CLASS II. AMPHIBIA[50].
AMPHIBIA differ markedly from Pisces in the fact that in the more
abundant and familiar forms the skin is naked, and that when the
integument is prolonged into median fins they are devoid of fin-rays.
The notochord may persist, but bony vertebral centra are always
developed. These are sometimes biconcave, sometimes procoelous,
sometimes opisthocoelous. There is only one sacral vertebra, except in
rare cases. The cartilaginous cranium persists to a considerable
extent but is more or less replaced by cartilage bone, and overlain by
membrane bone. The basi-occipital is not completely ossified, and the
skull articulates with the vertebral column by means of two occipital
condyles formed by the exoccipitals.
There is a large parasphenoid, but there are no ossifications in the
basisphenoidal, presphenoidal, and alisphenoidal regions. In most
cases the epi-otics and opisthotics are ossified continuously with the
exoccipitals.
The palato-pterygo-quadrate bar is firmly united to the cranium, so
the skull is autostylic. The palatines and pterygoids are membrane
bones. Teeth are nearly always borne on the vomers and commonly on the
maxillae and premaxillae. There are no sternal ribs, and the sternum is
very intimately related to the pectoral girdle. There are no obturator
foramina. The limbs are as in the higher vertebrata, divisible into
upper arm, fore-arm, and manus (wrist and hand), and into thigh, shin,
and pes (ankle and foot) respectively. The posterior limb is, as a
rule, pentedactylate, but in nearly every case the pollex is vestigial
or absent.
_Order 1._ URODELA[51].
The Urodela are elongated animals with a naked skin, a persistent
tail, and generally four short limbs.
The vertebral centra are opisthocoelous or biconcave, and there are
numerous precaudal vertebrae. Portions of the notochord commonly
persist in the intervertebral spaces. In the skull there is no
sphenethmoid forming a ring encircling the anterior end of the brain,
its place being in many cases partly taken by a pair of
orbitosphenoids. There is no quadratojugal, and the quadrate is more
or less ossified. The mandible has a distinct splenial, and the
articular is ossified.
There is no definite tympanic cavity. The hyoid apparatus is
throughout life connected to the quadrate by ligament, and a large
basilingual plate does not occur. The ribs are short structures with
bifurcated proximal ends. In the pelvis the pubis remains
cartilaginous, and there is a bifid cartilaginous epipubis. The bones
of the fore-arm and shin remain distinct, and the manus never has more
than four digits.
_Suborder_ (1). ICHTHYOIDEA.
The vertebrae are amphicoelous, but the notochord remains but little
constricted throughout the whole length of the vertebral column. Three
or four branchial arches nearly always persist in the adult. The
cartilages of the carpus and tarsus remain unossified.
The Ichthyoidea may be subdivided again into two groups:--
A. _Perennibranchiata_, whose chief distinguishing skeletal characters
are that the skull is elongated, the premaxillae are not ankylosed, the
maxillae are vestigial or absent; there are sometimes no nasals, and
the palatines bear teeth;
e.g. _Siren_, _Proteus_, _Menobranchus_.
B. _Derotremata_, whose chief distinguishing skeletal characters are
that there are large maxillae and nasals; teeth are borne by both
maxillae and premaxillae; there are no palatines; and both pectoral and
pelvic limbs are always present;
e.g. _Amphiuma_, _Megalobatrachus_, _Cryptobranchus_.
_Suborder_ (2). SALAMANDRINA.
The vertebrae are opisthocoelous. The skull is broad, and teeth are
borne by both premaxillae and dentaries. Nasal bones are present. The
remains of only two branchial arches are found in the adult. The
carpus and tarsus are more or less ossified.
This suborder includes the Newts (_Molge_), Salamanders
(_Salamandra_), and _Amblystoma_.
_Order_ 2. LABYRINTHODONTIA[52].
These are extinct Amphibia with a greatly developed dermal
exoskeleton, which is generally limited to the ventral surface. The
body and tail are long and in some cases limbs are absent. The teeth
are pointed and often have the dentine remarkably folded. The
vertebrae are amphicoelous, and are generally well ossified. The skull
is very solid, and has a greatly-developed secondary roof which hides
the true cranium and is very little broken up by fossae. Paired
dermal supra-occipitals are found, and there is an interparietal
foramen. The epi-otics and opisthotics form a pair of bones distinct
from the exoccipitals. Four simple limbs of moderate length are
generally present, and in some cases all four limbs are
pentedactylate. Among the better known genera of Labyrinthodonts are
_Mastodonsaurus_, _Nyrania_, and _Archegosaurus_.
_Order 3._ GYMNOPHIONA[53].
These animals form a group of abnormal worm-like Amphibia having an
exoskeleton in the form of subcutaneous scales arranged in rings. The
vertebrae are biconcave and are very numerous; very few however belong
to the tail. The skull has a complete secondary bony roof, the
mandible bears teeth and has an enormous backward projection of the
angular. The hyoid arch has very slender cornua and no distinct body,
it is attached neither to the cranium nor to the suspensorium. The
ribs are very long and there are no limbs or limb girdles.
_Order 4._ ANURA.
These are tailless Amphibia, which except in a few instances, are
devoid of an exoskeleton. The vertebrae are as a rule procoelous, and
are very few in number. The post-sacral part of the spinal column
ossifies continuously, forming an unsegmented cylindrical rod, the
urostyle. Remains of the notochord persist, lying _vertebrally_, i.e.
enclosed within the centra of the several vertebrae, and not as in
Urodela lying between one vertebra and the next. The skull is very
short and wide. The mandible is almost always, if not invariably,
toothless.
The frontals and parietals on each side are united so as to form a
pair of fronto-parietals, and a girdle-like sphenethmoid is present.
The quadrate is not generally ossified. A predentary or
mento-meckelian bone is commonly present in the mandible, and a single
bone represents the angular and splenial. The branchial arches are
much reduced in the adult, and the distal ends of the cornua unite to
form a flat basilingual plate of a comparatively large size.
Ribs are very little developed. Clavicles are present. The ilia are
very greatly elongated. The anterior limb has four well-developed
digits and a vestigial pollex, and is of moderate length; the radius
and ulna have fused. The posterior limb is greatly elongated and is
pentedactylate; the tibia and fibula are fused, while the calcaneum
and astragalus are greatly elongated, and it is largely owing to them
that the length of the limb is so great. The group includes the Frogs
and Toads, the predominant Amphibia of the present time.
FOOTNOTES:
[50] T.H. Huxley, _Amphibia_ (_Encyclopaedia Britannica_).
[51] See R. Wiedersheim, _Morphol. Jahrb._ Bd. III. 1877, p. 459.
[52] See A. Fritsch, _Fauna der Gaskohle_, Prague, 1883-85-86, also
writings of Cope, Credner, Huxley, H. v. Meyer, Miall.
[53] See R. Wiedersheim, _Anatomie der Gymnophionen_, Jena, 1879.