In this class there are 47 genera, and 400 species. The specific characters are taken from the number of rays in the fins, the form of the tail, the cirri or filaments at the mouth, the colouring of the body, the form of the scales, and other circumstances. The fifth class, that in which the INSECTS are included, comprehends 86 genera, disposed into seven orders, which are founded on the number and texture of the wings. I. COLEOPTERA, or Hard-winged Insects. Insects having the wings covered by two crustaceous cases. This order is the most extensive, including 30 genera, and 893 species. It includes all the insects commonly known by the name of beetles. II. HEMIPTERA, or _Half-winged Insects_, having the shells or cases semicrustaceous, not divided by a straight line as in the coleoptera, but overlapping each other at the margin; the beak curved inwards; 12 genera, 353 species. The cockroach, cricket, locust, and cochineal-insect, are examples. III. LEPIDOPTERA, or _Scaly-winged Insects_, having four wings, which are covered with imbricated scales; the tongue spiral and coiled up, the body hairy. In this order there are only 3 genera, _Papilio_, _Sphinx_, and _Phalaena_, the butterflies and moths; but the species are 780. IV. NEUROPTERA, or _Net-winged Insects_, with four naked, transparent, or reticulated wings; the tail generally destitute of a sting. There are 7 genera, and 83 species, among which are the dragon-fly, the may-fly, and the scorpion-fly. V. HYMENOPTERA, or _Thin-winged Insects_, with four naked membranous wings; some species, however, being wingless. The females have the tail armed with a sting. This order contains 10 genera, and 313 species, of which may be mentioned as examples, the wasp, bee, ichneumon-fly, and ant. VI. DIPTERA, or _Two-winged Insects_, having only two wings, and being furnished with a balance or club behind each wing. There are 10 genera, and 262 species, among which are the common house-fly, the flesh-fly, and the gnat. VII. APTERA, _Wingless_. Insects destitute of wings in both sexes. They are arranged under 14 genera, and consist of 300 species. In this order there are three divisions: some have six feet, as the flea, the louse, and the white ant; others have from 8 to 14 feet, as the spider, scorpion, crab, and lobster; while others have a still greater number, as the centipede. The generic characters are derived from the antennae, the jaws, the head, the thorax, the wings, the elytra or wing-covers; and the specific, from the colours and other circumstances. The number of species is 2984. The sixth class, that of VERMES or WORMS, is a very heterogeneous one, and to later authors has supplied materials for several classes. Linnaeus divides it into five orders: