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Tuatara: Volume 10, Issue 3, September 1962

A Classification of Echinoderms

page 138

A Classification of Echinoderms

Figure 1 gives the main features of a classification of the radially symmetrical groups of echinoderms, presented in outline at the recent meeting of ANZAAS in Sydney. A detailed exposition of the reasoning on which the classification is based will be given elsewhere (1), and a broader, but less technical, explanation has also been prepared (2). The proposals are based on the results of a study of growth gradients in post-larval stages, which have led inter alia to the isolation of surviving members of several groups hitherto known only from Palaeozoic fossils, namely Somasteroidea (3, 4, 5), Platyasterida (1), and Oegophiurida (1, 6). The living representatives of these ancient star-shaped echinoderms show that somasteroids, asteroids and ophiuroids are closely interrelated, all, for example, having caeca of the gut running far out into the arms, and the reproductive organs arranged serially along the arms. All the star-shaped groups can be demonstrated to have much closer affinities with the crinoids than hitherto supposed. They also show that the customary grouping of echinoids, holothurians and sea-stars in one subphylum ‘Eleutherozoa’ is unacceptable, for the assemblage is polyphyletic, and the characters supposed to unite the assemblage occur in fact only in demonstrably late asteroids and demonstrably early echinoids.

To express these conclusions, a revised classification is needed, from which ‘Eleutherozoa’ will disappear as a formal taxon. Its place will be taken instead by two distinct subphyla, the Asterozoa and the Echinozoa. The subphylum Asterozoa comprises so uniform an assemblage that all its members may be referred to a single class, for which the old name Stelleroidea is available. This class is regarded as comprising three subclasses, the Somasteroidea. Asteroidea and Ophiuroidea, between which no hard-and-fast lines can now be drawn, for they intergrade. The oldest members of the Asterozoa are the Somasteroidea, characterised by dominant pinnate growth axes, apparently inherited from crinoid-like ancestors, provisionally identified with biserial crinoids. In the later Asterozoa the pinnate gradients are gradually replaced by longitudinal adradial growth axes, leading to the condition seen in modern asteroids and ophiuroids.

page 139
Figure 1

Figure 1

page 140

The other subphylum, the Echinozoa, comprises more diverse groups, among which it is at present convenient to recognise three classes, the Echinoidea, Holothuroidea and Ophiocistioidea, though the exact relationship of the latter fossil group is uncertain. All Echinozoa exhibit dominant meridional growth patterns, and radially divergent axes never form, nor any trace of pinnate structure. Detailed data are given in the references cited.


1. Phylogeny of Sea-Stars (in press).

2. Ann. Rpt. Smithsonian Inst., 1962 (in press).

3. Science, 136, 633-6, 1962.

4. Univ. Kansas Pal. Contrib., Echinodermata, 6, 1962.

5. Zoolog. Zhurn. (Akad. Nauk SSSR), 41 (9), 1962.

6. Pub. Seto Mar. Biol. Lab., 10 (2), (in press).