海星是魚嗎？
Despite their name, starfish are not fish at all. Scientists prefer to call them sea stars. There are about 2,000 species, and all of them live in the ocean.
Sea stars live underwater, but that is where their resemblance to fish ends. They don't have gills, scales or fins. Instead of blood, they use seawater to pump nutrients around their bodies through what is called a "water vascular system".
Sea stars move using tiny tube feet on the underside of their bodies. An adult sunflower sea star can travel at an astonishing one meter per minute on 15,000 tube feet. The tube feet also help sea stars hold on to their prey.
Sea stars are related to sand dollars, sea urchins and sea cucumbers. They all belong to a group called echinoderms, whose bodies are arranged in five equal parts around a center. This doesn't mean every sea star has five arms, though. Some species have 10, 20 or even 40. And if a sea star loses an arm, it has the amazing ability to grow a new one.
海星的英文叫「starfish」，名字裡雖然有「fish（魚）」，但牠根本不是魚。科學家比較喜歡叫牠們「sea star」。海星大約有 2,000 種，全都生活在海裡。
海星生活在水下，但牠和魚的相似之處也就到此為止。牠沒有鰓、沒有鱗片，也沒有鰭。海星體內沒有血液，而是靠一套叫做「水管系統」的構造，用海水把養分輸送到全身。
海星靠身體底下許多細小的管足移動。一隻成年的向日葵海星能靠 15,000 隻管足，以每分鐘一公尺的驚人速度前進。管足還能幫海星牢牢抓住獵物。
海星和沙錢、海膽、海參是親戚，牠們都屬於棘皮動物。棘皮動物的身體以中心為軸，分成五個相等的部分。不過，這不代表每隻海星都只有五條腕，有些種類有 10 條、20 條，甚至 40 條腕。而且，海星如果失去一條腕，還有重新長出來的驚人能力。