appropriate ancestor for both protostomes and deuterostomes. Today the nearest approximations to a worm of this grade are found among the mollusks—aplacophorans and chitons—although these living groups have derived features that would have been absent in the Vendian. The more complex horizontal trails or shallow burrows appearing later imply a more efficient hydrostatic system and thus probably an ample hemocoel. Again, worms of this grade would probably be regarded as simplified mollusks, were they to be found alive and shoehorned into a living phylum.
If the sequence of introductions of design elements was truly parsimonious, the ancestral arthropods branched from this sort of proto-mollusk. A plausible scenario would begin with a seriated form, probably preferring hard grounds and supplementing a fundamentally peristaltic creeping locomotion by lateral body projections that served as accessory gripping mechanisms. Sclerotization may have begun as protection in such a habitat, since burrowing would not be possible, but flexibility of the body wall and peristaltic efficiency would have been sacrificed as it became heavier. Locomotory functions shifted to the lateral appendages, and a segmented anatomy evolved as series of muscles, nerves, and blood vessels developed to serve them. Flexibility of the trunk was maintained by jointing of the integument; this permitted vertical flexure when creeping over uneven substrates and perhaps lateral flexure to enhance locomotory power as needed through the use of the body wall musculature (Valentine, 1989). The jointing enhanced the segmented architecture. Body spaces continued to be developed on the site of the blastocoel: a hemocoel to provide hydrostatic functions and an enlarged organ hemocoel to bathe the heart. Jointing of the appendages occurred as they lengthened within a rigidifying exoskeleton. By the Tommotian, arthropod-type appendages were sufficiently well developed to permit their use in furrowing the substrate, presumably in search of food, which created a characteristic type of trace fossil (Crimes, 1989), probably reflecting an evolutionary radiation of jointed body types onto soft substrates.
The number of independent lineages that reached the arthropod condition has been in dispute. An analysis of the branching order of 12S rRNA within the arthropods (Ballard et al., 1992) suggests that ancestors of the myriapods branched most deeply and were followed by branchings that produced the ancestors of onychophorans (a lobopodal group that does not have jointed appendages) and then of the chelicerates, crustaceans, and hexapods in that order. However, an 18S rRNA tree (Wheeler et al., 1993) suggests that the onychophorans are sisters to a monophyletic arthropod clade, a view supported by a consensus tree