Protozoa epithelial cells

Single cells may deposit CaCOs extracellularly. This occurs in the formation of the test in protozoa (Pautard, 1970) and the spicules of calcareous sponges (Jones, 1970). Single cells also have the capacity to form portions or entire skeletons of echinoderm larvae in vitro (Okazaki, 1975) and cells which do not form distinct epithelia regenerate spines of echinoderms extracellularly (Heatfield and Travis, 1972). These results suggest the possibility that cells which are not in an epithelial layer may also deposit the meshwork of the echinoderm test and spines extracellularly. 

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