Spicule formation patterns and expression patterns of scl-CA1 and scl-CA2 in S. ciliatum. (a-d, f-h): double ISH with differentially labeled probes. (a-e) small S. ciliatum individual, longitudinally cut and viewed from atrial side (scale bar: 100 μm). (b, c, d) Details from boxed areas in a. (b) Predominant scl-CA2 expression in elongated, diactine-forming sclerocytes. Note scl-CA1 expression in presumably tetractine-formation and in some elongated sclerocytes (compare to (e)). (c) scl-CA1 expression in tetractine forming sclerocytes. (d) scl-CA1 expression in sextets of sclerocyte cells, presumably early triactine formation of initial radial tubes. (e) Calcein staining showing spicule growth of 18 h. (f-h) Expression of scl-CA1 and scl-CA2 in sclerocytes of spicule formation in the radial tubes (scale bars: 100 μm). (g) scl-CA2 expression (purple) in founder cells. (h) Incidentally observed expression of scl-CA1 and scl-CA2 in two sclerocytes (presumably beginning diactine formation).