Evolution and systematics

The name Acoela comes from two Greek words that mean "without a body cavity"; it refers to a distinguishing feature of this order (or phylum) of tiny wormlike multicellular marine invertebrates. Species in this group have no true body cavity or coelom. A true coelom is a fluid-filled body cavity formed from mesodermal tissue. It lies between the outer body wall of epidermal tissue and the gut or digestive tract.

The Acoela have long been included as an order within the phylum Platyhelminthes, the flatworms, and the class Turbel-laria, the marine flatworms. On the other hand, very recent morphological, developmental, and molecular studies, including comparisons of 18S ribosomal DNA from 61 species representing 25 animal phyla, support the hypothesis that the Acoela are a separate phylum representing the most primitive living animals showing bilateral symmetry, or regularity of body form on the right and left sides (Ruiz-Trillo et al.). Bilateral symmetry is a basic characteristic of all triploblastic animals, those with three tissue layers in their embryonic stages: the outer ectoderm, middle mesoderm or mesenchyme, and inner endoderm. In contrast to this pattern, diploblastic animals have only two layers of tissue as embryos, lacking the mesodermal layer.

The Acoela may be direct descendants of the earliest line of animals to diverge from diploblastic organisms with the beginnings of triploblastic features: a middle tissue layer and bilateral symmetry. The DNA studies suggest that the acoels or their direct ancestors diverged from the diploblasts much earlier than did the main line of triploblastic animals (including all living triploblasts other than Acoela), in pre-Cambrian times, well before the so-called Cambrian Explosion era of

540-500 million years ago, when most if not all modern animal phyla appeared.

The work of Ruiz-Trillo and his colleagues has been challenged by Tyler and his working group, who maintain that the order Acoela should be considered the earliest or most primitive group within phylum Platyhelminthes. Lundin considers the Acoela to have likely evolved from ancestors within or related to the Nemertodermatida.

Nevertheless, other features of the Acoela that support their being a phylum unto themselves include their simple nervous system, as compared with platyhelminth species, and their manner of embryonic development. According to a study by Raikova et al., which compared the brains of acoels with those of other platyhelminths, the acoel brain and nervous system are simpler and much different in structure from those of other platyhelminths, suggesting major differences between the two groups and supporting the classification of Acoela as a separate phylum.

As of 2003, Acoela is included as an order together with Nemertodermatida in the species complex Acoelomorpha within the phylum Platyhelminthes on the basis of similarities between Acoela and Nemertodermatida. According to the European Register of Marine Species, there are 20 families within the order Acoela.

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