|MadSci Network: Zoology|
Hi Steven, Sea anemones and their relatives use special stinging cells called cnidocytes (pronounced "ny'-do-sites") to catch their food. The animal's tentacles are loaded with batteries of these cnidocytes, and snag/harpoon/ poison small animals that swim or blunder into them. Each cnidocyte can be used only once, and here's an explanation for why: Inside the cnidocyte is a capsule called a cnida. Inside the cnida is a long, hollow, coiled thread called a nematocyst ("nee-ma'-toe-sist"). The nematocyst is the actual "dart" you asked about. The business end of the nematocyst can be armed with hooks, spines, or barbs, and some can inject toxins into the prey animal. Anyway, when the cnida is triggered to fire, the nematocyst is rapidly everted (turned inside out) and shot into the flesh of the prey. In some cases, the nematocyst actually leaves the body of the anemone, and remains dug into the flesh of the prey -- ouch! Once the thread is everted, it cannot be retrieved and recoiled back into the capsule, so nematocysts are definitely one-use-only structures. Fortunately, sea anemones make lots of cnidocytes, and many different kinds of nematocysts. Even if most of the cnidocytes are blown out during a feeding bout, the anemone can probably replace its arsenal within a day or so. Thanks for the question! Allison J. Gong Mad Scientist
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