How the Echinoderm Changed Its Symmetry

On the search for common mechanisms to generate the archetypal forms

Rudolf J. Prokop - Vaclav Petr


(Originally published in Bulletin of the Czech Geological Survey, vol. 73, n. 4, 351-354. Praha 1998)


"Modern man lives isolated in his artificial environment, not because the artificial is evil as such, but because of his lack of comprehension of the forces which make it work - of the principles which relate his gadgets to the forces of nature, to the universal order. It is not central heating which makes his existence 'unnatural,' but his refusal to take an interest in the principles behind it."

(Arthur Koestler)

"All animal phyla are unique, but some are more unique than others" is a well-known sentence of Nichols (from his book: "The Uniqueness of Echinoderms", 1976) about the phylum which is unique in many ways. The best known mystery of the phylum is the dominant five-fold radial symmetry, the "pentamery phenomenon". Everybody knows that recent echinoderms (sea lilies, sea stars, sea urchins, brittle stars, sea cucumbers) are obviously phylogenetically interrelated but although they share many common characteristics, no intermediate forms between the classes are known. A number of different propositions have been used to designate echinoderm homologies but pentamerism always presented a deep mystery that had not been explained satisfactorily by any of the previous authors because phylogenetic reconstructions have been deduced only from the palaeontological material, molecular analyses of recent specimens and historical speculations based almost exclusively on the theory of natural selection. But today, a fascinating pioneering attempt to solve that mystery and to reveal phylogenetic relations and evolution of the fundamental body plan in echinoderms is presented by the American palaeontologist and echinoderm specialist, Frederick Hatfield Clark Hotchkiss from Harvard, Massachusetts (esp. in his last work, HOTCHKISS 1998). His really modern and unifying methods combine palaeontology, molecular genetics and developmental biology using a contemporary theory of morphogenetic fields. Why we believe that this way is so utterly important?

Let us return back in time to Richard Owen, originator of the distinction between homology and analogy, who was interested in Cuvier's work on functional adaptation as well as German non-materialistic science (idealism of romantic Naturphilosophie) of form. The latter, transcendental aspect of the biological form (intrinsic structural order of it), was prime for Owen. He suggested that organismal morphologies are variants on perfect or ideal forms (Archetypes or "primal patterns"). He also proposed "secondary causes" (metagenesis) which were the means of "translating the Word into flesh" (paraphrased New Testament, John 1, 14). In one of his works (se the text-fig. 2), Owen was figured being flanked by Georges Cuvier and Lorenz Oken. Symbolically, this historical picture represented a synthesis (whether complete or not is a question) between functionalism and transcendentalism in vertebrate palaeontology.

Even some of today's post-modern neo-Darwinists are aware of the problem of the principles ruling the body architecture of animal bauplans but generally are unable to understand that the material genes are not the only determinants of living things and that the true essence rather lies in the immaterial factors that govern development (morphogenesis). Nevertheless, convergent evolution, in which very similar animals arise from quite different evolutionary histories, attracts more and more attention. It is extremely important to recognize that in morphogenesis, constraints and regularity themselves cannot be explained in terms of natural selection. Why?

It is generally agreed that August Weismann was fleshing out the essentials of neo-Darwinism and replaced the original Darwin's "mysterious laws of the correlation of growth" (mentioned in the Origin of Species) with a conception of inherited material elements (now genes, themselves immortal to the core). His theory of such a simple material determinant, in other ways fully compatible with the Darwin's Theory of Natural Selection, suggested a surprisingly easy explanation of the form of any organism. It was very useful, because it inspired modern palaeontological research in the fossil record and has given our natural science its lacking historical depth. Neo-Darwinism (Weismannism and the Modern Synthesis) became the governing mode of thought among both biologists and palaeontologists. But its reductionism, understanding the form as a mere "frozen accident" preserved by the action of natural selection, had and has nothing to say about how new forms of organisms arise. Neo-Darwinian morphology changed to a strictly historical science, stressing only the role of genes and genealogies, and elevating historical process to a primary explanatory principle in natural sciences.

