Vertebrate animals are characterized by having the brain and principal trunk of the nervous system included in a bony articulated case, composing the skull and vertebral column. They have all red blood, and a muscular heart. The mouth is furnished with two jaws moving vertically. They have distinct organs of sight, hearing, smell, and taste. The limbs are never more than four in number. The sexes are in all cases separate.

They are divisible into the following five classes:

* With warm blood: heart with two auricles and two ventricles.

I. Mammalia

Viviparous animals, suckling their young : breathing by lungs. Body generally covered with hair, and furnished in most cases with four feet.

II. Aves

Oviparous animals; breathing by lungs.

Body covered with feathers; furnished with two wings and two feet.

** With cold blood: heart with one or two auricles, and one ventricle.

III. Reptilia

Oviparous animals; breathing by lungs. Body covered with scales or shelly plates. Impregnation effected by sexual union : the young undergoing no metamorphosis.

IV. Amphibia

Oviparous animals; breathing by lungs and gills. Body covered with a soft naked skin: feet without claws. Eggs impregnated after exclusion: the young undergoing metamorphosis.

V. Pisces

Oviparous animals; breathing by gills only. Body covered with scales, and furnished with fins instead of feet.