1. Phylum Cnidaria

2. Phylum Cnidaria Characteristics
1. Radial Symmetry
2. Diploblastic – meaning that they only have two tissue layers, the ectoderm (outer layer, or epidermis) and endoderm (inner layer, or gastrodermis) and no true organs.
The “jelly” is the middle is not a true mesoderm and is called the mesoglea.
3.Have a mouth and a primitive stomach called a gastrovascular cavity. They don’t have an anus, however, so any undigested food has to come back out through the mouth.
4. Have a nervous system, called a nerve net, but no real brain.
5. Have specialized stinging cells called nematocysts used for defense and feeding.
6. Have a hydrostatic skeleton (the water serves as a skeleton).

3. Nematocysts
When you get “stung” by a jellyfish, it’s the nematocysts that are stinging you. Think of it as a tiny, spring-loaded javelin. The pointy part has hollow tips with a paralytic neurotoxin. And you aren’t the only one getting stung! Cnidarians use their nematocysts to paralyze small fish and crustaceans (small animals with shells) to use for food.

4. Movement
Cnidarians fill their hollow center up with water and then shoot it out again, using their muscle-like nerve ring, to provide them with locomotion. Because of this, they can only actively move in the direction their “bell” is facing.

5. Bioluminescence
Many cnidarians “glow”, flash, or emit light when threatened. The light is used to startle the predator.
The light is made by a chemical reaction of luciferase (an enzyme) and luciferin (the light maker).

6. Class Hydrozoa – The Hydra
Freshwater organism that only has a polyp stage, no medusa stage.
Usually found hanging on the underside of floating plants in streams or ponds.
Monoecious and reproduce asexually by budding.

7. The Hydra

8. Class Scyphozoa - Jellyfish

9. Reproduction in Jellyfish
Jellyfish are dioecious, either male or female, and reproduce sexually. Jellyfish mature through a process referred to as alternation of generations. This means that for the first part of their lives, they exist in the polyp stage, then change into the medusa stage later on.
The larva of a cnidarian is called a planula.
This planula develops into a polyp, the sessile, or sedentary stage of the animal.
This polyp then forms buds which detach from the polyp itself and form the free-swimming medusa stage that we are familiar with.

10. Class Cubozoa: Box Jellies

11. Big Stings Come in Small Packages!
The Irukandji Jellyfish (Carukia barnesi) is a box jelly that inhabits the waters of Australia. This is a deadly jellyfish, which is only 2.5 centimeters (with bell and tentacles) in diameter, which makes it difficult to spot. The Irukandji is believed to be the most venomous creature in the world. Movie

12. Phylum Cnidaria, Class Anthozoa - Coral
Coral reefs are the most complex, species-rich,
and productive marine ecosystems in the world.
They provide protection to coastlines from storm damage, erosion, and flooding by reducing wave action.
Serve as protection to many fish and other aquatic species.
Boost the economy in coastal areas due to increased tourism.
Source of building materials.
Within the past thirty years, the number of coral worldwide has massively declined due to human actions such as pollution, harvesting for money, and coastal development. The biggest threat to coral is global warming because the waters are becoming too hot for the coral to survive and they end up “bleached”.

14. Quick Review 1. Which of the following are true about cnidarians?
a. They have stinging cells.
b. They can regenerate lost body parts.
c. They live only in fresh water.
d. They include sponges, corals, and hydras.
2. Cnidarians are either in ______________form or in ______________form. Both body types have _______________ symmetry.
3. The sea anemone is a polyp. True or False?
4. A cnidarian will always have the same body form for its entire life. True or False?
5. All cnidarians have a nerve ___________, which controls movement of the body and tentacles.
6. Jellyfish have a nerve _____________, which coordinates swimming.