10. The fishes are divided into six ORDERS.
| ORDERS OF FISHES. | ||
| I. | Apodal | Have bony gills; and no ventral fins, as the eel. |
| II. | Jugular | Have bony gills; and the ventral fins situated in front of the pectoral fins, as the cod, haddock, and whiting. |
| III. | Thoraic | Have bony gills; and the ventral fins situated directly under the pectoral fins, as the perch and mackerel. |
| IV. | Abdominal | Have bony gills; and the ventral fins on the belly behind the pectoral fins, as the salmon, herrings, and carp. |
| V. | Branchiostegous | Have their gills destitute of bony rays. |
| VI. | Chondropterygeous | Have cartilaginous fins, as the sturgeons, sharks, and skate. |
11. The fifth class of animals comprises the INSECTS. These are so denominated from the greater number of them having a separation in the middle of their bodies, by which they are, as it were, cut into two parts. The science which treats of them is called ENTOMOLOGY.
Insects have, in general, six or more legs, which are, for the most part, nearly of equal length and thickness. Sometimes, however (as in the mole-cricket), the forelegs are very thick and strong, for burrowing into the ground; sometimes the hind thighs are long and thick, for leaping; or flattened, fringed with hairs, and situated nearly in an horizontal position, to serve as oars for swimming.
Most of the insect tribes are furnished with wings. Some, as the beetles, have two membranous wings, covered and protected by hard and crustaceous cases, called elytra; some, as the wasps and bees, have four wings without elytra; others, as the common houseflies, have two wings; and others, as the spiders, are entirely destitute of these members.
They are furnished with antennæ, which are usually jointed, and moveable organs, formed of a horny substance, and situated on the front and upper part of the head. These serve as instruments of touch, or of some sense which is to us unknown. The eyes of insects are formed of a transparent substance, so hard as to require no coverings to protect them. Their mouth is generally situated somewhat beneath the front part of the head, and in a few of the tribes is below the breast; and the jaws are transverse, and move in lateral directions. These are furnished with feelers, and other organs, of various arrangement and structure, which constitute the foundation of arrangement in some of the systems of entomology. All insects breathe, not through their mouth, but through pores or holes along the sides of their bodies; or, as in the crabs and lobsters, by means of gills. The skin of insects is, in general, of hard or bony consistence, divided into plates or joints which admit of some degree of motion, and is generally clad with very short hairs.
Nearly all insects go through certain great changes at different periods of their existence. From the egg is hatched the larva, grub, or caterpillar, which is destitute of wings; this afterwards changes to a pupa, or crysalis, wholly covered with a hard shell, or strong skin, from which the perfect or winged insect, bursts forth. Spiders, and some other wingless insects, issue from the egg nearly in a perfect state.
12. Linnæus has divided the animals of this class into seven ORDERS.
| ORDERS OF INSECTS. | ||
| I. | Coleopterous | Have elytra or crustaceous cases covering the wings; and which, when closed, form a longitudinal division along the middle of the back, as the chafer. |
| II. | Hemipterous | Have four wings, the upper ones partly crustaceous, and partly membranous; not divided straight down the middle of the back, but crossed, or incumbent on each other, as the cock-roach. |
| III. | Lepidopterous | Have four wings covered with fine scales, almost like powder, as the butterflies and moths. |
| IV. | Neuropterous | Have four membranous and semi-transparent wings, veined like net-work; and the tail without a sting, as the dragon-fly and ephemera. |
| V. | Hymenopterous | Have four membranous and semi-transparent wings, veined like network; and the tail armed with a sting, as the wasp and bee. |
| VI. | Dipterous | Have only two wings, as the common house-flies. |
| VII. | Apterous | Have no wings, as the spiders. |
13. The sixth and last class of animals consists of WORMS, or vermes. These are slow of motion, and have soft and fleshy bodies. Some of them have hard internal parts, and others have crustaceous coverings. In some of the species eyes and ears are very perceptible, whilst others appear to enjoy only the senses of taste and touch. Many have no distinct head, and most of them are destitute of feet. They are, in general, so tenacious of life, that parts which have been destroyed will be re-produced: These animals are principally distinguished from those of the other classes by having tentacula, or feelers.