Phylum Cnidaria

Characteristics
Members of this phylum live in both fresh and salt water, have tissues and a simple nervous system, and have stinging cells called nematocysts. They display radial symmetry and have two embryonic cell layers (the epidermis and the gastrodermis) separated by jellylike mesoglea.

Classification
The three classes are grouped according to body plan. Members of the class Hydrozoa have both the polyp and medusa stage in their life cycle. Examples include the Portuguese man-o-war. Members of the class Anthozoa have only the polyp stage in their life cycle. They are slightly more complex than hydrozoans. Examples include sea anemones. Members of the class Scyphozoa have only the medusa stage in their life cycle. They are the most large and complex members of this phylum. Examples include the common jellyfish.

Protection
Members of this phylum use nematocysts for protection.

Movement
Most adult cnidarians are free floating. In the larval stage, members are free swimming.

Feeding & Digestion
Cnidarians employ stinging cells called cnidocytes to catch food. There is both extracellular and intracellular digestion.

Circulation
Circulation is mainly accomplished through diffusion.

Excretion
Excretion is accomplished through diffusion.

Respiration
Respiration is accomplished through diffusion.

Nervous System
Cnidarians posses simple muscles and nerves. The statocyst is a gravitational sensory organ. The ocellus is a simple photoreceptor organ.