members of the
sub-family of vertebrates, still their structure is easily seen to
spring from a single form--all are modifications of some common
pattern, the differences arising from the necessities of the life of
the animal, as manifested through the desire and necessities of the
species.

Science shows the direct relationship between the Vertebrates, and the
Invertebrates by means of several connecting-links, the most noticeable
of which is the Lancelot, a creature resembling the fish-form, and yet
also closely resembling the lower (invertebrate) forms of life. This
creature has no head, and but one eye. It is semi-transparent, and
possesses _cilia_ for forcing in the water containing its food. It has
something like gills, and a gullet like the lower forms. It has no
heart, the blood being circulated by means of contracting vessels or
parts. Strictly speaking, it has no back-bone, or vertebra, but still
Science has been compelled to class it among the vertebrates because is
has a gristly cartilage where the back-bone is found in the higher
forms. This gristle may be called an "elementary spine." It has a
nervous system consisting of a single cord which spreads into a
broadened end near the creature's mouth, and which may therefore be
regarded as "something like a brain." This creature is really a
developed form of Invertebrate, shaped like a Vertebrate, and showing
signs of a rudimentary spine and nervous system of the latter. It is a
"connecting-link."

The lowest forms of the true Vertebrates are the great families of
Fishes. These Fish families include fishes of high and low degree, some
of the higher forms being as different from the lowest as they (the
highest) are different from the Reptile family. It is not necessary to
go into detail regarding the nature of the fish families, for every
student is more or less familiar with them.

Some peculiar forms of fish show a shading into the Reptile family, in
fact they seem to belong nearly as much to the latter as to their own
general family. Some species of fish known as the _Dipnoi_ or
"double-breathers," have a remarkable dual system of breathing. That
is, they have gills for breathing while in the water, and also have a
primitive or elementary "lung" in the shape of an air-bladder, or
"sound," which they use for breathing on land. The Mud-fish of South
America, and also other forms in Australia and other places, have a
modification of fins which are practicall