Biography Wikipedia
Wikipedia:
Hydroid may refer to:
Marine biology
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.Hydroids are a life stage for most animals of class Hydrozoa, small predators related to jellyfish.
Botany
In mosses, hydroids form the innermost layer of the stem of long, colourless, thin walled cells of small diameter.The cells are dead and lack protoplasm.They function as water and nutrients conducting tissues analogous to tracheids. However, they lack any pitting or surface texture.
Hydroids have been found in some Rhynie chert plants, including Aglaophyton, where they were initially mistaken for xylem vessels.