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.
FIG. 1 LUNG OF THE FISH CERATODUS
Partly laid open to show its chambered structure
(After Günther)
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]).