The class mammalia comprehends the following orders: 1. Carnivora (_flesh-eating_, as the lion, tiger, etc.); 2. Ruminantia (_animals that chew the cud_, as the camel, ox, sheep, and others); 3. Pachydermata (_thick-skinned_, as the elephant, horse, etc.); 4. Rodentia (_gnawing_, as the beaver, squirrel, mice, etc.); 5. Edentata (_toothless_, as the ant-eater and armadillo); 6. Quadrumma (_four-handed_, as the ape and monkey tribe); 7. Cherioptera (having _winged arms_, as bats); 8. Marsupialia (_pouched_, as the kangaroo and opossum); 9. Cetaceæ (whales, dolphins, and the various seals). The last-mentioned of these divisions includes members (and those the largest) of a tribe assigned in popular language to a distinct division of the animal world--fishes. But the whale and other creatures of its order possess the distinguishing attribute of the mammalia--that is, they afford their nutriment from the breast. The animals belonging to the ruminating and pachydermatous orders are further distinguished as ungulata, or hoofed, from the well-known characteristic of their extremities. The domesticated animals that are used as food by man are almost exclusively derived from this class. The animals included within the other orders of mammalia are designated as unguiculata, from their extremities terminating in claws, or nails. The invertebrate animals, or those which have no spinal bone, all have white blood. They are scientifically divided into molluscous animals, in which the muscles are attached to the skin, with or without the protection of a shell--such as snails and slugs; articulated animals, in which the covering of the body is divided into rings or segments, to the interior of which the muscles are attached--comprehending all insects and worms; and radiated animals, in which the organs of motion or sensation radiate from a common centre--such as star-fish.