Phylum Echinodermata

Abstract

The phylum Echinodermata comprises the Echinozoa (seacucumbers and sea-urchins), Homalozoa (fossil carpoids), Crinozoa (sea-lilies and various fossil groups) and Asterozoa (sea-stars). Most living representatives exhibit conspicuous radial or meridional symmetry of the body, though this is not a fundamental character of the phylum, for it is lacking from the older fossils, only imperfectly exhibited in others, and generally absent from the larval stages. Echinoderms are exclusively marine animals, and are among the most abundant elements of the fauna of the sea floor. Their relationships to other phyla are unknown, for echinoderms were already well differentiated at the onset of Palaeozoic times, and their distinctive characters seem to be unparallelled in any other group. Some representative echinoderms are illustrated in Fig. 767.