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JELLY ANIMALS: CNIDARIANS AND CTENOPHORES GENERAL OVERVIEW Cnidarians and Ctenophores are invertebrates, animals without backbones. Like all animals, they are composed of many cells, must eat to gain energy and can move from place to place during some part of their lives. Both these types of animals have radial symmetry: their body parts radiate out from the center and each slice is a duplicate of the other. This allows them to sense prey or enemies from any direction but with no front,also makes it difficult to coordinate their bodies to move towards a special spot. Animals that are radially symmetrical are generally slow-moving and have very slowmoving or anchored prey. Both Cnidarians and Ctenophores share other characteristics: They are made up of only 2 tissue layers: an outer protective covering containing nerve cells in a net-like arrangement and an inner layer containing digestive cells and reproductive organs, their only organ system. Cnidarians and Ctenophores may be up to 95% water! The space between the two tissue layers is filled with a jelly-like substance for support and movement. In the middle of both kinds of animals is a sac-like gut with only one opening: food goes in and waste goes out the same opening! They do not have Cephalization like most animals, which means they lack a definite head end, with feeding and sensory structures. I. Cnidarians (Phylum Cnidaria) This phylum includes stinging hydroids like the Portuguese-Man-of-War, jellyfishes, and corals and sea anemones. Cnidarians may spend their lives as one of two body forms: A. The medusa resembles an umbrella and floats like a tentacle-fringed bell in the water, surrounding the central mouth. Medusa jellyfish are planktonic (which means they cannot swim against a current) and move by pulsating the jelly in their bell. B. The polyp is tubelike and is usually attached to the ocean bottom; it may be solitary or part of a colony. Polyp forms move by pushing the water in their guts to slowly ooze across a surface. FEEDING: Stinging cells are filled with toxins that can stun or kill prey. The harpoon-like nematocysts lie coiled inside out within the stinging cells and shoot outward on contact or chemical stimulation. Once captured, tentacles bring prey into the hollow gut for digestion. Waste is excreted out same opening! IMPORTANCE: Cnidarians major predators on zooplankton and larger inverts and fish; often eaten as prey themselves, especially by sea turtles. Many Cnidarian are toxic to humans: Thousands of humans are stung each year by PORTUGUESE MAN OF WAR, SEA WASPS (the most deadly), and FIRE CORAL. Cnidarians are used in many biological & medical studies and colorful HYDROCORALS are often collected for saltwater aquariums. Coral reefs are largest structures built by living organisms on the planet and support an incredible species diversity. Coral colonies can provide borrowing spaces and protection for a wide range of animals and plants. They protect island margins from high waves and surge and are prime eco-tourism spots and the wealth of sea life in and around reefs supports commercial fishing industries II. Comb Jellies (Phylum Ctenophora) This phylum includes planktonic jelly animals that are generally small and almost transparent. Many spend their lives in the darkness of the deep ocean and can biolumenesce to make their own living light for prey capture or avoidance. Ctenophores move by beating 8 rows of tiny comb-like cilia along their bodies, which shine like rainbows in the darkness of the ocean. FEEDING: Instead of having many tentacles and stinging cells for catch their food, Ctenophores have 2 long tentacles, often coated with a paralyzing toxin. When its prey comes to close, it may get tangled in the tentacles and be slowly reeled into the Ctenophores mouth. Some Ctenophores have teeth-like spikes lining the opening of the mouth and open their mouths wider than their bodies to gobble down prey bigger than themselves. IMPORTANCE: Ctenophores are major planktonic predators and are often eaten as prey themselves. Ctenophores are not as dangerous to humans as Cnidarians: the toxin coating the long tentacles needs a break in a person's skin to seep in.