The lower metazoans comprise animals that are thought to lie relatively close to the evolutionary root of animals as a whole. While they all have their own specializations, it is possible, and to be hoped for, that they retain some features that were characteristic of the very earliest stages of metazoan and bilaterian evolution. Although it is widely accepted that the sponges are basal within the animals, broadly followed by the ctenophores and cnidarians, the relationships of the bilateri-ans remain highly controversial. Molecular systematics places the bilaterians into three groups: the Deuterostomia (including, among others, echinoderms and chordates); the Ecdyso-zoa (principally arthropods and nematodes), and the Trochozoa (annelids, mollusks, most flatworms, and several other small groups, including the brachiopods). At least one group of the flatworms may still be basal within the bilateri-ans, but the implications this position may have for the evolution of the lower metazoans are presently unclear. The fundamental questions about bilaterian evolution, including the origin of the coelom and segmentation, remain unanswered, and, indeed, as vigorously contested as ever.
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