In dividing the animal kingdom into two sub-kingdoms, vertebrates and invertebrates, we were at first guided by one distinction only. We simply inquired whether the animal had a backbone or not. This has already led us to find out other distinctions. We know now that those animals which have no backbone, and are therefore called invertebrates, are further distinguished by having no internal bony skeleton, no skull, and no brain and spinal cord, such as are found in the vertebrate animals.
But there are other points of difference which we must now consider. All the vertebrate animals have red blood, although in some (the reptiles, Batrachia, and fishes) the blood is cold. In mammals and birds the blood is warm, but all the invertebrate animals have cold, colorless blood.
Think next of the mouths of a mammal, a bird, a snake, a frog, and a fish. They all move vertically-that is, up and down. Now many of the invertebrates have no jaws at all. Whenever they have, the jaws always move horizontally, not up and down.
Again, we have seen that some of the vertebrates breathe through lungs, others through gills. But howdoes the air, in every case, reach these breathing organs? It is taken in at the mouth. Even the fish takes in the water with its mouth, and passes it backwards over the gills, which rob it of the air it contains. Not one single invertebrate animal uses its mouth in breathing. Some breathe through holes in their sides, others through long slits in the neck, others, again, through the tail. But they have neither lungs nor gills.
Let us next turn our attention to the limbs. None of the vertebrate animals have more than four limbs, but those invertebrates which have limbs at all, usually possess more than four. Thus all insects have six legs; spiders eight; shrimps, crabs, and lobsters have ten. Many of the insects, in addition to six legs, have also two pairs of wings.
Now I want you, lastly, to consider the young of the various members in the vertebrate group.
You could not mistake a kitten for a young snake, a bird, a fish, or even for some other mammal. Why? Because the young ones always resemble their parents. The baby boy has the form of a man; a calf is merely a little cow; a chick is a tiny hen. They grow bigger as they grow older, that is all. This is true of all vertebrates with the single exception of the Batrachia. These all commence life as tadpoles, in the water, and are not like their parents.
A young invertebrate is often a very different sort of creature from its parents. You remember that the parentinsect lays the egg, and that the larva or grub which comes from it is totally unlike its parent. Think of the caterpillar side by side with the butterfly; the ugly grub or maggot of the bee, fly, or wasp, side by side with the parent insects.
The egg produces the grub, the grub becomes the pupa, the pupa changes to the perfect insect.
Lesson 24