Enigmas of Echinoderm Nervous Systems
There are three reasons why it is difficult to place echinoderm nervous systems in an evolutionary context. In the first place, although the phylum is clearly deuterostome, there is very little beyond speculation to relate the origins of it to earlier metazoan groups. This argument applies equally, of course, to the chordates themselves since there is evidence that they shared a common ancestor with echinoderms. The second reason is that the primative echinoderms were more likely related to the sea-lilies (Crinoidea) than to the other 5 extant classes (Ophiuroidea-brittlestars, Echinodeasea urchins, etc., including the new class Concentricycloidea). There is a fundamental difference between the crinoids and the rest, in that the former live mouth up and the rest, effectively, mouth down. A consequence of this is that the role of the nervous systems are radically different, with the ectoneural nervous system being much less significant in the crinoids and the aboral being dominant. The problem here, of course, is that almost all we know about echinoderm nervous systems is derived from more advanced non-crinoids. The third reason is the most enigmatic. Echinoderms start off as perfectly respectable bilaterally symmetrical larvae from which a star is born. There are no really definitive studies which show for certain how the larval nervous system is related to that of the adult. Furthermore, there has never been a satisfactory suggestion for the advantages of the pentameric symmetry of the phylum, and there has been no effective escape from this body form for 500 million years. It must, thus, be possible that the radially symmetrical nervous system of the adult is basically a secondarily evolved system.
KeywordsNerve Cord Muscle Tail Giant Fiber Segmental Ganglion Larval Nervous System
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