They may look like day-glo flora, but cnidarians are actually carnivorous fauna. Cnidarians are either polyps, such as sea anemones or coral, or free-floating medusae, such as jellyfish.
Polyps have a tube-like body with an opening on top that is surrounded by outward- and upward-facing tentacles.
Medusae are usually bell-shaped animals with a concave oral surface, or mouth, and tentacles that dangle downward from the rim of an umbrella-like body.
Four classes make up the phylum Cnidaria:
Cnidarians live in aquatic environments and inhabit all depths, from the sandy substrate up to the surface. They can be found from the Great Barrier Reef rising off the coast of northeastern Australia to the continental shelves of bone-chilling arctic oceans, and just about every saltwater marine habitat in between.
Some jellyfish are even found in freshwater lakes, such as the freshwater jellyfish (Craspedacusta sowerbyi), which haunts several lakes in New Zealand. Not to worry though, these jellyfish are harmless to humans and feed mainly on zooplankton.
The body of polyps and medusae are diploblastic, meaning they have two layers—the ectoderm and the endoderm. The ectoderm is the outermost germ layer and the endoderm is the innermost germ layer. Between these two cell layers is a jiggling mass of clear material called mesoglea. The mesoglea serves as the glue that binds together the two cell layers in polyps and makes up the greater part of the animal in the case of the jellyfish. The common name, jellyfish, refers to the sheer bulk of gelatinous mesoglea that gives them their buoyancy.
Cnidarian bodies have only one opening that serves as both mouth and anus. Food enters through the mouth and excess food and waste leave through the mouth as well.
The phylum Cnidaria was formerly known as Coelenterata, but that name has been abandoned because it included the comb jelly, a tentacle-less marine predator that is not actually a cnidarian. Although the term Coelenterata has fallen out of use, its Greek meaning is significant—hollow gut. This expression alludes to the single sac-like body all members possess. Instead of organs, cnidarians have a gastrovascular body cavity that serves double duty by managing gas exchange and digestion.
Cnidarians have no need for a brain or a heart; instead, these animals have specialized tissues to coordinate basic bodily functions. A decentralized net of nerves and receptors allows the animals to negotiate their environment, whether they are moving about, detecting danger, or making a meal of any prey that swims or strolls within reach.