The great sub-kingdom of vertebrate animals includes such widely differing creatures as the horse, pigeon, herring, frog, and snake-creatures which it would be impossible to study in one group. We must find out in what points the various members of the sub-kingdom differ, and that will help us to a further classification.
Boys and girls living in the country may have seen the young calf sucking milk from its mother"s body. All must have seen the little kittens doing the same thing.
The milk comes from "teats" which the little one sucks. The Latin name for "teat" is mamma, and as all animalswhich suckle their young are provided with teats, we give them the name of mammals.
If we put our hand on any of these animals, we feel that their bodies are warm. They have warm blood. A fish and a frog always feel cold and clammy, because their blood is not warm. You know too that the colour of the blood of a horse, a sheep, a cow, or a pig is red.
If we examined the heart of a sheep, a horse, a rabbit, or any one of these mammals, we should find it to consist of four distinct chambers-two auricles and two ventricles, exactly the same as our own. All mammals, too, breathe through lungs as we do. We ourselves belong tothe class-mammals.
You have all seen the hen with her little chickens. How does she rear them? She, and birds of all kinds, lay eggs, and sit close upon them to hatch them with the warmth of their own body. The little ones break the shell and come out when fully formed, and the parent bird feeds them, but not with milk. She searches about for little morsels of solid food, which from the first the young ones are able to eat.
Birds, like mammals, have two pairs of limbs, but the front pair are specially fitted for flying. We call them wings.
If we place our hand on a bird of any sort, we feel that its body is warm. It has warm blood, like the mammals, and like them, too, it breathes through lungs, and the heart consists of four chambers.
Mammals have almost every variety of covering for their bodies, but birds of all kinds are clothed with feathers.
Now I want you to think of the snakes of our earlier lessons. These animals belong to the class called reptiles, which includes lizards, tortoises, and turtles, and the great crocodiles and alligators of the tropics. The word reptile means creeping thing. It is not a good name, for all these animals do not creep, and some of them are extremely nimble and agile.
Reptiles produce their young from eggs. But although they breathe through lungs, as birds and mammals do, their blood is cold, and the heart has generally three chambers only. Some, however, of the more vigorous among the reptile family have four chambers in the heart. That is to say, there is a partial division of the ventricle, which amounts to the same thing.
The frog-like animals form a class by themselves, and are known as Batrachia. They include frogs, toads, and newts. They are hatched by the heat of the sun from eggs which the mother has laid in the water. They commence life in the water, breathing, like fishes, through gills. Theheart has only two chambers-an auricle and a ventricle.
You know that the young frog, or tadpole, gradually loses its fish-like form, develops lungs instead of gills, and legs to walk and hop about on the earth. It then leaves the water and enters upon a new life, breathing air throughlungs, like all other land animals. Its blood is cold; the heart of the fully-developed frog has three chambers- two auricles and one ventricle.
Last in the list of vertebrate animals come the fishes. They produce their young from eggs. They breathe all their life through gills, and the heart has only two chambers-an auricle and a ventricle. They are cold- blooded animals, with no covering for the body but scales.
They have a sort of rudimentary limbs called fins, which serve to propel them through the water.
Lesson 21