The animals that we have been learning about so
far are all marine invertebrates. Invertebrates
are animals without backbones, and they are organized around
their intestinal tracts -- the sac that captures and
processes food, or a tube that starts with
the mouth and ends with an opening near the tail end of the
animal. This basic organizing feature allows for an
amazing number of body forms.
The phylum chordata is the one whose members are most
visible to us.
Animals in this phylum are characterized by a supporting
rod of connective tissue running down the length of the
body. In the earliest chordates this was just a rod
between a nerve cord and a digestive tract, all of which ran
down the length of the body from the head Some of these
early chordates still have descendents alive today.
The early chordates did not have bones -- they had
cartilage. Cartilage is made of -- Guess what --
Collagen!!! Do you remember which animal
was the first to have collagen? Click here to review
However, the most successful and numerous representatives of
this phylum are in the sub-phylum of the vertebrates.
Vertebrates are animals with backbones and bony. internal
skeletons, animals such as fishes, amphibians, reptiles,
mammals, and birds. The notochord is seen in their
embryos, and is encased inside the vertebrae as they develop
Before vertebrates ventured onto the land, however, they
were already a big success in the oceans. The Devonian
Age is called the Age of fishes, and the fossil record is
rich in fishes of all sizes.
Here is a
picture of a fossil of a fish.
Take a minute
to look at this fossil fish. See how the spine runs
down the body and how the other bones attach to it.
There are more than 25,000 species of fish in the waters of
the earth today. In lesson 9 we will think about the
vertebrates that live on land.