The echinoderms are a large group of exclusively marine animals, most of
which exhibit a marked five-fold radial symmetry (Pls. 35, 36). Living
echinoderms have well-developed nervous and digestive systems, a
distinct body cavity, and are a relatively complex group of organisms.
The typical echinoderm has a skeleton composed of numerous calcareous
plates which are intricately fitted together and covered by a leathery
outer skin (the _integument_). The echinoderm body often exhibits a
typical star-shaped form, but some types may be heart-shaped,
biscuit-shaped, or cucumber-shaped.
Members of this phylum range from Cambrian to Recent in age and are
abundant as fossils in many of the marine formations of Texas.
The phylum Echinodermata has been divided into two subphyla, the
Pelmatozoa (those forms that were attached to sea floor by a stem or a
stalk) and the Eleutherozoa (the stemless unattached echinoderms).