The only clearly diagnostic characters of this class as compared
with Fishes are the following:
have no fin-rays.
2. When limbs are present they contain the same skeletal
elements as those of the higher Vertebrata
Certain other structural peculiarities are common to the
whole of the Amphibia
, and are very characteristic of them
without being diagnostic. Thus:
- The body is usually devoid of any exoskeleton, and
when scales, or scutes, are present in recent Amphibia, they
are concealed within the skin (Caecilia, Ephippifer). In the
extinct Labyrinthodonta, the dermal armor is confined to the
ventral region of the body.
- The vertebral centra are always represented by bone.
- The sacrum rarely consists of more than one vertebra,
though there are individual exceptions to this rule, as in Menopoma.
- The suspensorial apparatus of the mandible is continuous
with the skull, which has two occipital condyles, and no
completely ossified basi-occipital.
- There are no sternal ribs.
are divisible into the following groups:
- A distinct and often long tail; the vertebrae amphicoelous or opis.
thocoelous; the proximal elements of the tarsus not elongated.
- Two or four limbs; no scutes or scales.
I. Saurobatrachia or Urodela.
- External branchiae or gill-clefts persistent, or disappearing
only in advanced age; no eyelids; vertebrae amphicaelous;
carpus and tarsus cartilaginous.
- No branchiae or branchial clefts in the adult; eyelids present;
carpus and tarsus more or less ossified; vertebrae commonly
- Limbs absent, or all four present. Three large pectoral osseous
plates and an armor of small scutes on the ventral surface of the body;
vertebrae amphicoelous; walls of the teeth more
or less folded.
- Tail obsolete in the adult.
- Limbs absent; numerous minute dermal scutes imbedded in
the integument of the serpentiform body.
- All four limbs present, and the proximal elements of the
tarsus much elongated; the body short, and the integument
devoid of small scutes, though dermal osseous plates are
sometimes developed in it.
IV. Batrachia or Anura.
The integument in most Amphibia
is soft and moist, as in
the Frog, where numerous glands open upon its surface. The Gymnophiona
are exceptional, among existing Amphibia
in possessing small, rounded, flexible scales, like the cycloid
scales of fishes, imbedded within the wrinkled integument.
In certain Batrachia
(Ceratophrys dorsata, Ephippifer
), flat dermal bony plates are developed in the
dorsal integument, and become united with some of the subjacent
vertebrae. Many of the extinct Labyrinthodonta
probably the whole of the members of that group, possessed
an exoskeleton which appears to have been confined to the
ventral surface of the body. Under the anterior part of the
thorax there is a sort of plastron composed of one median and
two lateral plates. The median plate is rhomboidal. The
lateral ones are somewhat triangular, and unite with the
anterolateral margins of the median plate by one side, sending
a process upward and backward from their outer angles.
The outer surfaces of all these plates exhibit a sculpture, which
radiates from the centre of the median plate and from the
outer angles of the lateral plates. These plates are in close
relation with the pectoral arch, and probably represent the
interclavicle and clavicles.
Minute bony plates cover the surface of the throat in a
small African Labyrinthodont, Micropholis
. I have not met
with dermal ossicles in this position in other Labyrinthodonts.
But in Archegosaurus, Pholidogaster, Urocordylus, Keraterpeton,
, the integument between
the thoracic plates and the pelvis presents regularly-disposed
rows of small elongated ossicles, which, for the most part,
converge from without, forward and inward, toward the middle
line. No trace of these appears upon the tail, nor in
any part of the dorsal region of the body, nor on the limbs.
The endoskeleton of the Amphibia
is least complete in Archegosaurus
, where the centra of the vertebrae are represented
only by bony rings, the ribs and the neural arches
being well ossified. In other Labyrinthodonts of the same
(Carboniferous) epoch, however, such as Anthracosaurus
centra of the vertebrae are completely ossified biconcave disks,
very like the centra of the vertebrae of Ichthyosaurus