d _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.

CLASS II. Birds.--Warm-blooded animals which have feathers and lay eggs.

CLASS III. Reptiles.--Cold-blooded animals which have scaly skins, like
lizards, snakes, and turtles.

CLASS IV. Batrachians.--Cold-blooded animals which have naked skins,
like frogs, toads, and newts.

All the foregoing classes, except a few of the Batrachians, breathe air
in lungs, and almost all, except snakes, have legs; none now living can
fly, except bats and birds; but bats are Mammals.

CLASS V. Fishes.--Cold-blooded animals which have either scaly or naked
skins, but no fur or feathers; which live in the water, breathe it with
their gills, and swim in it with fins.

CHAPTER V

CITIZEN BIRD

The apple trees were in full bloom the day that the Doctor again found
time to be with the children. It was exactly the kind of a day that
birds like. The ground was soft enough to let the earthworms come up to
breathe, so that Robins could catch them easily, and the air was full of
all kinds of insects newly out from their long winter sleep in their
soft cocoon beds, much to the delight of the Swallows and Flycatchers.

It was also a beautiful day for House People to watch their bird
neighbors; for it was mild but not too bright, and every one knows how
it hurts the eyes to look at flying birds with the sun shining in them.

Olive, Dodo, and Nat went out first and found Rap waiting. The Doctor
followed, carrying something in his hand in a black