Some present leading structuralists, esp. the British biologist Brian C. Goodwin (e.g. in his book entitled like one of the famous Kipling's Just So Stories: "How the Leopard Changed Its Spots", 1994, paraphrased here), stress that all cells of a given body contain identical copies of DNA but during morphogenesis develop differently. Further, that proteins have incomplete autonomy in self-bending and self-shaping into the form of a living organism and that there are unicellular organisms that will partially regenerate their complicated surfaces even if the nucleus itself is removed. The new "generative paradigm" (as opposed to "evolutionary paradigm", see GOODWIN 1984) would turn scientists from looking for the historical "just so stories" to search for Aristotle's "formal cause" or archetypes of form, establishing a "supra-molecular" causal principle (surprisingly, some scientists, esp. the British biologist Rupert Sheldrake, go even further, suggesting not only the existence of morphogenetic fields themselves but also of a Spirit that is both the goal and the ground of the entire Universe, a creative consciousness that transcends the Universe, that is the source of its existence and of the laws that govern it).

So the basic problem is that of the functional unity of an organism and transcendental morphology, in other words it is the problem of the concept of history and structure, process and pattern, external behaviour and internal meaning, function and form - of the two distinct explanatory modes in the biological thought, symbolized often by Cuvier and Geoffroy Saint-Hilaire's famous debate (1830). It ought to be stressed that in the discussed work by Frederick Hotchkiss in the journal Paleobiology (domain of macroevolutionists), this problem has been resurrected, thoroughly analyzed and successfully solved for the Phylum Echinodermata. Reading the work of William Bateson ("Materials for the Study of Variation", 1894) who proposed limitations on historical definitions of homologies because it is difficult to apply them to structural similarities between different parts of the same body, and in relation to his own numerous previous studies on echinoderm ray homologies, Hotchkiss points directly to Bateson_s rules of symmetry to explain that after a duplication of one ray in an unsegmented unirayed ancestor, two appendages were added to form a three-rayed bauplan (occurring on the left side of the ancestor) which was immediately compensated by morphogenetic regulation of bilaterality via the homeotic expression of a mirror-image pair of rays on the opposite side. This is very briefly his interpretation. But why is it so important and how does it relate to the solutions of previous authors?

One of the best previous accounts is that of the British palaeontologist D. G. Stephenson who presented (1976) an "ideal pattern" or "paradigm" for the origin of the pentamerism in echinoderms - one ray pointing upstream to monitor the current, one pair extending laterally to catch food and the last pair of rays extending downstream to warn of predators. In other words, Stephenson rather presented an historical speculation about the function, while Hotchkiss analyzes the bauplan itself. In fact, Stephenson's "ideal pattern" or "paradigm" is a possible external behaviour, while Hotchkiss wants to get at the interior meaning itself - inside the black box. Moreover, Hotchkiss says that pentamerism may have arisen independently in parallel lineages because of the possibility proposed by Goodwin that robust morphogenesis can repeat itself. It is now evident that similarities, in which the same bauplans arise independently in different lineages, are unexplainable by traditional historical accounts. Hotchkiss sheds light on the invalidity of majority of the current beliefs in form as a "frozen accident". It is extremely important that, like J. W. Goethe, Frederick Hotchkiss goes beyond both Cuvier and Geoffroy in search for reconcilliation, and, like Owen, he goes beyond both Cuvier and Oken to a fully dynamic conception of biological form and its history that would give a picture of the greatest possible clarity.


Fig. 1: Richard Owen, flanked by Georges Cuvier and Lorenz Oken (from Owen's "On the Principal Forms of the Skeleton and the Teeth", 1856). The picture symbolizes a synthesis between function and form, process and pattern, or functionalism and transcendentalism in vertebrate palaeontology.

Fig. 2: A mystery of the phylum Echinodermata is its dominant five-fold radial symmetry, the "pentamery phenomenon".

A Selected Bibliography of Brian C. Goodwin and Frederick H. C. Hotchkiss

GOODWIN, B. C. (1984): A relational or field theory of reproduction and its evolutionary implications, p. 219-241. In: M.-W. HO & P. T. SAUNDERS (Eds.): Beyond Neo-Darwinism, Academic Press. London.

GOODWIN, B. C. (1984): Changing from an evolutionary to a generative paradigm in biology, p. 99-120. In: J. W. POLLARD (Ed.): Evolutionary Theory: Paths into the Future. 271 pp., Wiley. Chichester, New York, Brisbane, Toronto, Singapore.

GOODWIN, B. C. (1994): Homology, Development, and Heredity, p. 229-247. In: B. K. HALL (Ed.): Homology: The Hierarchical Basis of Comparative Biology, 483 pp., Academic Press. San Diego, New York, Boston, London, Sydney, Tokyo, Toronto.

