rizes some of the more robust findings by listing some phyla from which rRNAs were studied to the right and the inferred branching patterns of the rRNAs on the left, with the implied appearance of some design elements in metazoan body plans indicated. A few of the design elements require comment. For present purposes, the unqualified terms "hemocoel" and "coelom" refer to fluid-filled spaces usually used as hydrostatic skeletons—the former being developed from the topological position of the blastocoel, the latter being developed within mesoderm. Spaces devoted only to serving organs (commonly as ducts or as buffering against solid tissues) are termed "organ hemocoels" or ''organ coeloms." "Seriation" refers to longitudinal repetition of organs, muscles, or other features but not necessarily in a correlated fashion, while "segmentation" refers to serial repetition with correlation among organ systems.

Many of the metazoan relationships indicated in Figure 4 are quite conventional, such as the sequence of choanoflagellates/poriferans/cnidarians/platyhelminths, the sister-group relationship of platyhelminths with higher metazoans, and the sister-group relationship of protostomes with deuterostomes. Less conventional, though hardly shocking, is the sister-group relationship of the arthropods with the unresolved cluster of protostomes, which includes mollusks and annelids. This is a particularly important branching, because it suggests that the last common ancestor of the protostomes was hemocoelic and that the coelom(s) of the deuterostomes arose independently of the coelom(s) of the protostomes. This branching also suggests that arthropod and annelid segmentation arose independently. The next deeper branching, between protostomes and deuterostomes, suggests that while their last common ancestor was not coelomic and need not have been hemocoelic, it did have a blood-vascular system. Other interpretations are possible but they are less parsimonious.

Late Precambrian Body Plans. The list of phyla on the right of Figure 4 indicates the body plans of the living organisms studied but certainly does not indicate the body plans of the common ancestors deeper within the branching pattern. On present evidence all of the branching between the choanoflagellate/poriferan and the arthropod/unresolved protostome branches occurred during the Precambian. The bilaterian traces of the Vendian must have been made by descendants of the platyhelminth/higher metazoan branch. The simple earliest trails require active worms with some hydrostatic system, possibly a tissue skeleton, or fluid in part; the relief indicated by these traces implies a non-flat body and that, in turn, a blood-vascular system. Such a worm, a vascularized "roundish flatworm," acoelomate (except possibly for organ spaces), probably



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