國家地理:水母為何如此迷人(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.
海月水母與包括獅鬃水母與海蕁麻在內(nèi)的近親，都屬于真水母。它們都是被分類為缽水母綱的動(dòng)物，和珊瑚一樣同屬刺絲胞動(dòng)物門。（門是一個(gè)非常廣泛的分類范疇，以至于人類、魚、蛇、青蛙和所有其他有脊椎骨的動(dòng)物都屬于脊索動(dòng)物，而樽海鞘有時(shí)與水母劃分在一起）成熟的真水母外觀看來像是倒過來的盤子或是鼓起來的降落傘。它們藉由收縮鐘狀體上的肌肉在水中推進(jìn)，觸手上的刺絲胞可以將微小帶刺的管子射向漂浮的獵物。要將食物卷入口中時(shí)，它們會(huì)使用彩帶狀的附器（口腕）。有幾種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.
令人聞風(fēng)喪膽的僧帽水母（又稱為葡萄牙戰(zhàn)艦）也和珊瑚有親緣關(guān)系，但屬于另一個(gè)不同的子群：管水母。管水母有著特殊的集體生活形式。看起來只有一只的僧帽水母，其實(shí)是從同一個(gè)胚胎發(fā)展而來的群體。這個(gè)胚胎并不只是愈長愈大，而是長出新的“身體”，每個(gè)身體都扮演不同的功能，例如有些會(huì)發(fā)展成觸手，有些則成為生殖器官。
