SUBPHYLUM D. VERTEBRATA.
THE animals included in this great group all possess an internal axial
skeleton forming the vertebral column or back-bone; and a dorsal
spinal cord. The vertebral column is developed from the skeletogenous
layer, which surrounds the spinal cord together with the notochord and
its sheath; and in the great majority of cases the notochord becomes
more or less modified and reduced in the adult. In some cases the
notochord remains unmodified and the skeletogenous layer surrounding
it is not segmented to form vertebrae, but in every case the neural
arches which protect the spinal cord are segmented. The notochord
never extends further forwards than the mid-brain.
All true vertebrates possess a cranium or skeletal box enclosing the
brain.
(I.) CYCLOSTOMATA.
The mouth in living forms is suctorial and is not supported by jaws.
In some fossil forms the character of the mouth is unknown.
_Order I._ MARSIPOBRANCHII[24].
In these animals limbs and limb girdles are always completely absent.
They have no exoskeleton except horny teeth.
The endoskeleton, excluding the notochord, is entirely cartilaginous
or membranous. The axial skeleton consists of a cartilaginous cranium
without jaws, succeeded by a thick persistent notochord enveloped in
a sheath. The notochord in living forms is unsegmented, but segmented
cartilaginous neural arches are present in some cases. A complicated
series of cartilaginous elements occurs in relation to the mouth,
gills, and sense organs. The median fins are supported by
cartilaginous pieces, the radiale. The order includes the Lampreys and
Hags.
_Order II._ OSTRACODERMI[25].
The forms included in this group have long been extinct, being known
only from beds of Upper Silurian and Lower Devonian age. They differ
much from all other known animals. The exoskeleton is always greatly
developed and includes (1) large bony plates covering the anterior
region; (2) scales covering the posterior region. The plates are
deeply marked by canals belonging to dermal sense organs. Jaws are
unknown, and arches for the support of the appendicular skeleton are
rudimentary or absent. The tail is heterocercal (see p. 60).
_Suborder_ (1). HETEROSTRACI.
The exoskeleton consists principally of calcifications forming dorsal
and ventral shields which cover the head and abdominal region; the
dorsal shield is formed of a few plates firmly united, the ventral
shield of a single plate. The shields are composed of three layers,
the middle layer being traversed by canals belonging to the dermal
sense organs which open to the exterior by a series of pores. The tail
is sometimes covered by scales. The orbits are widely separated and
laterally placed. Paired appendages are absent. These curious forms
are found in beds of Upper Silurian and Lower Devonian age. One of the
best known genera is _Pteraspis_.
_Suborder_ (2). OSTEOSTRACI.
The exoskeleton as in the Heterostraci consists of shields and scales,
the shields being divisible into three layers. The anterior part of
the body is covered dorsally by a single large shield which differs
from those of the Heterostraci in having the inner layer ossified. The
middle layer contains canals for the passage of blood vessels, but the
exoskeleton shows no impressions of dermal sense organs. The posterior
part of the body is covered by large quadrangular scales. Paired
appendages are absent, but median dorsal and caudal fins occur
supported by scales, not fin-rays. _Cephalaspis_, the best known of
these animals, occurs in beds of Lower Devonian age.
_Suborder_ (3). ANTIARCHA.
The exoskeleton is formed of bony plates, the dorsal and ventral
shields each consisting of several symmetrically arranged pieces. The
tail may be covered with small scales or may be naked. The head is
articulated with the trunk, and its angles are drawn out into a pair
of segmented paddle-like appendages, covered with dermal plates. The
orbits are close together. A dorsal fin and traces of mouth parts
occur in _Pterichthys_, but the endoskeleton is unknown. The best
known forms _Pterichthys_[26] and _Asterolepis_ occur in beds of Lower
Devonian age.
GENERAL ACCOUNT OF THE SKELETON OF MARSIPOBRANCHII.
The Marsipobranchii are worm-like animals. The living forms include
two families, the Myxinoidei (Hags)--genera _Myxine_ and
_Bdellostoma_--and the Petromyzontidae (Lampreys).
Three species of _Petromyzon_ are known, _P. fluviatilis_, _P.
marinus_ and _P. planeri_. The larval forms were for a long time
thought to belong to a separate genus and were called _Ammocoetes_.
The Myxinoids, although very highly specialised in their own way, are
at distinctly a lower stage of development than the adult Lamprey, and
come nearer to the larval Lamprey or Ammocoete.
SPINAL COLUMN.
[Illustration FIG. 4. A, DORSAL; B, LATERAL AND C, VENTRAL VIEW OF THE
SKULL OF _Petromyzon marinus_ × 1 (after PARKER).
1. horny teeth.
2. labial cartilage.
3. anterior dorsal cartilage.
4. posterior dorsal cartilage.
5. nasal capsule.
6. auditory capsule.
7. dorsal portion of trabeculae.
