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Transcript
LESSON 17 E X P L O RI NG THE HYDRA The Hydra: UpOose and Personal The Hydra, one of the simplest multicellular organisms, is a member of the phylum Cnidaria. The Hydra is a freshwater animal. Its body is a thin, hollow cylinder with five to seven tentacles extending from its mouth. Hydra come in many different colors, including tan, gray, green, and brown. Adult Hydra are typically 6 to 13 millimeters long and are capable of ~tretching out or contracting. • Adult Hydra The Hydra's most common method of reproduction is by budding, a form of asexual reproduction. A small growth, or bud, forms on the adult Hydra's body through a series of cell divisions. The bud soon develops tentacles and breaks away. The new Hydra that breaks off can then live independently. Hydra also can reproduce by regeneration, another form of asexu al reproduction. They can grow new bodies from small . pieces that h ave been detached . When fall arrives and cooler conditions prevail, Hydra usually develop sex organs and reproduce sexually. This is because th e eggs that Hydra budding (continued) STC/MS™ OR G ANI SMS- FR O M MA C R O T O M I C RO 199 LESSON 17 EXPLORI N G THE HYDRA are produced are able to survive the colder conditions, while the parents cannot. The Hydra has no circulatory system with a heart and blood vessels. B.ecause its body is only two cell-layers thick, the Hydra's cells can get the nutrients it needs through a process called "diffusion." This occurs as the Hydra moves through the water and the cells interact with the environment, exchanging oxygen and food for carbon dioxide and other Epidermal cells wastes through the cell membranes. The Hydra's outer layer of cells protects the organism. Its inner layer of cells secretes enzymes that digest the Hydra's food. The Hydra does not have a central nervous system. Instead, it has a "nerve net" through which impulses that control the Hydra's muscle contractions are carried. These contractions allow the Hydra to expand, contract, and move about. Sting cell Nematocyst I, Sting cell with t hread of nematocyst discharged I ....... . ....) Notice the coiled thread inside the sting cell on the left. It has been discharged from the sting cell on the right. 200 STCJMS™ ORGAN I SMS- FR O M M AC R O TO MI C RO LESSON 17 EXPLORI NG T H E HYDRA Paralyzing Poisons and Tight Embraces le e lith The Hydra feeds on small organisms, such as Daphnia. Specialized cells that make up part of the outer layer of a Hydra contain stinging structures called "nematocysts." When an organism such as Daphnia brushes against the Hydra's tentacles, This Hydra has captured and paralyzed a minnow. Is it biting off more than it can chew? the nematocysts shoot out threads. These threads pierce the prey and release a paralyzing poison. digestive cavity, wave about to stir the digestive Other nematocysts release threads that surfluid. Undigested pieces are released through round the prey and hold it tight. The tentacles the Hydra's mouth. Digested nutrients move then,_ move the prey toward the Hydra's mouth into the cells of the Hy dra by diffusion. and force it into the its digestive cavity. The Hydra spends much <?f its time attached The Hydra secretes digestive enzym es into to a surface by its base. However, when it's time to move , the Hydra moves in style . It simply this cavity from cells in the inner layer. These floats or travels from one location to another by enzymes break down the prey into usable doing somersaults! D nutrients. Whip-like flagella, which line the The Hydra is somewhat of a gymnast, often performing somersaults to move from one place to another. STC/MS™ OR GAN I SMS- F RO M M ACRO T O M ICRO 201