The polyp may reproduce asexually by budding other polyps or may continue the sexual cycle by budding immature medusae called ephyra. Ephyra develop into mature medusae. These tentacles are armed with stinging cells called cnidocytes containing small, barbed harpoon-like structures called nematocysts. Captured prey are pushed through the mouth into the gastrovascular cavity, where extracellular digestion occurs followed by phagocytosis of small food particles and some intracellular digestion. The polyp stage dominates the hydrozoan life cycle, although both polyps and medusae occur in most species.
Amoeboid cells circulate in the mesoglea.
Ectodermal cells include cnidocytes and muscular contractile cells. Endodermal and glandular cells secrete enzymes into the gastrovascular cavity for extracellular digestion. The hydra are small, common hydrozoans that live in shallow, freshwater ponds. They are usually less than 1 cm tall and prey on smaller invertebrates among the filaments and leaves of freshwater algae and plants. Hydra have no medusa stage.