find suitable food, although this food is often a kind quite different from that of the insect in its adult state. Some insects, however, as bees, wasps, ants, and earwigs, pay great attention to their young. The stage of development at which insects issue from the egg is very different in different tribes. In general, however, they undergo very remarkable metamorphoses. A worm, inhabiting a muddy pool, becomes a winged creature that sports in the air; a crawling caterpillar, that greedily devours some kind of herbage with its horny jaws, eating vastly more in proportion to its size than an ox, is converted into a splendid butterfly, flitting from flower to flower, and feeding only on nectareous juices. The first stage of insect-life, after the egg is hatched, is called the larva1 stage, and in this the young insect generally

increases much in size. Caterpillars are the larvæ of butterflies, moths, and hawk-moths; grubs are the larvæ of beetles; maggots are the larvæ of two-winged flies, such as the housefly and blow-fly. The next stage into which insects pass is the pupa 2 stage, in which those that undergo complete metamorphoses are almost quite inactive, and take no food. The pupa of a butterfly, a moth, or a hawk-moth is called a chrysalis 3 or chrysalid, from the golden spots with which many such pupae are adorned. Chrysalids are often enclosed in a horny membranous case. Many larvæ, when about to change into the pupa state, spin cocoons or silken envelopes in which they undergo their transformation, and which serve for the protection of the inactive and helpless pups. Silkworms are the larvae of certain species of moth, which, on undergoing their transformation into pupa, spin for themselves cocoons, the thread of which is silk. The cocoon of the common silkworm exhibits externally a loose gauze-like covering, within which is a compact and close oval ball; yet all this is one continuous thread, which may sometimes be unwound to the length of one thousand feet. The perfect insect is formed within the covering of the pupa, attaining there its full proportions, and ready to use its wings, its legs, and all its other organs within a few moments after it has emerged into the open air.

Fig. 69.-Purple Emperor Butterfly:
The larva and pupa or chrysalis are shewn

below-the larva to the right, and the
chrysalis to the left.

Insects are all animals of small size. The largest are tropical, amongst which are butterflies of almost a foot across the expanded wings. Insects abound far more in warm than in cold climates. The different species are very numerous, those of beetles alone exceeding in number all the kinds of vertebrated animals.

A few species of Insects are important for their usefulness to man; but a far greater number are remarkable for the injuries they inflict by the destruction of herbage and crops or of articles of food or raiment. Of noxious insects, locusts may perhaps be regarded as the chief. Of insects useful to man, bees and silkworms deserve to be first named, and after them the cochineal insect and blistering-flies. There are a few others to

which we are indebted for substances useful in medicine and the arts. The instincts and habits of insects are very various and interesting. Many volumes have been written on those of bees and ants alone.

Of the orders into which the class of Insects is divided by systematic naturalists, we must be contented with very briefly noticing only the most important. Taking first the insects which undergo complete metamorphoses, and in their perfect state have the mouth fitted for gnawing, tearing, and masticating, we find the order Coleoptera,1 distinguished by having the first pair of wings modified into hard sheaths -called elytra, or wing-cases-to cover the second pair when not in use, and to protect the upper side of the abdomen, the wings being folded crosswise under these wing-cases. Coleopterous insects are sometimes collectively called beetles, although many of them are familiarly known by other names, as chafers, weevils, lady-bugs, &c. The glowworm and fireflies belong to this order, as do also the valuable cantharides, or blistering-flies.-The order Orthoptera 2 differs from Coleoptera chiefly in having the wing-covers of a soft substance, somewhat resembling parchment, and the wings folded longitudinally in a fan-like manner. this order belong locusts, grasshoppers, and crickets; also those curious insects which, from their resemblance to leaves, twigs, and other objects, are known as leaf-insects or walking-leaves, walking-sticks, &c.— The order Neuroptera 3 consists of insects having mouths similar to those of the orders already noticed, and four nearly equal and membranous wings, all adapted for flight, not folded in any way when at rest, and divided by their nervures-ribs or veins-into a delicate network. To this order belong dragon-flies, may-flies, ant-lions, and termites or white ants. These last-named insects have social habits resembling those of the true ants. The great ant-hills of Africa and South America are constructed by them.-The order Hymenoptera contains a vast number of

4

1 From Greek coleos, a sheath, and pteron, a wing.

2 From Greek orthos, straight, and pteron, a wing.

3 From Greek neuros, a nerve or string, and pteron, a wing.

4 From Greek hymen, a membrane, and pteron, a wing.

То

species, among which are ants and bees. They have the mouth furnished with mandibles for cutting and tearing, but other parts of it formed for suction. The wings are four in number, membranous, not folded when at rest, the first pair larger than the second, the wings of the same side united in flight by little hooks. Many species of this order have stings, and they are the only insects which have.

Among the orders of insects which undergo complete metamorphoses, but, in their perfect state, have the mouth formed only for suction, the first that demands notice is the order Lepidoptera, to which butterflies, moths, and hawk-moths belong. The mouth has a long trunk, coiled up when not in use; the wings are four in number, membranous, and covered with minute closely set scales, of very various forms. Great beauty appears in the whole order.-The order Diptera 2 contains all the twowinged insects commonly known as flies, also midges, gnats, &c. The number of species is immense. The mouth is formed for suction alone.

The insects which undergo no metamorphosis are comparatively few. They are all destitute of wings. Many of them are parasitical on other animals.

Mollusca.

