HOW A REPTILE IS KNOWN
DEFINITION OF REPTILES BY THEIR VITAL ORGANS
The relations of reptiles to other animals may be stated so as to make
evident the characters and affinities which bind them together. Early in
the nineteenth century naturalists included with the Reptilia the tribe
of salamanders and frogs which are named Amphibia. The two groups have
been separated from each other because the young of Amphibia pass
through a tadpole stage of development. They then breathe by gills, like
fishes, taking oxygen from the air which is suspended in water, before
lungs are acquired which afterwards enable the animals to take oxygen
directly from the air. The amphibian sometimes sheds the gills, and
leaves the water to live on land. Sometimes gills and lungs are retained
through life in the same individual. This amphibian condition of lung
and gill being present at the same time is paralleled by a few fishes
which still exist, like the Australian _Ceratodus_, the lung-fish, an
ancient type of fish which belongs to early days in geological time.
This metamorphosis has been held to separate the amphibian type from
the reptile because no existing reptile develops gills or undergoes a
metamorphosis. Yet the character may not be more important as a ground
for classification than the community of gills and lungs in the fish and
amphibian is ground for putting them together in one natural group. For
although no gills are found in reptiles, birds, or mammals, the embryo
of each in an early stage of development appears to possess gill-arches,
and gill-clefts between them, through which gills might have been
developed, even in the higher vertebrates, if the conditions of life had
been favourable to such modification of structure. In their bones
Reptiles and Amphibia have much in common. Nearly all true reptiles lay
eggs, which are defined like those of birds by comparatively large size,
and are contained in shells. This condition is not usual in amphibians
or fishes. When hatched the young reptile is completely formed, the
image of its parent, and has no need to grow a covering to its skin like
some birds, or shed its tail like some tadpoles. The reptile is like the
bird in freedom from important changes of form after the egg is hatched;
and the only structure shed by both is the little horn upon the nose,
with which the embryo breaks the shell and emerges a reptile or a bird,
growing to maturity with small subsequent variations in the proportions
of the body.
[Illustration: FIG. 1 LUNG OF THE FISH CERATODUS
Partly laid open to show its chambered structure (After Guenther)]
THE REPTILE SKIN
Between one class of animals and another the differences in the
condition of the skin are more or less distinctive. In a few amphibians
there are some bones in the skin on the under side of the body, though
the skin is usually naked, and in frogs is said to transmit air to the
blood, so as to exercise a respiratory function of a minor kind. This
naked condition, so unlike the armoured skin of the true Reptilia,
appears to have been paralleled by a number of extinct groups of fossils
of the Secondary rocks, such as Ichthyosaurs and Plesiosaurs, which were
aquatic, and probably also by some Dinosauria, which were terrestrial.
Living reptiles are usually defended with some kind of protection to the
skin. Among snakes and lizards the skin has commonly a covering of
overlapping scales, usually of horny or bony texture. The tortoise and
turtle tribe shut up the animal in a true box of bone, which is cased
with an armour of horny plates. Crocodiles have a thick skin embedding a
less continuous coat of mail. Thus the skin of a reptile does not at
first suggest anything which might become an organ of flight; and its
dermal appendages, or scales, may seem further removed from the feathers
which ensure flying powers to the bird than from the naked skin of a
frog.
THE REPTILE BRAIN
Although the mode of development of the young and the covering of the
skin are conspicuous among important characters by which animals are
classified, the brain is an organ of some importance, although of
greater weight in the higher Vertebrata than in its lower groups.
Reptiles have links in the mode of arrangement of the parts of their
brains with fishes and amphibians. The regions of that organ are
commonly arranged in pairs of nervous masses, known as (1) the olfactory
lobes, (2) the cerebrum, behind which is the minute pineal body,
followed by (3) the pair of optic lobes, and hindermost of all (4) the
single mass termed the cerebellum. These parts of the brain are extended
in longitudinal order, one behind the other in all three groups. The
olfactory lobes of the brain in Fishes may be as large as the cerebrum;
but among Reptiles and Amphibians they are relatively smaller, and they
assume more of the condition found in mammals like the Hare or Mole,
being altogether subordinate in size. And the cerebral masses begin to
be wider and higher than the other parts of the brain, though they do
not extend forward above the olfactory lobes, as is often seen in
Mammals. In Crocodiles the cerebral hemispheres have a tendency to a
broad circular form. Among Chelonian reptiles that region of the brain
is more remarkable for height. Lizards and Ophidians both have this part
of the brain somewhat pear-shaped, pointed in front, and elongated. The
amphibian brain only differs from the lizard type in degree; and
differences between lizards' and amphibian brains are less noticeable
than between the other orders of reptiles. The reptilian brain is easily
distinguished from that of all other animals by the position and
proportions of its regions (see Fig. 19, p. 53).
