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The Hox gene complement of acoel flatworms, a basal bilaterian clade.
Acoel development supports a simple planula-like urbilaterian.
Phylogenetic distribution of microRNAs supports the basal position of acoel flat worms and the polyphyly of Piatyhelminthes.
This is in contrast to the reports of Bresslau (2) and Costello (4) that the cleavage producing the second duet is dexiotropic, but in agreement with Apelt who examined 13 acoel species (5).
Our results show that acoel development differs from that of quartet-cleaving species, not only in the formation of duets rather than quartets of micromeres, but also in the absence of alternating cleavage, and in the plane of first cleavage, which divides the embryo into left-dorsal and right-ventral halves.
In the phylogenetic trees obtained by three different methods, the dicyemids formed a monophyletic unit with the myxozoans, nematodes, and acoel flatworms.
Phylogenetic position of acoel turbellarians inferred from partial 18S rDNA sequences.
However, orthonectid species have been previously reported from Swedish waters, the habitat of Xenoturbella bocki, such as Rhopalura philinae from the gastropod Philine scabra (Lang, 1954) and Intoshia paraphanostomae from acoels
(Westblad, 1942; Dorjes, 1979; Kozloff, 1992).
(1990) and Hooge and Rocha (2006) studied, respectively, freshwater rhabdocoels and acoels from the state of Sao Paulo.
Acoels, a group of marine flatworms, are represented by specimens sampled only in the state of Sao Paulo, mainly in sandy beaches, on algae or rock coasts (Tables 1 and 2).