Mature nematocysts of the sea anemones Rhodactis rhodostoma and Anthopleura elegantissima contain a fluid that has a high concentration of solutes and is extraordinarily rich in calcium (ca. 500-600 mmol/kg wet weight); this contrasts with the surrounding cytoplasm which is rich in potassium but poor in calcium. The undischarged capsule is surrounded by a membrane that probably acts as a selective permeability barrier between the cytoplasm and the nematocyst fluid. During discharge the nematocyst moves to the surface of the nematocyte and comes into contact with the external sea water medium. Calcium, which may be bound to proteins in the undischarged state, is rapidly lost from the fluid; at the same time, sea water enters the capsule. In vitro experiments have already shown that calcium loss increases the osmotic pressure of the capsular fluid, causing an influx of water from the exteral medium; this influx appears to increase the hydrostatic pressure inside the capsule to the point that the thread everts explosively.
Lubbock, R., Gupta, B. L., & Hall, T. A. (1981). Novel role of calcium in exocytosis: Mechanism of nematocyst discharge as shown by x-ray microanalysis. Proceedings of the National Academy of Sciences of the United States of America, 78(6 I), 3624–3628. https://doi.org/10.1073/pnas.78.6.3624