8. lateral distal mandibular.
9. lingual cartilage.
10. branchial basket.
11. cartilaginous cup supporting pericardium.
12. sheath of notochord.
13. neural plate.]
In Myxinoids and larval lampreys, the notochord is enclosed in a thick
chordal sheath, in connection with which in the tail region there
occur cartilaginous pieces forming neural arch elements. In the trunk
region, however, no cartilage occurs in connection with the spinal
column, the only cartilage present being that forming the radiale of
the dorsal fin. On the other hand in most species of lamprey
(_Petromyzon_) cartilaginous pieces forming imperfect neural arches
(fig. 4, B, 13) are found lying in the tough skeletogenous layer
dorsal to the notochord, and extending throughout the whole length of
the trunk and tail. Two of these pieces, which are probably homologous
with the neural plates (see p. 72) of Elasmobranchs, occur to each
_neuromere_, or segment as determined by the spinal nerves. The
dorsal and caudal fins are supported by paired cartilaginous radiale
which are connected proximally with the skeletogenous layer.
THE SKULL.
In Myxinoids the cranium is a mere cartilaginous floor without side
walls or roof, and the trabeculae[27] end without growing forwards
into cornua. In Lampreys the trabeculae grow forwards and send up
plates of cartilage which meet above (fig. 4, 7) and form side walls
and a roof for part of the brain case. In Lampreys a labial suctorial
apparatus is well developed, including a large ring-like piece of
cartilage (fig. 4, 2) which supports the oral funnel and bears a large
armament of horny teeth. In Myxinoids on the other hand the labial
skeleton is small and consists merely of barbels round the mouth.
The olfactory organ of Myxinoids has a very curious skeleton. It is
covered with a kind of grating of cartilage which is prolonged in
front into a tube composed of a series of imperfect cartilaginous
rings. In Lampreys the olfactory organ opens merely by a short
membranous passage. In correlation with the small development of the
labial suctorial apparatus in Myxinoids the lingual apparatus is very
greatly developed. The tongue in _Myxine_ has been said to 'dominate
the whole body' (Parker). It is supported by a great median
cartilaginous bar which when followed forwards first becomes bifid and
still further forwards becomes four-cleft.
The horny teeth in Myxinoids are chiefly borne on the very large
supralingual apparatus. They form a double series arranged in the form
of an arch. In _Myxine_ there are seven large teeth and nine small
ones on each side. In _Bdellostoma_ the teeth of the two rows are more
equal in size. In _Bdellostoma_ and _Myxine_ it has been shown that
imperfect calcified teeth occur below the horny teeth.
In Lampreys the lingual apparatus (fig. 4, C, 9) is well developed,
but not excessively so. It consists of a long median cartilaginous bar
which ends in front with a semicircular piece of cartilage supporting
the median part of the tongue.
In both Myxinoids and Lampreys there is a complicated branchial basket
apparatus, but while in Myxinoids the basket apparatus is
interbranchial, formed deep within the head near the hypoblastic
lining of the throat, in Lampreys it is extra-branchial and formed
outside the head cavities (fig. 4, 10). The two sides of the basket
apparatus in _Myxine_ are not symmetrical. In the interbranchial
basket apparatus of Myxinoids the hyoid and first and second branchial
arches can be recognised. Traces of the interbranchial skeleton of
Myxinoids can be detected in Lampreys, and similarly in Myxinoids,
there are indications of the extra-branchial skeleton of _Petromyzon_.
The branchial basket in Lampreys forms at its posterior end a kind of
cup which supports the pericardium (fig. 4, 11).
A remarkable Cyclostome named _Palaeospondylus_[28] has recently been
described from the Scottish Old Red Sandstone. It differs however from
all living Cyclostomes, in having a spinal column formed of distinct
vertebrae with well-developed neural arches. The caudal fin is well
developed and the dorsal radiale are forked as in lampreys. The skull
is well calcified and the auditory capsules are specially large. The
mouth is very similar to that of lampreys, being circular and without
jaws; it is provided with barbels or cirri. There is no trace of limbs
and the average length is only about 1-1-1/2 inches.
FOOTNOTES:
[24] See W.K. Parker On the skeleton of the Marsipobranch fishes,
_Phil. Trans._ 1883, London.
[25] See A. Smith Woodward, _Catalogue of Fossil Fish in the British
Museum_, Part II., 1891. A. Smith Woodward, _Nat. Sci._ vol. I. 1892,
p. 596.
[26] See R.H. Traquair, _Ann. Nat. Hist._, ser. 6, vol. II. 1888, p.
485.
[27] See p. 17.
[28] R.H. Traquair, _Ann. Nat. Hist._ vol. VI. 1890, p. 485; _P. Phys.
Soc. Edinb._ vol. XII. 1892-93, pp. 87-94, and 312-320. A. Smith
Woodward, _Nat. Sci._ vol. III. p. 128, 1893.