Coming now to the primary division of the animal kingdom called Mollusca, we return to the consideration of animals far lower in organisation than those which have last been under notice, and ascend as by a new path to higher tribes; for there is not in nature a regular unbroken gradation, as some have fancied, from the lowest to the highest forms of animal life. But in the highest Mollusca we have a nearer approach than in any of the other animal sub-kingdoms, to the highest of all, that of Vertebrata. Many of the lower Mollusca-those forming the order Polyzoa3 -were until recently ranked among Zoophytes, and they have a strong general resemblance to the polypes already noticed as belonging to the great group of Calenterata. They are, like them, of minute size, and many have a similar coralline fabric. They also resemble them in multiplying by gemmation and forming compound animals. In the lowest groups of Mollusca, we find only one principal nervous ganglion. In the higher groups, there are several ganglia lying somewhat irregularly in different parts of the body, and communicating by nervous threads with a larger mass. It is only in the highest Mollusca that a distinct head is found, bearing organs of special sense. The lowest groups have no distinct heart, but all the higher possess it. The bilateral symmetry of form, almost universal in articulated and in vertebrate animals, is comparatively

1 From Greek lepis, a scale, and pteron, a wing.
2 From Greek dis, twice, and pteron, a wing.
3 From Greek polys, many, and zōon, an animal.

rare in the Mollusca. The shell with which many species are protected, has not in the least degree the nature of an external skeleton, no muscles being attached to it but those which are necessary for the opening and shutting of its own valves. It is a mere calcareous exudation from the mantle, a thick, soft, flexible skin, with which, in all the Mollusca, the whole body is invested. Many Mollusca have no shell; many have the shell in a single piece, which is often a spiral tube closed at one end, and gradually widening to its open extremity. Such shells are called univalves.1 Snails, whelks, and periwinkles afford familiar examples of the most common form of univalve shells. Somewhat different forms may be seen in limpets, cowries, &c. The shells of many molluscs are composed of two valves, as those of oysters, mussels, &c., which the animal is able to open and shut at pleasure. Such shells are called bivalve.2 In a small number of molluscs, the shell is composed of a greater number of pieces, and such shells are said to be multivalve, but the multivalve molluscs differ little from univalves in their general organisation.

All molluscs have the power of locomotion, at least in the earliest stage of their existence; although many of the lower tribes soon lose it, and become permanently fixed in one spot. The modes of locomotion are various. Many molluscs move by means of a muscular structure in some particular part of the mantle, termed the foot, which in some, as in the cockle, is an organ for springing, in others for burrowing, in many for crawling, and in some for swimming. In the highest of all orders of molluscs, the Cephalopoda —of which the cuttle-fish and squid are examples the head is furnished with tentacles armed with suckers, and by means of these the animal drags itself along, whilst the body is also often provided with fin-like expansions, which serve for swimming. The respiration of Mollusca generally takes place by gills; but slugs and snails, which do not inhabit water, have a pulmonary sac or cavity lined with a vascular network.

Vertebrata.

The highest primary division of the animal kingdom is that of Vertebrata, comprising the five classes of Fishes, Amphibians, Reptiles, Birds, and Mammals, the first-named being the lowest, and the last the highest of all. All the classes of Vertebrata agree in what may be called a general type of structure, exhibited in its highest perfection in man, and of which some account has already been given under

1 From Latin unus, one, and valva, a valve.

2 From Latin bis, twice, and valva, a valve.

3 From Latin multus, many, and valva, a valve.

4 From Greek cephale, the head, and pous, podos, a foot.

PHYSIOLOGY. One of the most marked characteristics of the vertebrate animals is that indicated by their common name; they have an internal bony skeleton, to which the muscles are affixed, and of which the principal part is the backbone, terminated at the anterior extremity by the skull. They have in general four limbs, corresponding to the arms and legs of man, but very variously modified as organs of mere locomotion, and as organs also of prehension; and assuming not only the characters of arms and legs, terminated by hands or feet, but also of fins, wings, &c. Some vertebrate animals have only one pair of limbs instead of two, a few have no limbs at all. No vertebrate animal has any other number of limbs than two or four. The limbs are always formed of bones jointed to the backbone and one to another, surrounded with muscles, and covered with skin. All vertebrate animals have a distinct head, a skull containing the brain, and a spinal cord extending through the centre of the backbone. They have a system of circulation dependent on the action of a muscular heart with two or more cavities. Those which live in water breathe by gills, those on dry land by lungs. The digestive system, although exhibiting a variety of modifications as great as the external form, presents always the same general characters. The mouth, for example, is always situated in the front of the head, and has two bony jaws articulated to the other bones of the head. A tongue is also always present in the mouth, an organ to which nothing truly corresponding is to be found in any invertebrate tribe.

Fishes. The class of Fishes consists of animals that all live in water, and at all stages of their existence breathe by gills. In number—both of individuals and of different kinds-fishes are supposed to exceed all other vertebrate animals together. Not only the sea, but lakes and rivers abound in them, even the waters of hot springs and the pools of caverns have their peculiar species. Their form is adapted to easy progression through water. The four limbs which belong to the vertebrate type are generally all present, assuming the form of fins —the first pair, situated on the breast, being the pectoral 1 fins (p, fig. 70), and the second the ventral 2 (v, fig. 70), their normal position being on the belly. The latter are liable to great changes of position, and are often found close under the pectoral fins, and sometimes before them, quite on the throat of the fish, in which position they are called jugular3 fins. Fishes, however, have also other fins, not so closely connected with the proper skeleton, and placed singly on the middle line, both above and below. One of these, the caudal or tail fin (c, fig. 70), is the principal organ of locomotion, the whole muscular power of great part of the

1 From Latin pectus, the breast.
2 From Latin venter, the belly.

3 From Latin jugulum, the throat.

4 From Latin cauda, a tail.