One of the distinguishing features of cnidarians is the presence of cnidocytes (NIE-duh-siets), which give the phylum its name. Cnidocytes are specialized cells used for defense and capturing prey. Figure 33-4 shows a type of cnidocyte organelle called a nematocyst (nuh-MAT-uh-sist), which has a long filament coiled up inside it. In some cnidarians, the cnidocytes are concentrated in the epidermis, especially on the tentacles. When an object brushes against the "trigger" on a cnidocyte, the nematocyst inside it suddenly pushes the filament out of the cell with great force. Some nematocysts have filaments with sharp tips and spines that puncture the object and inject poison. Others have filaments that adhere to the object by wrapping around it.
The relationship between structure and function is clearly seen in the way cnidarians feed. The tentacles capture small animals with their nematocysts and paralyze them with the poison they inject. The tentacles then push the prey into the gastrovas-cular cavity through the mouth. After enzymes inside the gas-trovascular cavity break up the prey, cells lining the cavity absorb the nutrients. Undigested food and waste are expelled through the mouth.
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