Hydroids are a life stage for most animals of the class Hydrozoa, small predators related to jellyfish.Some hydroids such as the freshwater Hydra are solitary, with the polyp attached directly to the substrate.The original polyp is anchored to a solid substrate and forms a bud which remains attached to its parent.Some species have the polyps budding directly off the stolon which roots the colony.The polyps are connected by epidermis which surrounds a gastrovascular cavity.
Hyastenus bispinosus
(
Arrow crab
) on
Aglaophenia cupressina
(Stinging hydroid). Small
decorator crabs
, like this one, are often found wearing snipped-off tips of stinging hydroids on their heads to fend off predators.
