Open circulations arose prior to Cambrian times perhaps in trilo-bites when arthropod bodies formed. Some trilobites were slow pelagic swimmers, others faster predators, and some crawled, consuming organic matter from sediments when trapped in the mud of early oceans to deposit their exoskeletons later to be found in Burgess Shale. Among the trilobites, worm like strings of body segments having heads and tails possibly feathery gills occurred (Ref: Trilobites).

Today open circulations, some quite modified, occur in at least three major groups: Arthropods, Echinoderms and Mollusks. Molluskan body cavities are coeloms. Coeloms resemble hemo-coels, but they arise differently in development and are highly modified, even though in some mollusks the cavities comprise large portions of their circulation. According to one popular theory, the coelomate theory, mollusks evolved from a coelomate ancestor along with the annelids, the segmented worms, as both show embryonic spiral cleavage, a specific type of cell division during development producing a similar larval form called a tro-chophore. Such academic questions will probably be clarified after more genetic work. Molluskan cavities will not be considered here.

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