CHAPTER II. How a Reptile Is Known

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.

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.