These organisms have an internal skeleton, including vertebrae (spinal column), jaws, and appendages. Most of the skeleton is made up of bone. Their appendages are well suited to swimming and some have extensions of the skeleton in these appendages.
All members of this group live in water, but some can withstand periods of drought when their aquatic habitat dries out, leaving them in the mud.
These organisms have gills as well as an internal air sac (lung) that they use for gas exchange.
One species has been called a "living fossil."


Two subclasses within Phylum Chordata