory fenestra
has already been described. The foramen for the optic
nerve (Fig. 7. //.) appears low down in the middle region
of the lateral (sphenoidal) wall of the cranium. The tri-
geminal foramen (f.) is similarly placed in the hinder
part of the sphenoidal wall, and the orifice for the facial i»
a little behind and below this. The external foramina for
the glossopharyngeal and vagus nerves are on the pos-
terior aspect of the skull, the former low down and exter-
nal, marking the original separation between auditory
mass and basilar plate ; the latter higher up, and at the
side of the foramen magnum in the line of junction of the
occipital and auditory regions.

103. Examining the inner surface of the sknll, the pitui-
tary body is seen to be guarded in front %nd behind by small
cartilaginous ridges in which some bony deposit takes place.
These markedly resemble the anterior and posterior clinoid
lidges bounding the " sella turcica " of Mammalia ; and the re-
semblance is strengthened by the entrance of the internal caro-
tid arteries on either Bide of the pituitary body. With such
facts may be coupled the likeness of the occipital articulation
with the atlas to that of Mammalia.

104. The palato-mandibular cartilages, constituting
the main part of the oral apparatus, are loosely swung from
the sides of the basis cranii by two short ligaments. The
upper jaw, or palato-quadrate arcade, is attached by the
ethmopalatine ligament to the antorbital region ; by the
metapterygoid ligament (m. pt) to the postorbital trabecu-
lar angle behind the trigeminal foramen, and in front of
the spiracle (sp.) ; and by the sjrmplectic ligament to the
hyomandibular behind. The metapterygoid and the sym-
plectic ligaments are very closely connected, and some
fibres of the former are attached to the hyomandibular
cartilage. The hinder or quadrate end of the upper jaw
{qu.) forms a condylar surface for articulation with the
lower jaw or articulo-meckelian element.



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38 MORPHOLOGY OF THE SKULL. [CHAP.

105. The upper jaw is narrow behind, broad and
convex outwardly in the middle, and bends inwards to be
united with its fellow by a strong ligament. The lower
jaw is very similar to the upper; it is deep and outwardly
convex behind, for attachment of the oral muscles. Teeth
are borne on the mucous membrane of the fore and inner
part of both jaws. The fourth and fifth labials (/. 4 and
5), set on the upper and lower jaws obliquely, make an
acute angle by the approach of their pointed hinder ends,
considerably within the angle of the jaw. When the
mouth is opened the angle between them becomes obtuse.
There is a small cartilage in the anterior wall of the
spiracle.

106. The large phalangiform hyomandibular (Ji. m.), the
upper segment of the hyoid arch, is articulated with the side
>yall of the cranium beneath the pterotic ridge. The spiracle
is in front of it, while distally it is connected with the
quadrate region by the symplectic ligament. ITie stout and
elongated ceratohyal (c. hy,) is articulated with the distal
facet of the hyomandibular, and is locked within the
everted postero-inferior edge of the mandible, being strongly
bound to it by the mandibulo-hyoid ligament (m. A. I).
At its ventral end the ceratohyal is bilobate, and articu-
lates by its anterior lobe with the broad basihyal plate,
which it also slightly underlies behind.

107. The succeeding four arches have supero-trans-
verse pharyngobranchials {ph, br.), which are in contact
with their fellows in the middle line beneath the vertebral
column ; lateral epibranchials {e, br,) and ceratobranchials
(c. br,), and infero-median hypobranchials (h. br.) di-
rected backwards. Each epibranchial has a small forward
spur at its upper end. Normally the pharyngobranchials
are turned somewhat backwards ; but when the pharynx
is distended they may be directed transversely or even
a little forwards. Each pharyngobranchial has at its lower
end a backward spur. The fifth branchial arch is much
smaller than the rest, having its pharyngobranchial con-
fluent with the fourth, and no hypobranchial. The median



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II.] THE ADULT SKATE. 39

basibranchial (ft. br.) lies between the last three arches, in
contact anteriorly with the hinder hypobranchiab, and
with the fifth pair of qeratobranchials behind.

108. There is a vascular plexus (pseudobranchia) on
the anterior side of the spiracular opening. The five gill-
clefts are the external openings of as many pouches, the
anterior and posterior walls of which bear transversely
plaited branchial laminae. Internally each pouch opens
into the pharynx. In the partition between each pouch
is a series of cartilaginous rods, or branchial rave, most
of them simple, extending from the hyoid and nrst four
branchial arches towards the skin. Those in relation with
the hyoid are pectinately branched ; three of them are in
the hyomandibular region.

109. Four extrabranchial cartilages (ex. Jr.), parallel
to the ceratobranchials, are superficial to the correspond-
ing sets of branchial rays. They are pedate below, and
very greatly resemble the scapulo-coracoid cartilages
situated immediately behind them (see Fig. 6, e. cr,).
The fifth branchial arch has no gill-pouch behind it, no
rays, and no extrabranchial.

8kuU of the adult Skate.

110. The most notable change in the appearance of
the skull is produced by the great elongation of the pre-
nasal rostrum (Fig. 8, p, n), which has the first labials
(t 1) on either side of its fore extremity. The lateral
portions of the intemasal plate where they adjoin the
nasal capsules are continued into the rostrum, so that the
cranial floor is continuous with a long concavity on the
hinder part of the rostrum. All traces of the external
branchiae have disappeared; and cartilaginous branchial
rays, about a dozen m the anterior wall of each gill- sac,
extend from the arches outwards and backwards. The
antorbital plate has become a thick backwardly and out-
wardly curved rib-like bar, pointed behind, lying outside
and above the upper jaw.



