Schematic representation of the hypothetical evolution of the cnidarian nematocyst. The secretome of early eukaryotes exhibited secretory vesicles that contained enzymes used for extracellular digestion. It evolved structural proteins with biomechanical properties that finally gave rise to a variety of protozoan extrusive organelles, which exhibit sophisticated discharge mechanisms. Some of these organelles are structurally similar to nematocysts, suggesting that nematocysts might have evolved from the extrusomes of a non-cnidarian precursor. Note that core components of the phototransduction cascade evolved in protozoans , as indicated by red vesicles and lettering. A nematocyte-sensory receptor complex evolved only in multicellular cnidarians/metazoans (Adapted from Figure 6 of ).