The Evolution of the Echinoderms ' 



By H. Barraclough Fell 



Victoria University of Wellington, New Zealand 



[With 3 plates] 



Sea stars, sea urchins, and sea cucumbers are obviously interrelated 

 in some way, for tliey sliare many common characters. Their origins 

 and ancestry, however, have long remained a baffling enigma, for no 

 intermediate forms between sea stars and other echinoderms have 

 hitherto been recognized, and the relationships suggested by knov^^n 

 fossils have seemed to conflict with those suggested by embryology. 

 Recently a promising line of inquiry, based on comparative study of 

 growth-gradients, has given a very different aspect to the problem. 

 The investigation, reported in detail elsewhere (see references), has 

 led to the isolation of several surviving members of supposedly ex- 

 tinct groups of sea stars, and the information yielded by these now 

 suggests that sea stars (both brittlestars and starfishes) arose from 

 a different stock from that which gave rise to sea urchins and sea 

 cucumbers. This, in turn, implies that the conventional phylogeny 

 of echinoderms is in need of some revision. A brief sketch of these 

 ideas follows. 



CONVENTIONAL CLASSIFICATION 



The Echinodermata are, of course, well known for their conspicuous 

 radial symmetry. They are also to l^e reckoned among the most 

 nmnerous animals in the sea, as evidenced by the large numbers taken 

 in trawls, and by photographs of the sea floor. Fossils show that they 

 have been important faunal elements since the Cambrian. Although 

 they are so notable for radial symmetry, we now know of at least 

 one quite varied group of Paleozoic echinoderms with no trace of 

 radial symmetry, the Homalozoa. Thus, a modern diagnosis of the 

 Echinodermata would not include radial symmetry as a fundamental 

 feature, stressing rather the unique crystalline calcite features of the 

 skeleton, and the presence of a multipurpose hydrovascular system. 



^ Address delivered to the Jubilee Congress of the Australian and New Zealand Associa- 

 tion for the Advancement of Science held at Sydney in August 1962. 



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