The term cnidaria derives from the Ancient Greek word knídē (κνίδη “nettle”), signifying the coiled thread reminiscent of cnidocytes. The word was first coined in 1766 by the Swedish … See more

Sexual
Cnidarian sexual reproduction often involves a complex life cycle with both polyp and medusa stages. For example, in Scyphozoa (jellyfish) … See more

Many cnidarians are limited to shallow waters because they depend on endosymbiotic algae for much of their nutrients. The life … See more

Cnidarians form a phylum of animals that are more complex than sponges, about as complex as ctenophores (comb jellies), and less complex than bilaterians, which include almost all … See more

Basic body forms
Most adult cnidarians appear as either free-swimming medusae or sessile polyps, and many hydrozoans species are known to alternate between the two forms.
Both are radially symmetrical, like a wheel and a tube … See more

Cnidarians were for a long time grouped with Ctenophores in the phylum Coelenterata, but increasing awareness of their differences … See more

Fossil record
The earliest widely accepted animal fossils are rather modern-looking cnidarians, possibly from around See more