all the rest. If they were not all alike in some things, we could not
call them all animals. One of the things in which all the animals I have
named are alike is, that they all have skeletons. One of the things in
which all their skeletons are alike is, that they have backbones.
Backbones are the chains of bones that run along the back from the head
to the tail. Backbones are called by the Wise Men _vertebrae_; animals
that have backbones are named _Vertebrates_; and animals that lack
backbones are named _Invertebrates_."

"Tell us the names of some Invertebrates, please, Doctor," said Rap.

"Well, all sorts of insects are Invertebrates, and so are lobsters and
crabs, oysters and clams, worms, starfishes, jelly-fishes, corals, and
even sponges. Then there are some too small to see without a microscope.
But never mind about Invertebrates now. I only want you to remember that
all beasts, birds, reptiles, and fishes are Vertebrates, and that there
are five principal classes of them. If I should tell you as much about
them as I have about the Building of a Bird, you would see that they are
all built on what we call the Vertebrate plan of structure. Here is a
chart of the Classes of Vertebrates--you can study it this afternoon,
till you learn it by heart."


VERTEBRATES

_Animals with Backbones_

CLASS I. Mammals.--Warm-blooded animals which have fur or hair, bring
forth their young alive, and nurse them. House People are Mammals.