海星是鱼吗？
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.
海星的英文名叫“starfish”，名字里虽然有“fish（鱼）”，但它根本不是鱼。科学家更喜欢叫它们“sea star”。海星大约有 2000 种，全都生活在海里。
海星生活在水下，但它和鱼的相似之处也就到此为止了。它没有鳃，没有鳞片，也没有鳍。海星体内没有血液，而是靠一套叫作“水管系统”的结构，用海水把养分输送到全身。
海星靠身体下面许多细小的管足移动。一只成年的向日葵海星能靠 15000 只管足，以每分钟一米的惊人速度前进。管足还能帮海星牢牢抓住猎物。
海星和沙钱、海胆、海参是亲戚，它们都属于棘皮动物。棘皮动物的身体以中心为轴，分成五个相等的部分。不过，这并不代表每只海星都只有五条腕，有些种类有 10 条、20 条，甚至 40 条腕。而且，海星如果失去了一条腕，还有重新长出来的惊人本领。