Phylum Cnidarian - Advanced

Includes: Jellyfish, Corals, Anemones, Hydra

Many cnidarian species exist in two different body forms during their life cycle: the free-swimming form, called the medusa (a jellyfish for instance), and the stationary form called the polyp (which resembles a tiny sea anemone).

Both forms have a single opening that serves as the mouth and anus and is typically surrounded by a ring of feeding tentacles packed with stinging cells called cnidocytes (the “c” is silent). These stinging cells shoot out a barbed shaft called a nematocyst to catch prey. The nematocyst can even inject toxins.

Cnidarians have a defined top and bottom, and are made up of two layers of tissue that include nerve and muscle cells. With these nerves and muscles, ancestral cnidarians were the first animals to move.

Cnidarian Features

  • Two tissue layers with nerve and muscle tissue
  • Nematocysts which are structures contained in special cells called cnidocytes that are ejected for both defense and offense
  • 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

Cnidocyte: a specialized cell containing the nematocyst that defines the phylum.
Nematocyst: a coiled, hollow, usually barbed shaft, which quickly turns outward (is everted) from its capsule when stimulated by predator or prey. Nematocysts inject a complex stew of chemical agents into their prey.
Pelagic: living in the waters of the open ocean.