of any proper bony framework, within or without. Many of them, however, as shell-fish, have an external shell as a means of defence, but it does not form the point of support to the muscular system, like the bones of Vertebrata and the external envelope of Articulata. The functions of sensation and locomotion are, with some exceptions, of a low order, whilst those of digestion, circulation, and respiration are highly developed. Cuttlefish, squid, cockle, snail, oyster, clam, quahaug.

v, one of the valves of the shell; vl, its hinge; mm, lobes of the mantle, one of which is folded back; c', nfuscles of the shell; br, gills; b, mouth; t, tentacula; f, liver; i, intestine; a, anus; co, heart.

IV. The RADIATA are so called from the circular arrange

Radiata. Sea-anemone.

A

A, represents the animal as seen from above. In the centre is the mouth surrounded by tentacula with which its food is seized and conveyed into it. B, a section exhibiting its internal organs. a, cavity of stomach, which is surrounded by b, a series of chambers, in which the germs of the young are developed.

ment of their organs around a central spot, at which the mouth is situated. The animals of this division are commonly denominated Zoöphytes. The organs of sensation, of locomotion, of circulation, of respiration, and the nervous system, are very imperfectly developed, and cannot be always detected, and in many of the lower tribes scarce any trace of organization is to be discovered. Starfish, sea-anemone, polypes, sunfish, coral animals, infusoria.

CHAPTER VI.

FIRST BRANCH.-VERTEBRATA OR VERTEBRAL ANIMALS.

THESE are divided into four classes:

1. Mammalia or Mammals.

2. Birds.

3. Reptiles.
4. Fishes.

The first two classes are distinguished by having a fixed temperature above the ordinary temperature of the medium in which they live. The last two by having a temperature varying but little from that of such medium. Hence the former are called warm-blooded, and the latter cold-blooded.

Of the warm-blooded animals, those of the first, class produce their young alive, and nourish them during infancy by their own milk, secreted by organs called mamma or breasts. Hence their name. This includes man, quadrupeds, seals, whales. Those of the second class, or Birds, produce their young by means of eggs, hatched usually by the heat of their own bodies, and support them by food, which they provide for them as soon as they come out of the egg. These two classes resemble each other also in the general structure of the organs of respiration and circulation.

* A different disposition of the classes of Vertebral animals has been suggested, increasing their number to nine or ten, by a subdivision of those of Reptiles and Fishes. Doubtless there may be sufficient grounds for this new arrangement, but they are such as can only be intelligible to well-instructed naturalists, and it has seemed preferable, therefore, to adhere to the old and generally understood divisions

The first of the classes of cold-blooded animals, Reptiles, contains those which breathe air only and cannot exist without it, as lizards, frogs, tortoises, serpents; the second, or Fishes, those which receive the air through the medium of water by means of branchiæ or gills.

SECTION I.

CLASS I. MAMMALIA.

Of the structure of man, the most perfect example of this class, an account has already been given, and we shall get the most intelligible idea of that of the other Mammalia by tracing the degree and manner in which they depart from him.

We have already adverted to the manner in which the structure of man is modified in the formation of the monkey tribe, p. 33, (Fig. 6.) The locomotion of these animals is chiefly that of climbing. On level ground they scramble along rapidly enough, but clumsily and imperfectly, whilst among the branches of trees they make their way with astonishing ease and rapidity. This is owing partly to the presence of hands upon all four of their extremities, but quite as much to the different distribution of force between the upper and the lower limbs. Man is comparatively a poor climber. His chief strength is in his legs. It is with difficulty that he raises his body with his arms. The monkey's greatest strength is in his arms, and the advantage which this gives him in climbing will be seen at once.

proper carnivorous animals, like the cat tribe, and in the gnawing tribe, like the squirrel. These, although differing from each other in so many particulars, are yet somewhat alike in the manner in which they differ from man. The fore limbs are brought nearer together and are directed forward; the chest is narrow, especially at its upper part; and the shoulder and arms have much less freedom and variety of motion. The fingers are not separate, but enveloped and confined by the skin; there is no thumb, and claws are substituted for nails. The wrist and heel are removed farther from the fingers, and, as a consequence, motion, which is wholly on the four extremities, is performed upon the ends of the

Fig. 11.

Jerboa.

fingers and toes instead of the hand and foot. Hence the arm and thigh are both shorter; and the elbow, or the joint corresponding to the elbow, and the knee, are carried up toward the body. The fore limbs are still, however, capable of performing some of the offices of hands, as we see in the common cat, and more especially in the squirrel. The panther and jerboa are also examples of the same structure.

Still considerable freedom of motion remains. Many of these animals use their fore paws as hands with some dexterity, and are excellent climbers, as the squirrels, though they have not the power of grasping which distinguishes the monkeys. Some of them naturally, and others by force of education, are capable of assuming the erect posture, by throwing themselves backward upon their heels and bending the knee and hip joints at very acute angles.

All the animals of these tribes are not constructed alike; for, whilst there are some not far removed from the monkeys in their structure and capacity for motion, others approach to the more complete quadrupeds.

As familiar examples of these last, namely, the complete quad

rupeds, we may take the ruminating and the single hoofed animals, the camel, (fig. 12,) the ox, and the horse. In these the chest is still more contracted, the arms or fore legs are brought more closely together,—all freedom of motion in the shoulder joint is lost; -the bones of the fingers are united into a solid mass, and form a round limb terminated by one or more hoofs. The same

takes place in the hind limb. The heel is further raised from the ground, and the animal can no longer throw himself back upon it, but, as in a horse that rears, always supports himself on the ends of the toes.

In this figure the single line drawn around the skeleton indicates the outline of the living animal; vc, the vertebræ of the neck; vd, of the back; vl, of the loins; vs, of the sacrum; vq, of the tail; c, the ribs; o, shoulder-blade or scapula; h, arm; cu, forearm, a single bone corresponding to the two bones between the elbow and wrist in man, between these two bones is the joint corresponding to the elbow; ca, bones of the wrist me, of the hand; ph, of the fingers; fe, of the thigh; ro, of the knee; ti, of the leg, in place of two in man; ta, of the ankle; mt, toes.

By a comparison of this skeleton with that of man (p. 23) and with the account of the gradual transition of form which takes place from the structure of man to that of the complete quadruped, a conception will be obtained of the changes there referred to.

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