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What does the dreaded "E" word mean, anyway
Natural History, Feb, 2000 by Stephen Jay Gould
The major group of mesozoans, the Dicyemida, live as microscopic parasites in the renal organs of squid and octopuses. Their adult anatomy could hardly be simpler: a single axial cell (which generates the reproductive cells) in the center, enveloped by a single layer of ciliated outer cells (some ten to forty in number) arranged in a spiral around the axial cell, except at the front end, where two the tissues of the host.
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The zoological status of the dicyemids has always been controversial. Some scientists, including Libbie H. Hyman, who wrote the definitive, multivolume text on invertebrate anatomy for her generation, regarded their simplicity as primitive and their evolutionary status as intermediate in the rising complexity of evolution. As she noted in 1940, "Their characters are in the main primitive and not the result of parasitic degeneration." But even those researchers who viewed the dicyemids as parasitic descendants of more complex free-living ancestors never dared to derive these ultimately simple multicellular creatures from a very complex metazoan. For example, Horace W. Stunkard, the leading student of dicyemids in the generation of my teachers, thought that these mesozoans had descended from the simplest of all Metazoa above the grade of sponges and corals--the platyhel-minth flatworms.
Unfortunately, the anatomy of dicyemids has become so regressed and specialized that no evidence remains to link them firmly with other animal groups, so the controversy of persistently primitive versus degeneratively parasitic could never be settled until now. But newer methods of gene sequencing can solve this dilemma, because even though visible anatomy may fade or transform into something unrecognizable, evolution can hardly erase all traces of complex gene sequences. If genes known only from advanced Metazoa--and known to operate only in the context of organs and functions unique to Metazoa--also exist in dicyemids, then these creatures are probably degenerated metazoans. But if, after extensive search, no sign of distinctive metazoan genomes can be detected in dicyemids, then the Mesozoa may well be intermediate between single and multicelled life after all.
In the October 21, 1999, issue of Nature, M. Kobayashi, H. Furuya, and P. W. H. Holland present an elegant solution to this old problem ("Dicyemids Are Higher Animals"). These researchers located a Hox gene--a member of a distinctive subset known only from metazoans and operating in the differentiation of body structures along the antero-posterior (front to back) axis--in Dicyema orientale. These particular Hox genes occur only in triploblastic, or "higher," metazoans with body cavities and three cell layers, and not in any of the groups (such as the Porifera, or sponges, and the Cnidaria, or corals and their relatives) traditionally placed "below" triploblasts. Thus, the dicyemids are descended from "higher," triploblastic animals and have become maximally simplified in anatomy by adaptation to their parasitic lifestyle. They do not represent primitive vestiges of an early stage in the linear progress of life.
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