Members of this phylum are found in both fresh and salt water. Their form of defense is with stinging cells called nematocysts. These marine members are usually free floating as adults and swimming as larva. Many bodily functions are accomplished simply through diffusion. The three classes within the phylum are Anthozoa, Hydrozoa, and Scyphozoa. There are many different species within this phylum including: the jellyfish, the hydra, coral and sea anemones.
Their Nervous System:
The Cnidaria nervous system is simple. They have no brains. Instead of a central nervous system they have nerve nets, which are made up of sensory neurons. These neurons react to different stimuli with signals that cause other neurons to send other signals, like motor neurons that contract muscles. The cilia found in their neurons detects physical contact. These systems of web like neurons are necessary for the organisms coordination, movement, and reactions to certain odors that are vital for survival.