Zoology 10th Edition by Stephen Miller,John Harley

Exercise 4

Members of the phylum Echinodermata are characterized by the presence of pentaradial symmetry, an internal calcium carbonate skeleton, a water-vascular system, and a hemal system. The phylum is composed of five classes with extant members. Asteroidea includes the sea stars. They have arms that are broadly attached to a central disk. Ophiuroidea includes basket stars and brittle stars. They have arms sharply set off from a central disk. They lack suction disks on their tube feet. Echinoidea includes the sea urchins and sand dollars. They have round, flattened, or globular tests that are covered with spines, which are used in locomotion. Holothuroidea includes sea cucumbers. They have a muscular body with microscopic ossicles. They lack arms and have secondarily derived bilateral symmetry. Crinoidea includes sea lilies and feather stars. They have a crown of arms connected to a calyx. A stalk (sea lilies) or cirri (feather stars) is used for attachment to the substrate.
How is pentaradial symmetry manifested in members of each of the five echinoderm classes?