body being brought to bear in its stroke.

The fins on the back are

called dorsal fins (fig. 70, d1, first dorsal; d2, second dorsal), and that behind

trate towards the vertebræ. Fishes consume little air in respiration, and are cold-blooded animals, having in general a temperature little above that of the water in which they live.

The mouth of fishes is their only organ of prehension. In some it is extremely small, in others extremely large; in some destitute of teeth, in others furnished with a multitude of very minute teeth, in others, again, with a great number of large and strong teeth. In some, as lampreys, it forms a sucker, by which the fish can both affix itself and suck up the blood of the animal on which it preys. The teeth of fishes are more various in number, form, and position than those of any other animals. Some fishes feed on vegetable food, such as the leaves of water-plants; but most of them feed on animal food, of which there is no kind that does not seem to be particularly agreeable to some of them.-Fishes are generally oviparous, and the multitude of their eggs is prodigious, as may be seen in the roe of a herring or a cod; a few kinds are ovoviviparous, the eggs being hatched within the body of the parent, and the young produced in a living state.-Fishes are generally covered with scales, which are variously formed and arranged. Some kinds have large bony plates instead of scales. Of the uses of fishes to man, by far the most important is that of supplying him with food. It is impossible here to enumerate the fishes highly valuable in this respect. Some of them are to be found inhabiting both salt and fresh waters, in all parts of the world. The herring, cod, and salmon are, however, the three species of highest economical importance.

Fishes are divided into two great sections or sub-classes-OSSEOUS 2 FISHES, which have hard bones, and CARTILAGINOUS FISHES, of which the bones are cartilaginous, and destitute of true bony fibres. The Osseous Fishes are far more numerous than the Cartilaginous; the

1 From Latin ovum, an egg, and viviparus, bringing forth its young alive.
2 Bony, from Latin os, a bone.

most important of the latter section being the tribes of Sharks, Rays (Skates, &c.), Sturgeons, and Lampreys.

Amphibians.—The class Amphibians1 receives its name from the fact that many of the species belonging to it are capable of living either on dry land or in water. Many naturalists regard it merely as an order of the class of Reptiles, to which the name of Batrachian 2 Reptiles is given, the frog being taken as the type of the whole. The most essential differences between Amphibians and Reptiles are, that young Amphibians undergo metamorphoses, and that Amphibians breathe by gills alone in the earlier part of their life, whilst in their adult state most of them breathe by lungs alone, although some have both lungs and gills. Most of the Amphibians, in an adult state, have four limbs, although in some these are altogether wanting, and others have only one pair; but in the early stages of their life none of them have limbs at all, and their form is fish-like, of which we have a familiar example in the tadpole, the young of the frog.

Fig. 71.-Successive stages of the Frog:

In the order of the numbers-from the egg almost to the perfect form.

Frogs, toads, newts, and salamanders are the best known Amphibians; but some of those of other countries greatly exceed those of Britain in size.

Reptiles.-Reptiles 3 agree with Amphibians and Fishes in being coldblooded. They differ from Amphibians in breathing by lungs throughout the whole period of their existence. Three orders of reptiles are generally

1 From Greek amphibios, having a double life.
2 From Greek batrachos, a frog.

3 From Latin repo, to creep.

recognised by naturalists: Ophidians1 or Sérpents; Saurians, including lizards, crocodiles, and all species of lizard-like form; and Chelonians,3 containing tortoises, turtles, &c.

SERPENTS have no limbs, and in general the body is very much elongated. The body and tail are covered with scales, the head very often with plates. The vertebræ and ribs are extremely numerous, some serpents having more than three hundred pair of ribs. A serpent moves by means of its ribs and the scales of the abdomen, which are joined to the ribs by slender cartilages, and take hold of the surface over which the animal passes. Many serpents not only glide over the ground with great rapidity, but climb trees with comparative ease. The vertebræ are so formed and jointed together as to give great pliancy to the body. Serpents abound chiefly in the warm parts of the world, to which also the largest species belong, and those of which the venom is most deadly. Venomous serpents depend on their poisonfangs for the capture of their prey. The fangs are a pair of tubes firmly fixed into a movable bone in the front of the upper jaw. When not in use, they are laid back on the roof of the mouth h; but when the animal is irritated, and about to assail its enemy or its prey, they stand out like two lancets. Above them, towards the back of the head, is a large gland for the production of the poison, which passes down the fangs into the wounds which they make. Many serpents, however, are not venomous, but depend on other means for catching their prey, some feeding chiefly on insects or very small animals; and some, the largest and most powerful of all-as the Boa-constrictor and the Pythons of the East Indies-killing even large quadrupeds by mere muscular effort when they have laid hold of and coiled themselves around them.

LIZARDS may be regarded as exhibiting the perfection of the Saurian type. In them, and in chameleons, iguanas, &c., the body is covered with scales; but in crocodiles and alligators it is covered with bony plates, and these animals have therefore been separated from Saurians by some naturalists, and constituted a distinct order under the name of Loricata.1 They are all large reptiles, much larger than any of the scale-covered Saurians, and formidable from their strength and voracity. They inhabit the lakes and rivers of warm countries. The scale-covered or true Saurians are also chiefly inhabitants of warm countries, and most of them love dry situations and sunshine. Some climb trees, and generally live in them, pursuing there their insect prey. A few of them, as the Iguana or Guana of the West Indies, feed on leaves and fruits. The

1 From Greek ophis, a serpent.

2 From Greek sauros, a lizard.

3 From Greek chelone, a tortoise.

4 That is, mailed, from Latin lorica, a coat of mail.

Iguana, although of repulsive appearance, is highly esteemed as an article of food, and is the only Saurian reptile which is so used, except by savage tribes.

