Relatives of jellyfish and corals, hydroids are very small, predatory animals that often look like jellyfish or branching plants. They gather food with tiny tentacles armed with stinging cells. There are about 3,800 known species, living in marine or freshwater habitats. One particular species, the hydra, holds within its genomic code the key to biological immortality. The hydra does not age, and can even regrow its head if decapitated.