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The heart is at first like that of Fishes, consisting of only two chambers, and having only the function of driving the blood to the gills. When the lungs are developed, however, the heart undergoes considerable change, becoming like that of a Reptile. It now consists of three chambers, and is so constructed that the impure blood mixes in it with the pure blood coming from the lungs, and that the body is supplied with this mixture. As a consequence of this, at any rate in part, the Frog is, like the Fishes and Reptiles, cold - blooded. In other words, its temperature is very little higher than that of the air or water in which it lives.

The nervous system of the Frog is well developed, con

Fig. 39.-Skeleton of the common Frog (Rana temporaria); d Vertebræ

of the back, with long side-processes.

sisting of a brain and spinal marrow, and of the nerves connected with these. There are also two large eyes, furnished with movable eyelids, and two ears, which can be detected externally.

The skeleton of the Frog (fig. 39) is of a much higher type than that of a fish ; but there are only three points which need be particularly pointed out. Firstly, the broad and flat skull is jointed to the backbone by two distinct joints. Secondly, the joints or vertebræ of the back (d) carry long side-processes, but the ribs are quite rudimentary and merely consist of gristle. Thirdly, the limbs are not in the least like the “fins” of fishes, but they exhibit the same bones as are present in the limbs of the higher animals.

The common Frog is of a reddish-brown or yellowishbrown colour, spotted with black; but it has, to a certain extent, the power of altering its colour according to the intensity of the light to which it may be exposed. It feeds upon small slugs, worms, and insects, and is in turn largely eaten by various animals, especially by Owls. It is an excellent swimmer, and mostly passes the winter under water, in a torpid condition. Owing to the length of its hind-legs, it is also a capital jumper; and it can produce the peculiar sound which is known as “croaking."

RECAPITULATION OF ESSENTIAL CHARACTERS. — The animal when young is a water-breather, and is provided with gills. In its adult state it breathes by means of lungs (with or without the original gills). Both pairs of limbs are usually present, and they do not present the form of “fins," possessing, on the contrary, the same bones as in higher animals. The blood is cold. The skull is jointed to the backbone by two joints, and the nose opens behind into the throat. The heart is threechambered in the adult, but two-chambered in the young. The skin is almost universally soft and destitute of scales. These characters distinguish the class Amphibia as a whole.

CHAPTER XXI.

CLASS REPTILIA.

The class Reptilia (Latin, repto, I crawl) comprises all the animals which are commonly called “Reptiles,” except the Amphibians, which would also be placed here by popular consent, but which we have seen really to constitute a distinct division. The living reptiles fall into the four very distinct groups of the tortoises and turtles, the snakes, the lizards, and the crocodiles and alligators. It is difficult to choose any single type which adequately represents the entire class, and which is at the same time

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readily obtainable for examination. We may, however, select one of the venomous serpents, such as the Rattlesnake, most of the remarks which will be made about this form applying with equal truth to the common Viper of Britain.

The Rattlesnake (Crotalus horridus) has entirely the form which we generally understand by the term “snakelike,” having a long, cylindrical body, tapering towards the tail (fig. 40). The body is completely destitute of limbs, and is covered with an armour formed of small overlapping horny scales. Some of the scales which cover the head are of larger size than the others, and there is also a row of oblong shields which are continued along the whole lower surface of the animal from the head to the end of the tail. The tail terminates in a singular organ known as the “rattle,” which the animal shakes when alarmed or intending to bite. The rattle is really a mere appendage of the skin, and is composed of a number of horny pyramidal joints, loosely united together. The animal, lastly, sheds its skin periodically.

The head is somewhat triangular, broadest behind (fig. 40), and supported upon a comparatively slender neck. The eyes have the peculiarity that they are not furnished with movable eyelids, but are covered by a transparent continuation of the skin. This gives the animal a peculiar fixed and stony stare. Between the eye and the nostril on each side is also to be noticed a deep depression or pit.

At the front of the head is placed the mouth, within which we shall find some of the most characteristic structures in the anatomy of the Rattlesnake. The tongue is forked, capable of being protruded from the mouth at will, and, when protruded, maintained in rapid vibration. The tongue, however, in spite of appearances, is a perfectly harmless organ, and the offensive weapons of the snake are to be found in the teeth (fig. 41). The two halves of the lower jaw are very loosely united together in front, so as to allow of their free separation, and they carry each a series of pointed conical teeth, directed backwards, and amalgamated with the substance of the jaw itself (1). Behind, each half of the lower jaw is united with the skull by a peculiar movable bone (2), which is known as the

“quadrate bone" (Latin, quadratus, four-sided). This bone is directed backwards behind the base of the skull,

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Fig. 41.-Skull of the Rattlesnake (after Dumeril and Bibron). 1 One-half

of the lower jaw, united to the skull by the quadrate bone (9); m Upper jaw carrying the poison-fang; p Series of teeth upon the palate.

and owing both to this circumstance and to its mobility, the snake can open its mouth to an extraordinary width, and swallow morsels of comparatively immense size.

Along the palate above, on each side, is also a row of small conical recurved teeth (p); but the upper jaws offer the most striking peculiarities. The upper jaw (fig. 41, m) is a short movable bone, so jointed to the skull that it can be raised and depressed at will. Each upper jaw carries a great curved tooth, firmly amalgamated with the bone of the jaw, and termed the "poison-fang.” The poison-fangs are not only of much greater size than the other teeth, but they are provided with a canal, which opens at the point of the fang in a minute aperture. The canal in the fang communicates above (fig. 42) with the tube or duct leading from a singular organ known as the “poison-gland." This organ (fig. 42, a) is a gland placed beneath and behind the eye, and secreting the peculiar fluid which renders the bite of the snake fatal. When the snake wishes to kill an animal for food, or when attacked by an enemy, it employs these formidable weapons in the following manner: The great poison-fangs

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