are valuable on account of their fur. The rapacity of man, however, has nearly exterminated these very interesting animals.

G. Classification of Animals

146. Vertebrates and invertebrates. All animals may be divided into two great groups, known respectively as vertebrates and invertebrates. To the first group belong the animals that have a "backbone or spinal column composed of a series of bones known as vertebra. To this group belong fishes, frogs, turtles, birds, rabbits, and human beings, for all of them have a spinal column made up of vertebræ. Insects, earthworms, and oysters, on the other hand, have no backbone; hence, they are called invertebrates (i.e. animals without vertebræ).

While

147. Summary of the classification of the invertebrates. the vertebrates, on account of their size, are more familiar to most people, in reality there are a great many more kinds of invertebrates than vertebrates. For example, over 300,000 different species of insects have been described, more than all other species of animals put together. The invertebrates are divided by zoölogists into ten or more branches or subkingdoms, some of the most common of which are named in the table on pages 192 and 193.

148. Summary of the classification of the vertebrates. The vertebrate branch of the animal kingdom is divided into five distinct classes. The striking characteristics of each of these classes will be seen by studying the table on page 194.

149. Reproduction among the vertebrates. - Among the animals belonging to the two lowest vertebrate groups, namely, the fish and amphibia, the female forms eggs within the body and deposits them in the water. Before these eggs can develop, however, they must be fertilized by sperm-cells produced by the male, and this is likewise true of all the higher animals and plants. The fertilized eggs develop into embryos by the process of cell division, and enough food is stored in the egg to supply the young animal until it can secure its own food. Much the same is true also in the case of rep

tiles, except that the eggs are usually laid in the sand and left to develop by the warmth of the sun. There are, however, certain exceptions to the general statements made above. Some of the sharks, for example, and certain of the snakes, instead of depositing eggs that develop into embryos in the water or on land, retain the eggs, and the young are born in a form much like that of the adult.

Very few of the animals belonging to the classes that we have been discussing (namely, the fishes, amphibia, and reptiles) ever take any care of their young. The great majority of birds, however, not only build nests in which to lay their eggs, but they also brood over their eggs until they are hatched, and then the parents feed the young until they are ready to fly.

A few of the lowest mammals, like most of the vertebrates named above, lay eggs. By all the common mammals, however, the eggs are not laid, but as was the case with certain sharks and snakes, the eggs develop into a form resembling the parent, before being born. All mammals at birth, unlike birds, are unable to eat the food that is used by their parents. Hence, a form of food that is easily digestible must be furnished. This is secreted by certain cells of the adults in the form of milk. The masses of cells that secrete milk are known as mammary glands, and because of the presence of these glands in all animals of this the highest group of vertebrates, this class is known as the mammals.

1 Cold-blooded animals are those animals in which the temperature of the blood changes with the temperature of their surroundings. Warm-blooded animals, on the other hand, maintain under normal conditions an almost constant temperature. The temperature of the human body, for example, is 98.6° F., which is usually higher than the temperature of the earth, air, and water; consequently when we touch a fish or frog, the animal feels cold.