Birds have the parts of the brain formed and arranged in a way that is
equally distinctive. The cerebral lobes are relatively large and convex,
and deserve the descriptive name "hemispheres." They are always smooth,
as among the lower Mammals, and extend backward so as to abut against
the hind brain, termed the cerebellum. This junction is brought about in
a peculiar way. The cerebral hemispheres in a bird do not extend
backward to override the optic lobes, and hide them, as occurs among
adult mammals, but they extend back between the optic lobes, so as to
force them apart and push them aside, downward and backward, till they
extend laterally beyond the junction of the cerebrum with the
cerebellum. The brain of a Bird is never reptilian; but in the young
Mammal the brain has a very reptilian aspect, because both have their
parts primarily arranged in a line. Therefore the brain appears to
determine the boundary between bird and reptile exactly.
REPTILIAN BREATHING ORGANS
The breathing organs of Birds and Reptiles which are associated with
these different types of brain are not quite the same. The Frog has a
cellular lung which, in the details of the minute sacs which branch and
cluster at the terminations of the tubes, is not unlike the condition in
a Mammal. In a mammal respiration is aided by the bellows-like action of
the muscles connected with the ribs, which encase the cavity where the
lungs are placed, and this structure is absent in the Frog and its
allies. The Frog, on the other hand, has to swallow air in much the same
way as man swallows water. The air is similarly grasped by the muscles,
and conveyed by them downward to the lungs. Therefore a Frog keeps its
mouth shut, and the animal dies from want of air if its mouth is open
for a few minutes.
Crocodiles commonly lie in the sun with their mouths widely open. The
lungs in both Crocodiles and Turtles are moderately dense, traversed by
great bronchial tubes, but do not differ essentially in plan from those
of a Frog, though the great branches of the bronchial tubes are
stronger, and the air chambers into which the lung is divided are
somewhat smaller. The New Zealand Hatteria has the lungs of this
cellular type, though rather resembling the amphibian than the
Crocodile. The lungs during life in all these animals attain
considerable size, the maximum dimensions being found in the terrestrial
tortoises, which owe much of their elevated bulk to the dimensions of
the air cells which form the lungs.
The lungs of Serpents and Lizards are formed on a different plan. In
both those groups of reptiles the dense cellular tissue is limited to
the part of the lung which is nearest to the throat. This network of
blood vessels and air cells extends about the principal bronchial tube
much as in other animals, but as it extends backward the blood vessels
become few until the _tubular_ lung appears in its hinder part, as it
extends down the body, almost as simple in structure as the air bladder
of a fish. Among Serpents only one of these tubular lungs is commonly
present, and the structure has a less efficient appearance as a
breathing organ than the single lung of the fish _Ceratodus_ (Fig. 1).
The Chameleons are a group of lizards which differ in many ways from
most of their nearest kindred, and the lungs, while conforming in
general plan to the lizard type in being dense at the throat, and a
tubular bladder in the body, give off on both sides a number of short
lateral branches like the fingers of a glove (Fig. 18, p. 51).
Thus the breathing organs of reptiles present two or three distinct
types which have caused Serpents and Lizards to be associated in one
group by most naturalists who have studied their anatomy; while
Crocodiles and Chelonians represent a type of lung which is quite
different, and in those groups has much in common. These characters of
the breathing organs contribute to separate the cold-blooded armoured
reptiles from the warm-blooded birds clothed with feathers, as well as
from the warm-blooded mammals which suckle their young; for both these
higher groups have denser and more elastic spongy lung tissue.
It will be seen hereafter that many birds in the most active development
of their breathing organs substantially revert to the condition of the
Serpent or Chameleon in a somewhat modified way. Because, instead of
having one great bronchial tube expanded to form a vast reservoir of air
which can be discharged from the lung in which the reptile has
accumulated it, the bird has the lateral branches of the bronchial tubes
prolonged so as to pierce the walls of the lung, when its covering
membrane expands to form many air cells, which fill much of the cavity
of the bird's body (see Fig. 16). Thus the bird appears to combine the
characters of such a lung as that of a Crocodile, with a condition which
has some analogy with the lung of a Chameleon. It is this link of
structure of the breathing organs between reptiles and birds that
constitutes one of the chief interests of flying reptiles, for they
prove to have possessed air cells prolonged from the lungs, which
extended into the bones.