The offspring of all the tribes are produced from _eggs_, which, after they have been deposited by the parent animals in a proper place, are hatched by the heat of the sun. The eggs of some of the species are covered with a shell: those of others have a soft and tough skin or covering, not much unlike wet parchment: and the eggs of several are perfectly gelatinous. In those few that produce their offspring alive, as the vipers, and some other serpents, the eggs are regularly formed, but are hatched within the bodies of the females. 8. This class is divided into two ORDERS. _ORDERS OF AMPHIBIA._ I. REPTILES Have four legs, and walk with a crawling pace, as the _tortoises_, _frogs_, and _lizards_. II. SERPENTS Have no legs, but crawl on their belly. 9. FISHES constitute the fifth class of animals. They are all inhabitants of the water, in which they move by certain organs called _fins_. These, when situated on the back, are called _dorsal fins_; when on the sides, behind the gills, they have the name of _pectoral fins_; when on the belly near the head, they are _ventral_; when behind the vent, they are _anal_; and that at the posterior extremity of the body is called the _caudal fin_. Fishes breathe by _gills_, which, in most of the species, are situated at the sides of the head. In some of the flatfish, however, as the skate and thornback, they are on the under part of the body. Fish rise and sink in the water, generally by a kind of bladder in the interior of their body, called an _air-bladder_. Some of them, as the skate and other flat-fish, do not possess this organ, and consequently are seldom found but at the bottom of the water. The bodies of these animals are usually covered with _scales_, which keep them from injury by the pressure of the water. Several of them are enveloped with a fat and oily substance to preserve their bodies from putrefaction, and also to guard them from extreme cold. 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.