Phylum Cnidarian

Includes: Jellyfish, Corals, Anemones, Hydra

Many cnidarian species have two different body forms during their lives. One is the free-swimming form, called the medusa (a jellyfish for instance). The other is an attached form called the polyp (which resembles a tiny sea anemone).

Both forms have a single opening that serves as the mouth and anus. A ring of feeding tentacles typically surrounds the opening. The tentacles are packed with stinging cells, which contain nematocysts that shoot out a barbed shaft to catch prey. The nematocyst can even inject toxins.

Cnidarians have a top and a bottom. Their bodies are made up of two layers of tissue that include nerve and muscle cells. This body, with both muscles and nerves, allowed ancestral cnidarians to be the first animals on the planet to move.

Cnidarian Features

  • Two tissue layers with nerve and muscle cells
  • Nematocysts, which are structures contained in special stinging cells that can catch food or defend from predators
  • Two main life forms: free-swimming medusa (e.g., jellyfish) or stationary polyp (e.g., anemone)

Cnidarian fact:

The longest animal in the world is a cnidarian! Praya, a pelagic jelly, can reach a length of 120 feet.

Word Bank

Nematocyst: the specialized capsule that contains a coiled, hollow, usually barbed thread. The thread explodes out from the capsule when stimulated by predator or prey. Nematocysts can inject toxins into their prey.
Pelagic: living in the waters of the open ocean