GOODWIN, B. C. (1994): How the Leopard Changed Its Spots: The Evolution of Complexity, Simon and Schuster. New York.

GOODWIN, B. C. - HOLDER, N. - WYLIE, C. C. (Eds.) (1983): Development and Evolution (The Sixth Symposium of the British Society for Developmental Biology), 437 pp., Cambridge University Press. Cambridge, London, New York, New Rochelle, Melbourne, Sydney.

GOODWIN, B. C. - SAUNDERS, P. (Eds.) (1992): Theoretical Biology: Epigenetic and Evolutionary Order from Complex Systems, Johns Hopkins Press. Baltimore.

GOODWIN, B. C. - TRAINOR, L. E. H. (1983): The ontogeny and phylogeny of the pentadactyl limb, p. 75-98. In: B. C. GOODWIN, N. HOLDER, & C. C. WYLIE (Eds.): Development and Evolution (The Sixth Symposium of the British Society for Developmental Biology), 437 pp., Cambridge University Press. Cambridge, London, New York, New Rochelle, Melbourne, Sydney.

HOTCHKISS, F. H. C. (1970): North American Ordovician Ophiuroidea: the genus Taeniaster BILLINGS, 1858 (Protasteridae). - Proceedings of the Biological Society of Washington, 83(5): 59-76. Washington, D.C.

HOTCHKISS, F. H. C. (1974): Studies on Paleozoic Ophiuroids and the Ancestry of the Asterozoa. PhD Dissertation, 178 pp. Yale University. - Diss. Abstr. Int. 35(6): 2922B.

HOTCHKISS, F. H. C. (1976): Devonian ophiuroids from New York State: reclassification of Klasmura, Antiquaster, and Stenaster into the Suborder Scalarina nov., order Stenurida. - Bulletin of the New York State Museum, 425: 1-39. Albany.

HOTCHKISS, F. H. C. (1977): Ophiuroid Ophiocanops (Echinodermata) not a living fossil. - Journal of Natural History, 11(4): 377-380. London.

HOTCHKISS, F. H. C. (1978): Studies on echinoderm ray homologies: Loven's law applies to Paleozoic ophiuroids. - Journal of Paleontology, 52(3): 537-544. Lawrence.

HOTCHKISS, F. H. C. (1979): Case studies in the teratology of starfish. - Proceedings of the Academy of Natural Sciences of Philadelphia, 131: 139-157. Philadelphia.

HOTCHKISS, F. H. C. (1980): The early growth stage of a Devonian ophiuroid and its bearing on echinoderm phylogeny. - Journal of Natural History, 14: 91-96. London.

HOTCHKISS, F. H. C. (1982): Ophiuroidea (Echinodermata) from Carrie Bow Cay, Belize. In: K. RUTZLER & I. G. MACINTYRE (Eds): The Atlantic Barrier Reef Ecosystem at Carrie Bow Cay, Belize, I: Structure and Communities. - Smithsonian Contributions to the Marine Sciences, 12: 387-412. Washington, D. C.

HOTCHKISS, F. H. C. (1993): A new Devonian ophiuroid (Echinodermata, Ophiuroidea) from New York State and its bearing on the origin of ophiuroid upper arm plates. - Proceedings of the Biological Society of Washington, 106(1): 63-84. Washington, D.C.

HOTCHKISS, F. H. C. (1995): Loven's law and adult ray homologies in echinoids, ophiuroids, edrioasteroids, and an ophiocstioid (Echinodermata: Eleutherozoa). - Proceedings of the Biological Society of Washington, 108(3): 401-435. Washington, D.C.

HOTCHKISS, F. H. C. (1998): A "rays-as-appendages" model for the origin of pentamerism in echinoderms. - Paleobiology, 24(2): 200-214. Lawrence.

HOTCHKISS, F. H. C. (1998): Discussion on pentamerism: the five-part pattern of Stromatocystites, Asterozoa, and Echinozoa. pp. 37-42. In: R. MOOI & M. TELFORD (Eds): Echinoderms, San Francisco. A. A. Balkema, Rotterdam.

HOTCHKISS, F. H. C. - SEEGERS, P. R. (1978): Variable symmetry in starfish. - Thalassia Jugoslavica, 12(1): 173-180.

Frederick Hatfield Clark Hotchkiss

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