ORDER I. _Chelonia_ (turtles and tortoises). Flattened reptiles with
body inclosed in bony case. No teeth or sternum (breastbone).
Examples: snapping turtle, box tortoise.
ORDER II. _Lacertilia_ (lizards). Body covered with scales, usually
having two-paired appendages. Breathe by lungs. Examples: fence
lizard, horned toad.
[Illustration: The common garter snake. Reduced to about one tenth natural
size.]
ORDER III. _Ophidia_ (snakes). Body elongated, covered with scales. No
limbs present. Examples: garter snake, rattlesnake.
ORDER IV. _Crocodilia._ Fresh-water reptiles with elongated body and
bony scales on skin. Two-paired limbs. Examples: alligator, crocodile.
Birds.--Birds among all other animals are known by their covering of
feathers and the presence of wings. The feathers are developed from the
skin. These aid in flight, and protect the body from the cold.
[Illustration: Adaptations in the bills of birds. Could we tell anything
about the food of a bird from its bill? Do these birds all get their food
in the same manner? Do they all eat the same kind of food?]
The form of the bill in particular shows adaptation to a wonderful degree.
A duck has a flat bill for pushing through the mud and straining out the
food; a bird of prey has a curved or hooked beak for tearing; the
woodpecker has a sharp, straight bill for piercing the bark of trees in
search of the insect larvae which are hidden underneath. Birds do not have
teeth.
[Illustration: Common tern and young, showing nesting and feeding habits.
(From group at American Museum of Natural History.)]
The rate of respiration, of heartbeat, and the body temperature are all
higher in the bird than in man. Man breathes from twelve to fourteen times
per minute. Birds breathe from twenty to sixty times a minute. Because of
the increased activity of a bird, there comes a necessity for a greater and
more rapid supply of oxygen, an increased blood supply to carry the
material to be used up in the release of energy, and a means of rapid
excretion of the wastes resulting from the process of oxidation. Birds are
large eaters, and the digestive tract is fitted to digest the food quickly,
by having a large crop in which food may be stored in a much softened
condition. As soon as the food is part of the blood, it may be sent rapidly
to the places where it is needed, by means of the large four-chambered
heart and large blood vessels.
The high temperature of the bird is a direct result of this rapid
oxidation; furthermore, the feathers and the oily skin form an insulation
which does not readily permit of the escape of heat. This insulating cover
is of much use to the bird in its flights at high altitudes, where the
temperature is often very low. Birds lay eggs and usually care for their
young.
[Illustration: African ostrich, one of the largest living birds.]