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Dear Richard, Friday, July 19, 2002 you wrote: RJ> Dear Tunicatists, RJ> With regard to the instances of ascidians with paired atrial RJ> siphons mentioned by Arjan Gittenberger and Karen Sanamyan, we RJ> would like to mention a fossil comparison. RJ> Namely, the 300 million-year-old mitrateJaekelocarpus oklahomensis , as recently re-described by us inNature, also had paired atrial siphons or, at least, paired RJ> dorso-lateral gaps whose presumed function was to let water out. SinceJaekelocarpus was probably a member of the tunicate stem group, RJ> this paired condition is likely to be more primitive than the RJ> condition with one atrial opening only. The paired atrial siphons in most ascidians is an abnormality. Something like two-heads snakes. It is hardly possible to take this abnormality as an argument for any kind of comparison. As to Dimeatus - this is highly adapted deep-water genus, and without any doubt large paired atrial orifices (as well as giant branchial orifice and many other features of this genus), is (recent) adaptations for using slow water currents for filter-feeding in abyssal depths. For more information on Dimeatus see my paper: Sanamyan, K., 2001, Dimeatidae, a new family of the Ascidiacea with description of a new species of the genus Dimeatus. Zoosystematica Rossica, 9(1), 2000: 37-41. -- Best wishes, Karen Sanamyan http://lithopssoft.com/zoo - ascidian taxonomic database