CNIDARIANS

WHAT ARE CNIDARIANS?

• Cnidarians are a phylum of invertebrates that include sea anemones, jellyfish, corals and hydras
• Most live in saltwater
• A few such as hydras, live in freshwater

BODY FORMS OF CNIDARIANS

• Cnidarians have two main body forms
• Polyp
• Sessile
• Commonly attached to the sea floor or another object
• Medusa
• Free-floating

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• Most cnidarians spend part of their life cycle in each form

CNIDARIAN CHARACTERISTICS

• Have radial symmetry
• Hollow body cavity
• A single opening is used to take in food and release waste
• Have stinging cells called nematocysts

CNIDARIAN CHARACTERISTICS

• More complex than poriferans
• They contain specialized tissues
• Have a simple nervous system that consists of a nerve net to help them carry out coordinated movements

By OpenAperture - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=32328508

EXAMPLES OF CNIDARIANS

• Corals
• Can be traced back in the fossil record for over 600 million years
• Live in warm, tropical waters
• Consist of polyps that live together called colonies
• Coral colonies build up large structures called coral reefs

EXAMPLES OF CNIDARIANS

• Sea anemones
• Sometimes referred to as sea flowers
• Look like plants anchored to the sea floor
• The ends of the tentacles are armed with stinging nematocysts to kill their prey
• Some species of sea anemones reach up to 2 meters in diameter

By Peter Southwood - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=26515052

EXAMPLES OF CNIDARIANS

• Hydras
• Smallest cnidarians
• Live in rivers, streams, lakes and ponds
• Not completely sessile
• Eat small invertebrates
• Hydras do not appear to die of old age.

EXAMPLES OF CNIDARIANS

• Jellyfishes
• Jellyfishes are found in most of the world’s oceans
• Soft-bodied
• Some can be seen through
• Most species have tentacles containing nematocysts
• These tentacles trap fish and other marine organisms for food