Very different from all other reptiles in structure and general appearance is the last order, that of CHELONIANS, all of which are popularly known by the names of tortoise and turtle-the tortoises being those which live on land, and have their feet adapted chiefly for walking; the turtles, those which live in water, and whose limbs are mere paddles for swimming. The Chelonians are all inhabitants of warm countries. Their numbers in some places are astonishingly great. The species both of tortoises and turtles are very numerous, and some of them are highly esteemed for food. The useful substance called tortoise-shell is the hard covering of animals of this order. This covering is really bony, and therefore differs from the outer covering of all other vertebrate animals, being formed from the ribs, the vertebræ, and the breast-bone expanded into plates, and firmly jointed into each other, so as to acquire great solidity, particularly in tortoises, and give most perfect protection to creatures which have no other means of defence. Tortoises feed exclusively on vegetable food, but some of the aquatic Chelonians pursue and prey upon other aquatic animals. The jaws of Chelonians are not furnished with teeth, but are hard, sharp, and horny; they act in a manner somewhat resembling the mandibles of birds. All the Chelonians are oviparous. They lay a great number of eggs, which are covered with a calcareous shell like those of birds, whereas those of other reptiles have only a parchment-like covering. The eggs of turtles are a favourite luxury in the countries in which they can be obtained.

Birds.-Birds are the highest class of oviparous animals, and the only class having warm blood. They exhibit a great similarity in their general structure, and differ very widely in appearance and characters from every other class of animals. To this class belong all the vertebrate animals which are capable of true flight, except bats. The fore pair of limbs in birds serve them only as wings, never in any degree as arms or legs; those few birds in which they are too small to raise the body in the air, generally employ them to aid their swift running upon land, as the ostrich, or for swimming under water, as penguins, which in this manner pursue fish. The body of birds is covered with feathers, and this is one character in which all birds agree, and in which they differ from all other animals. The jaws of birds are much elongated, so as to form the bill, the organ chiefly used in seizing food. The bones of the wing are essentially the same with those of the arms of man, but very much modified to accommodate them to their different use. The bones which represent the fingers are much condensed and partially obliterated. The surface

G

necessary for striking the air is provided by feathers different from those of other parts of the body, called wing-feathers or quills. In many birds, similar feathers appear in the tail, which is used for governing flight. The mechanical adjustments by which the wings are fitted for use as organs of flight, are very beautiful. Birds do not rise or sustain themselves in the air because of any special lightness of their bodies, which are in fact much heavier than the air, but by the action of their wings upon it, and its resistance to them. The legs of birds consist of parts corresponding to those of the legs of man, but the thigh is short, and concealed within the body, the apparent thigh corresponding to the human leg, and the shank, which serves the bird as a leg, to a part of the foot.—Some birds are very useful to man, as affording, in their flesh and their eggs, valuable articles of food; but the number really of importance in this respect is very small in comparison with the multitude of species, and the useful kinds belong mostly to two orders, Rasores and Natatores, to be presently noticed.

We can only indicate in a few sentences the orders of birds. The order Accipitres,1 Rapaces,2 or Raptores,3 contains birds of prey, the food of which is the flesh of animals, chiefly other birds and quadrupeds. They are characterised by great strength of claws and bill. The upper mandible of the bill is hooked and sharp. The claws also are generally hooked and sharp, and the feet formed for seizing prey. The wing is generally large and pointed. This order is divided into two groups, Diurnal and Nocturnal, the first containing eagles, falcons, hawks, vultures, &c.; and the latter consisting entirely of owls. -The order Insessores is a very large one, consisting of birds which have weak, slender feet, not adapted for seizing prey, but generally for perching on trees. Like the Birds of Prey, and most of the birds of other orders, they have four toes, three directed forward, and one backward. The bill is weak and little curved. The wings are of moderate size, and generally rounded. Some of the Insessores feed chiefly on insects, some on seeds or fruits, some indifferently both on animal and vegetable food. To this order belong crows, thrushes, linnets, larks, sparrows, hummingbirds, birds of paradise, &c.-The order Scansores 5 consists of birds very similar to Insessores, but having two toes behind and two before, so that their feet are peculiarly adapted for grasping and climbing. Parrots afford perhaps the best example of this order. Woodpeckers also belong to it. -The Rasores, or Gallinaceous Birds, have a bill of moderate size, short wings, a heavy body, weak toes, and short, stout claws. In general,

1 Latin, plural of accipiter, a hawk.

2 Latin rapax, rapacious.

3 Latin, plural of raptor, a seizer or plunderer.

4 Latin, sitters,' from sedeo, sessum, to sit.

5 Latin, climbers,' from scando, scansum, to climb.

6 Latin, scrapers,' from rado, rasum, to

scrape.

7 From Latin gallus, a cock.