Cnidarians and Ctenophores
{Cnidae (Greek) = nettle; Ctene = comb}
These two groups are often combined together as the Coelenterata. Cnidarians include sea anemones, corals hydroids, true jellies, and more, while ctenophores are more commonly known as comb jellies.
Adult cnidarians take either a polyp or a medusa body form, or have a lifecycle that alternates between the two. Species can also be solitary or colonial. Sea anemones and corals, which belong to Class Anthozoa, exist only as polyps at all stages of their lifecycle and are generally sessile. The true jellies, belonging to Class Scyphozoa, spend most of their lives as free-swimming medusas, with the polyp stage making up only a small portion of their life cycle. Hydrozoans (Class Hydrozoa) are a morphologically diverse group, and can be polyps or medusas and solitary or colonial as adults.
For ease of identification, the divisions used here are based more on body form rather than strict phylogenetic classifications; there is much overlap between these organizational systems, however.
Adult cnidarians take either a polyp or a medusa body form, or have a lifecycle that alternates between the two. Species can also be solitary or colonial. Sea anemones and corals, which belong to Class Anthozoa, exist only as polyps at all stages of their lifecycle and are generally sessile. The true jellies, belonging to Class Scyphozoa, spend most of their lives as free-swimming medusas, with the polyp stage making up only a small portion of their life cycle. Hydrozoans (Class Hydrozoa) are a morphologically diverse group, and can be polyps or medusas and solitary or colonial as adults.
For ease of identification, the divisions used here are based more on body form rather than strict phylogenetic classifications; there is much overlap between these organizational systems, however.
Anemones, Corals, and Sea Pens
Grouped here are the anemones and the anemone-like corals: the sessile polyps that have an upward-facing mouth ringed with tentacles. The corals can be soft-bodied like anemones, or may secrete a hard skeleton. Some species in this grouping house symbiotic algae in their tissues, which can give them a bright green colour.
Hydroids
Hydroids are the polyp forms of Class Hydrozoa. They usually form colonies, which can often appear deceivingly like plants, seaweeds, or even true corals. Most hydrozoans alternate between polyp and medusa forms, though the medusae are usually small and can be quite inconspicuous.
Jellyfish
Some species of jellyfish (more accurately known as 'jellies') can grow up to 2 m in diameter, yet as a group they are technically zooplankton because they are unable to swim against currents, and can only move with the prevailing currents and tides — which is why many often end up beached.
Here we group the medusa and other free-floating forms of the true jellies (Scyphomedusae) and the hydrozoans (Hydromedusae), as well as the comb jellies (Ctenophores).
Here we group the medusa and other free-floating forms of the true jellies (Scyphomedusae) and the hydrozoans (Hydromedusae), as well as the comb jellies (Ctenophores).
