But however curious the phenomena attending the development of the tadpoles of the Amphibian Reptiles may be to the observer who merely watches the changes perceptible from day to day in their external form, they acquire tenfold interest to the physiologist who traces the progressive evolution of their internal viscera; more especially when he finds that in these creatures he has an opportunity afforded him of contemplating, displayed before his eyes, as it were, upon an enlarged scale, those phases of development through which the embryo of every air-breathing vertebrate animal must pass while concealed within the egg, or yet unborn.[11]
CHAPTER II.
OPHIDIAN REPTILES, OR TRUE SNAKES.
Reptiles are, as has been said in the preceding chapter, Vertebrated Animals, breathing by lungs, having red and cold blood; that is to say, not producing sufficient heat to render their temperature superior to that of the atmosphere. Destitute of hairs, of feathers, of mammary glands, and having bodies covered with scales.
Snakes, properly so called, have the tympanic bone, or pedicle of the lower jaw, movable, and nearly always suspended to another bone, analogous to the mastoid bone, which is attached to the cranium by muscles and ligaments, a conformation which gives to these animals the vast power of distension they possess. Their trachea is long, their hearts placed far back, and the greater number have one very long lung and vestiges of a second. They are divided into non-venemous and venemous; and the latter are subdivided into venemous with maxillary teeth, and venemous with isolated fangs.
The Snakes prey almost exclusively on animals of their own killing; the more typical species attacking such as are frequently larger than themselves: and the maxillary apparatus is, as we have seen, modified so as to permit of the requisite distension. According to Professor Owen's clear and intelligible description, the two superior maxillary bones have their anterior extremities joined by an elastic and yielding fibrous tissue with the small and single intermaxillary bone; the lower maxillary rami are similarly connected. The opposite extremity of each ramus is articulated to a long and movable vertical pedicle formed by the tympanic bone, which is itself attached to the extremity of a horizontal pedicle formed by the mastoid bone, so connected as to allow of a certain yielding movement upon the cranium. The other bones have similar loose movable articulations, which concur in yielding to the pressure of large bodies with which the teeth have grappled.
The class of Reptiles is divided into three orders:—the Ophidians, comprehending the Snakes; the Saurians, the Lizards and Crocodiles; and the Chelonians, the Turtles and Tortoises.
Ophidians.
In Ophidians, commonly known under the name of Snakes, the body is long, round, and straight. They have neither feet, fins, nor other locomotive extremities. Their mouths are furnished with pointed hooked teeth. In the Boas and Pythons the teeth are slender, curved, bending backwards and inwards above their base of attachment. In others each maxillary bone has a row of larger ones, which gradually decrease in size as they are placed further back. These teeth are not contiguous, being separated by considerable intervals. The smaller non-venemous Serpents, such as the Colubridæ, have two rows of teeth in the roof of the mouth. Each maxillary and mandibular bone includes from twenty to twenty-five teeth. In the Rattlesnakes and some other typical genera of poisonous Snakes, the short maxillary bone only supports a single perforated fang. Their lower jaw is highly distensible; the opening beings longer than the skull. They have no neck; their eyelids are immovable; their skin is coriaceous, highly extensible, and scaly or granulous, covered with a thin caducous epidermis, which detaches itself in one entire piece, and is reproduced several times in one year. Their movements are supple and varied. In consequence of the sinuosity of their bodies,—for, though scale-clad, Snakes are without apparent means of progression,—they make their way with the utmost facility, by walking, leaping, climbing, or swimming.
According to the genus chiefly, the very numerous species inhabit either arid or moist places, the ground, or bushes and trees. Some pass much of their time in the water, and one family (that of the Hydrophidæ) is exclusively aquatic—even pelagic in the instance of one very widely diffused species, the Pelamis bicolor. In the Arboreal Snakes the tail is very long, and highly prehensile; in others, as the Vipers, it is short and without any prehensility. In the Sea Snakes (Hydrophidæ), it is laterally much compressed. Like other true reptiles, Snakes abound more especially in warm climates, and there are many kinds of them in Australia; but the order has not a single representative in New Zealand.