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40 MORPHOLOGY OF THE SKULL. [CHAP.

111. The following are, briefly, the chief points in
which the Skate's skull diflfers from that of the Dogfish : —

Fig. 8.



Skull of Skate, nearly adult. Au, periotic capsule; na. olfactory
capsule ; pn. prenasal cartilage ; 11, V, in the side wall of the cranium,
optic and trigeminal foramina ; pl.pLy qu. palato-quadrate bar ; mck, man-
dibular cartilage; m.pt, metapterygoid ; Km, hyomandibular ; i. h, I. inter-
hyal ligament; e,hy. epihyal; c.hy. ceratohyal; h.hy, hypohyal;
/i.6r. 1,2,5, hypobranchials ; above them are seen the cerato, epi, and
pharyngobranchials ; 21,4, labials.

the presence of a supero-median cranial fontanelle, and of
a large prenasal rostrum ; the second labials not coalescing
with the olfactory capsules ; the fourth labials becoming
involved in the inner fold of the nasal openings, so that
three pairs of labials protect these orifices ; the distinct-
ness and size of the antorbital cartilage ; the large size of
the caitilage {m,pt) in the anterior wall of the spiracle*;
the foim and forward direction of the hyomandibular, and
its dissociation from the rest of the hyoid arch, except by
a posteiior and superior interhyal ligament attached to
a distinct epihyal ; the smaller size of the hyoid arch, and
its close connection w^ith the first branchial ; the absence

^ The presence of one or more spiracular cartilages is constant in
Sharks ; a small one is found in the Dogfish.



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II.] SUMMAKY. 41

of a basihyal, and the junction of the first hypobranchials
across the middle line ; the coalescence of the fifth hypo-
branchials (h. hr, 5) to form a broad median basibranchial
plate ; the absence of the fifth pair of labials, and of the
extrabranchial cartilages.

112. Summary. The history of these skulls is singu-
larly uncomplicated ; but this does not necessarily indicate
that the type is a low one. Paired basal elements arise at
first, the trabeculae in the anterior portion of the cranial
floor, and the parachordals behind. By their union, and
by the chondrification of the greater part of the cranial
wall in continuity with them, a simple cartilaginous cra-
nium is formed, deficient in one or more regions above.
The ear-cartilages are impacted in the cranial mass behind,
and the nasal capsules have their own skeletal defence in
front, coalescing with the cranial cartilage as well as with
an intemasal element which is at first distinct from the
trabeculae, but afterwards fused with the cranial floor.
The small prenasal spur found in the Dogfish is repre-
sented by the great rostrum of the Skate; while the
antorbital region is characterised in the latter by the
development of an antorbital rod extending backwards.

113. The skeleton of the jaws and gills possesses from
its origin substantially its adult relations; and no part
of it is at any time fused with the cranium proper. The
upper and lower jaws are primarily continuous, although
the bars are sharply bent ; but they are speedily cut in
two. In the Skate an upper and posterior region of the
mandibular arch chondnfies early to form the spiracular
cartilage.

114. The suspension of the jaws is effected by the
intermediation of an element of the hyoid arch; but in
the Dogfish the hyomandibular is the segmented upper
extremity of the bar, while in the Skate it arises separately
in the antero-superior region of the arch. The stout
ceratohyal is in the Dogfish directed forwards and closely
related to the lower jaw and the distal end of the hyoman-



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42 MORPHOLOGY OF THE SKULL. [CHAP. II.

dibular ; while in the Skate the hyoid arch is feebler, but
is connected directly with the skull and the proximal end
of the hyomandibular. It has an epihyal as well as a
ceratohyal piece, and is in relation with the first branchial
arch.

115. The branchial arches commence as simple rods,
afterwards segmented into a series of pieces, the arches of
opposite sides being connected by basal cartilages ; the
Skate is distinguished by the separate origin of its upper
branchial elements. Branchial rays are found in the
partitions between the gill-pouches.

116. Two series of cartilages occurring less generally
in other types than in those which have been described
are named labial and extrabranchial. These are developed
between the primary slj:eletal elements and the skin, and
enter into relation with the rostrum, the nostrils, the jaws,
and the gill-clefts.



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CHAPTER III.

THE SKULL OF THE SALMON.

First Stage : Unkatched embryos, with simple facial arches,

117. The embryos here described have their neural
tube open both anteriorly and posterioriy. The long tape-
like body lies on a yelk-sac, with the umbilicus very little
constricted, so that its rim reaches to within a short dis-
tance behind the mouth. The involutions to fonn the
sense-organs are widely open ; yet the primary elements
of the skull can be distinguished, though merely by being
composed of more consistent tissue than the surrounding
parts. The rods or bars to be spoken of are formed of
the very small mother-cells of hyaline cartilage.

118. The enlargements of the neural tube to form the
cerebral vesicles are very slight, not constituting any obvious
bulging ; but the three vesicles are distinct, the first being
largest, and having the landmark of the pituitary body



Online LibraryWilliam Kitchen ParkerThe morphology of the skull → online text (page 4 of 31)