Definition of hydroid in US English:

hydroid

noun

Zoology
  • A coelenterate of an order which includes the hydras. They are distinguished by the dominance of the polyp phase.

    Order Hydroida, class Hydrozoa

    • ‘The most numerous inhabitants are hydroids and copepods.’
    • ‘There were dense walls of soft corals, sponges, anemones and hydroids, nudibranchs, creepy-crawlies and lots of fish.’
    • ‘Colonies may be mistaken for hydroids, corals, or even seaweeds.’
    • ‘Aside from its temporary and permanently resident fish, the underside is decorated with sponges, hydroids, tunicates, tube worms, cup corals and anemones.’
    • ‘On the walls are a riot of soft corals, sponges, hydroids and anemones in all shapes and sizes.’

adjective

Zoology
  • 1Relating to coelenterates of the hydroid group.

    • ‘Before long, similar green proteins were detected in many bioluminescent coelenterates including various medusae, apparently all luminescent hydroid polyps, and a few others.’
    • ‘Along the tops of the walls and gullies are many small hydroid clumps which, in turn, attract a range of delicately coloured nudibranchs that feed on them.’
    • ‘The hydroid tissues, in particular, are the general run of moss water storage cells and are probably only remotely related to the xylem of tracheophytes.’
    • ‘He had shown that hydroid jellyfish known as naked-eyed medusae reproduce not only by spewing eggs, but also by asexual budding, which he found marvelous to behold.’
    1. 1.1
      another term for polypoid (sense 1)

Origin

Mid 19th century: from hydra + -oid.

Pronunciation

hydroid

/ˈhīdroid/