![]() Myoepithelia are widely regarded as the evolutionary origin of bilaterian muscle cells ( Mackie, 1970 Rieger, 1986 Rieger and Ladurner, 2003). Cnidarians, in contrast, possess myoepithelia but also striated muscle cells, at least for some members of the Hydrozoa and Scyphozoa ( Seipel and Schmid, 2005 Schmidt-Rhaesa, 2007). Contractile epithelia sensu lato may be regarded as part of the ground pattern of the Metazoa.Īmong the Metazoa, only the basal Porifera and Placozoa lack muscle cells ( Jones, 1962 Nickel, 2010). We conclude that epithelial contraction plays a major role in sponges. Should mesohyl contraction be present, it is likely to be part of the antagonist system. Our results support the pinacoderm contraction hypothesis. We also tested both hypotheses by carrying out volumetric studies of 3D synchrotron radiation-based x-ray microtomography data obtained from contracted and expanded specimens of Tethya wilhelma. In the present study we addressed this by studying sponge contraction kinetics. The question of agonist–antagonist interaction in sponge contraction seems to have been completely neglected so far. No direct support exists for either hypothesis. Two competing hypotheses on the mechanisms behind this have been postulated to date: (1) mesohyl-mediated contraction originating from fusiform smooth muscle-like actinocytes (‘myocytes’) and (2) epidermal contraction originating in pinacocytes. Sponges constitute one of the two metazoan phyla that are able to contract their bodies despite a complete lack of muscle cells.
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