which were previously directed backwards along the roof of the mouth, are now erected, by the elevation of the

Fig. 42. The head of the Rattlesnake, dissected to show the poison-gland (a) and poison-fangs (f). After Duvernoy.

upper jaw-bone, to which they are attached. The fangs are then plunged with great force into the body of the animal which the snake wishes to kill. In performing this act, the muscles which cover the poison-gland compress it strongly, so that as the poison-fangs enter the flesh, a drop of the poisonous fluid is driven into the wound through the little perforation in the point of the poison-fang. The poison itself is a colourless, greasy fluid, and it produces death, at any rate in the majority of cases, with a rapidity proportioned to the size of the wounded animal.

The mouth opens into a comparatively simple digestive tube, which terminates in a vent placed on the under surface of the tail. The breathing-organs are in the form of voluminous lungs, of which one is rudimentary. It is to be remarked, therefore, that the animal is strictly an airbreather, and that at no time of its life does it possess gills, or organs adapted for breathing air dissolved in water. The heart consists of three chambers, and is so constructed that the impure (venous) blood, mixes to a greater or less extent with the pure blood which has come from the lungs ; and the body is then supplied with

this mixture. As one result of this, the temperature of the body is low, and the animal is "cold-blooded." The nervous system, lastly, consists of a brain and spinal cord, with the nerves which these give off.

It still remains to say a few words about the skeleton of the snake, and especially as to the peculiarities which are connected with its mode of locomotion. The brain is protected within a bony skull, and the spinal cord is protected by a very long and extremely flexible backbone, with which the skull unites by a single joint. There are no traces of limbs, and the snake progresses by gliding upon its belly, walking in reality upon the ends of its ribs. In accordance with this, the ribs are exceedingly numerous, and instead of a number of them being united to a breast-bone in front, this latter bone is absent, and the ribs are simply connected with the horny shields which cover the belly. The snakes creeps along the ground, therefore, by the movements of the numerous ribs, which it employs in progression somewhat in the same way that a centipede uses its legs.

is

The common Rattlesnake of North America is usually about three or four feet in length, sometimes more. Its general colour is brownish-yellow, with two rows of partially united brown blotches of an irregularly lozengeshaped figure, but the tail is black. The "rattle light brown in colour, and increases in length with the years of the animal, though not receiving an additional joint per annum, as is often stated. Full-grown specimens ordinarily possess a rattle of from sixteen to twentyfour joints. The snake shakes the rattle when alarmed or about to strike, but what the function of this singular organ may be, must, in spite of recent speculations, be regarded as still unknown. The Rattlesnake lives upon small animals, such as hares or squirrels, and birds, which it kills by its poisonous bite, and then swallows whole, the teeth permitting neither division of the food nor mastication. It is a sluggish animal, which remains torpid during the winter, and is most active and most poisonous in the hottest weather.

RECAPITULATION OF ESSENTIAL CHARACTERS.-The animal is an air-breather, and never possesses gills at any time of its life. The two sides of the heart communicate with one another (in most cases), and the body is always supplied with a mixture of pure and impure blood. The blood is cold. The skin usually develops horny scales. The lower jaw is jointed to the skull by means of a "quadrate bone," and the skull is united with the backbone by means of a single joint. The condition of the limbs varies, but in no case are more than two pairs present. These characters distinguish the class Reptilia as a whole.

CHAPTER XXII.

CLASS AVES.

THE class Aves (Latin, avis, a bird) includes only those most familiar and beautiful of animals, the Birds. Instead of finding any difficulty in selecting an example which is both common and at the same time exhibits the leading peculiarities of the class, it would not be an easy matter to choose a common bird in which the more important characters of the entire division are not present. We shall therefore select as our type the domestic Goose, or rather the Grey Lag Goose (Anser ferus) from which the domestic breed is descended.

Like all birds, the Goose is a genuine biped, supporting its body, in standing or walking, exclusively upon its hind-legs. As in the great majority of birds, also, the fore-limbs are converted into wings, and are employed in supporting the body of the animal in the air, or, in other words, in flight. The fore-limbs are thus useless for purposes of grasping, and all acts of this nature are performed by the beak-that is to say, by the jaws; though

some birds can likewise employ the hind-feet in seizing objects.

The entire body of the Goose is covered with a close covering of those peculiar appendages of the skin which are

Fig. 43.-The Grey Lag Goose (Anser ferus). After Yarrell.

known as feathers. The lower portion of the legs and the beak are unfeathered, but the head, neck, and body are protected by a dense plumage, which serves two purposes. In the first place, since it conducts heat very imperfectly, the plumage serves to retain and conserve the heat generated within the body of the animal. The Goose, therefore, though spending a portion of its time in a medium so cold as water, whilst it is a "hot-blooded" animal, is nevertheless able to keep its temperature up to as high as about 100° or a little over. In the second place, the plumage is kept oiled by the oily secretion of certain glands near the tail, and the bird is thus enabled to enter the water without getting wet.

The feathers carried by the wing are longer than those which cover the body, and are the organs which propel

the bird through the air. Those carried by the hand are longer than those supported by the fore-arm, and the whole can be made to beat the air by the downward stroke of the wing. The feathers of the tail, also, though of no great length in the Goose, are longer than those which cover the body, and can be spread out like a fan, so as to serve as a rudder and guide the bird in its course through the air.

Whilst the wings or fore-limbs are used in flight, the hind-limbs or legs are used both in walking upon the land and in swimming in the water. The legs are placed so far back (fig. 43) that the animal cannot walk lightly or gracefully, but on the contrary has a waddling and awkward gait upon the land. This same circumstance, however, enables the feet to act very efficiently as oars or paddles,

Fig. 44.-A, Head of the Grey Lag Goose; B, Foot of the domestic Goose.

when the animal is in the water, and this use of them is greatly facilitated by the peculiar form of the foot. The foot (fig. 44, B) consists of four toes, of which three are