Cnidaria

__** Cnidarians **__ http://wintersscience.wikispaces.com/cnidaria+video

1)Radially Symmetrical. 2)Body multicellular, few tissues, some organelles. 3)Body contains an internal cavity and a mouth. 4)Two different forms exist, medusa and polyp 5)Reproduction is asexual or sexual. 6)Has a simple net like nervous system. With nematocysts 7)Has a distinct larval stage which is plankton like 8)Lives in aquatic environments, mostly marine. 9)Mostly carnivorous otherwise filter feeders. 10)May have a minimal skeleton of chiton or calcium carbonate.
 * Characteristics of Cnidaria:-**

__**Cnidarians**__ include some of the most beautiful animals. They include the jellyfish, hydras, sea anemones, and corals. Most cnidarians are marine, but a few, such as the hydra live in fresh water. These simple, **__radially__** symmetrical animals are flower-like and in some ways resemble plants. Cnidaria is Latin for "stinging cells."

Cnidarians are just a little more complex than sponges. They have **__two cell__** layers that are organized into cell layers called tissues. Cnidarians have digestive, muscle, nerve, and sensory tissue. The cnidarian body is a hollow sack with only one body opening (a mouth). It is surrounded by **__tentacles__**. The tentacles may contain **__nematocysts__** (stinging cells). These cells contain poison arrows that can paralyze their prey. The tentacle then brings the prey to the mouth and into the simple digestive cavity. After the prey is digested inside the hollow sack, the food passes into the body cells, and waste must go back out of the mouth (called **__two-way__** digestion).

The cnidarian has two body forms or shapes. Some have an umbrella shape where the mouth and tentacles hang down. This is the medusa body plan. The **__medusa__** jellyfish has a simple nerve net that is found around the bell of the jellyfish. The **__nerve net__** allows the jellyfish to open and close so that the organism can move around. Coral, hydra, and sea anemone have the **__polyp__** form. A polyp does not move. Its tube-shaped body is attached to something with the mouth and tentacles pointed upwards. Some polyps live in **__colonies__**, like the coral. Coral live in tropical waters and over time build up reefs that are teeming with life. The dead coral provide a base onto which new coral attach and provide both food and habitats for other animals and plants. media type="youtube" key="AjM-WOeM3uA" height="385" width="640"