Phylum Cnidaria

• Name means nettle in Latin
• Examples: hydra, sea jellies, corals, sea anemones
• Alternates between two different body plans: Polyp and Medusa. Called Polymorphism.
• Polyp (hydra): tube with tentacles around mouth, sessile (ex. sea anemone)
• Medusa: umbrella shape, tentacles around mouth, motile, free-swimming (ex. sea jellies)
• Nematocysts (stinging cells): used for defense and to capture prey. Each one contains neurotoxins, which acts to paralyze prey. (How we classify cnidarians)
• All members of this phylum exhibit radial symmetry
• Most of the cnidarians are marine (ocean) with a few species that live in fresh water.
• These organisms are carnivores, and act as predators in their environment.
• The process of feeding begins with tentacls stinging their prey with nematocysts. Tentacles grab the prey, and the prey is pulled into mouth.
• The prey is pushed into the gastro-vascular cavity. In this structure, the cnidarian extrudes enzymes, allowing for extracellular digestion.
• Any material that is undigested will be expelled back out of the mouth. Their mouth is also their anus.
• All respiration occurs via diffusion. This is possible because the body is only two cell layers thick.
• All material passes through the body of the Cnidarians via diffusion.
• All wastes are excreted by diffusion as well.
• Cnidarians gather information from their environment using specialized sensory cells.
• Cnidarians do not have cephalization or a nervous system.
• Cnidarians are organized around a nerve net that is centered around the mouth.
• Sensory cells: chemoreceptors (chemical), thigmoreceptors (touch), photoreceptors (light) called ocelli, statocysts (balance) - tells which direction is down.
• The medusa stage is motile or free-swimming.
• The Polyp stage is sessile. Exceptions include the hydra that tumbles on its tentacles, and the sea anemone that grows on a pedal disk that can detach from its substrate. Most animals move their bodies using a hydrostatic skeleton.
• Asexual reproduction: budding from polyps or medusae
• Sexual: a. medusae release sperm and eggs b. some monoecious (male and female), some dioecious (produce sperm OR egg) c. Larvae free-swimming