国家地理:水母为何如此迷人(3)
Moon jellies and their cousins, which include lion's mane jellies and sea nettles, are known as true jellies. They belong to the class Scyphozoa, in the phylum Cnidaria, which also includes corals. (A phylum is such a broad taxonomic category that humans, fish, snakes, frogs, and all other animals with a backbone belong to the same one -- the chordates -- as do salps, which are sometimes lumped with jellies.) As adults, true jellies are shaped like upside-down saucers or billowing parachutes. They propel themselves through the water by contracting the muscles of their bells, and their tentacles are equipped with stinging cells that shoot out tiny barbed tubes to harpoon floating prey. To reel it into their mouths, they use streamer-like appendages known as oral arms. In some species the oral arms have mouths of their own.
海月水母与包括狮鬃水母与海荨麻在内的近亲，都属于真水母。它们都是被分类为钵水母纲的动物，和珊瑚一样同属刺丝胞动物门。（门是一个非常广泛的分类范畴，以至于人类、鱼、蛇、青蛙和所有其他有脊椎骨的动物都属于脊索动物，而樽海鞘有时与水母划分在一起）成熟的真水母外观看来像是倒过来的盘子或是鼓起来的降落伞。它们藉由收缩钟状体上的肌肉在水中推进，触手上的刺丝胞可以将微小带刺的管子射向漂浮的猎物。要将食物卷入口中时，它们会使用彩带状的附器（口腕）。有几种水母的口腕上就有嘴。
Jellyfish like the dreaded Portuguese man-of-war are also related to corals, but they're part of a different subgroup, the siphonophores, which practice an unusual form of collective living. What looks like a single man-of-war is technically a colony that developed from the same embryo. Instead of simply growing larger, the embryo sprouts new "bodies," which take on different functions. Some develop into tentacles, for example; others become reproductive organs.
令人闻风丧胆的僧帽水母（又称为葡萄牙战舰）也和珊瑚有亲缘关系，但属于另一个不同的子群：管水母。管水母有着特殊的集体生活形式。看起来只有一只的僧帽水母，其实是从同一个胚胎发展而来的群体。这个胚胎并不只是愈长愈大，而是长出新的“身体”，每个身体都扮演不同的功能，例如有些会发展成触手，有些则成为生殖器官。
