ORDER I. _The Elasmobranchs._ Fishes which have a soft skeleton made
of cartilage and exposed gill slits. Examples: sharks, skates, and
rays.
ORDER II. _The Ganoids._ Fishes which once were very numerous on the
earth, but which are now almost extinct. They are protected by
platelike scales. Examples: gars, sturgeon, and bowfin.
[Illustration: A bony fish.]
ORDER III. _The Teleosts, or Bony Fishes._ They compose 95 per cent of
all living fishes. In this group the skeleton is bony, the gills are
protected by an operculum, and the eggs are numerous. Most of our
common food fishes belong to this class.
ORDER IV. _The Dipnoi, or Lung Fishes._ This is a very small group. In
many respects they are more like amphibians than fishes, the swim
bladder being used as a lung. They live in tropical Africa, South
America, and Australia, inhabiting the rivers and lakes there.
Characteristics of Amphibia.--The frog belongs to the class of vertebrates
known as Amphibia. As the name indicates (_amphi_, both, and _bia_, life),
members of this group live both in water and on land. In the earlier stages
of their development they take oxygen into the blood by means of gills.
When adult, however, they breathe by means of lungs. At all times, but
especially during the winter, the skin serves as a breathing organ. The
skin is soft and unprotected by bony plates or scales. The heart has three
chambers, two auricles and one ventricle. Most amphibians undergo a
complete metamorphosis, or change of form, the young being unlike the
adults.
[Illustration: Newt. (From a photograph loaned by the American Museum of
Natural History.) About natural size.]
[Illustration: The leopard frog, an amphibian.]