Protanthea simplex. Photo by Robin Harvey, S.M.B.A.
Oban (Scotland) taken in Lock Melfort, Scotland.
Protanthea simplex. Photo by Ron Ates
Protantheae with flattened, disc-like proximal end. Column of the same structure as the tentacles with spirocysts and a more or less strongly developed longitudinal muscle layer, not capable of involution. No distinct sphincter. Tentacles not retractile. Longitudinal muscles of tentacles and radial muscles of oral disc ectodermal. Actinopharynx with longitudinal muscles and often with spirocysts, with weak siphonoglyphs. Mesenteries typically arranged in cycles, each pair, except the directives, with the longitudinal muscles facing each other. 8 mesenteries, the two pairs of directives and on each side 2 single mesenteries whose retractors face the ventral directives, perfect. Owing to asexual reproduction some more mesenteries may be perfect. Gonads usually on all perfect mesenteries. Ciliated tracts of the filaments absent. Cnidom: spirocysts, atrichs, basitrichs, microbasic p-mastigophors, microbasic amastigophors.
Definitions and contained taxa are those of Oscar Carlgren as published in 1949 : Kungl Svenska Vetenskapsakadamiens Handlingar, Series 4, Volume 1, Number 1
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