Phylum Echinodermata (2)
Echinoderms (see Figs. 4.29 and 4.30) are marine animals with calcareous skeletons composed of plates. Most have fivefold (pentameral) symmetry. They possess a water vascular system for locomotion, food gathering, respiration, and sensory functions and have a poorly developed circulatory system. Some have light-sensitive cells that function as simple eyes.
Class Crinoidea (middle Cambrian to Recent)
Class Cystoidea (lower Ordovician to Devonian)
Class Blastoidea (middle Ordovician to Permian)
Crinoids, cystoids, and blastoids were most common in the Paleozoic. Only one group, the crinoids (sea lilies), continues to recent times. The anatomy of these three classes of attached echinoderms is very similar. They have a root or anchor system attached to a stem or column of circular plates with a central hole through which run ligamentous fibers. Atop the column is a cup-shaped calyx that houses the internal organs. Arms covered with calcareous plates and pinnules trap food from the seawater and food groves in the arms direct it to the mouth. The major differences among the stalked echinoderms lie in the structure of the calyx. In crinoids, the calyx is composed of two or three groups of five plates each. Cystoids have irregular numbers of plates, and the body is not symmetrical. The plates are pierced with pores for respiration. Blastoids have three groups of five plates in the calyx, and ambulacral grooves make a five-rayed pattern.
This class includes sea urchins and sand dollars and ranges from Ordovician to Recent. Echinoids, which fall into two categories, irregular and regular, are most common in the Mesozoic and Cenozoic.