Is a jellyfish a fish?（水母是鱼吗？）

Is a jellyfish a fish?

水母是鱼吗？

Most certainly not. A jellyfish is a creature belonging to the phylum Cnidaria, a zoological category of aquatic animals known as coelenterates or cnidarians, which also includes corals and sea anemones.

肯定不是鱼！水母是一种刺胞动物，是一种水生动物的门类，叫做腔肠动物或是刺细胞动物。珊瑚和和海葵也属于这类动物。

Jellyfish live in water, and have bell- or saucer-shaped bodies made of a jelly-like substance, which is often transparent or semi-transparent. Around the edge of the body are stinging tentacles, which the jellyfish uses to catch its prey and which can also cause painful injuries to swimmers if they come into contact with them.

水母生活在水里，身体呈铃状或碟状，主要由胶状物组成，往往是透明或半透明的。身体的边缘，长有刺状的触须，主要用来捕猎。如果有人跟它们发生碰触，水母会用触须攻击，导致刺痛受伤。

One of the main differences between jellyfish and fish is that, while fish are vertebrates, having a backbone, all coelenterates are invertebrates. Jellyfish don't have a skeleton, a brain, or a central nervous system, and unlike fish, they reproduce both sexually and asexually at different stages of their life cycle.

水母和鱼之间的主要区别之一在于：鱼是脊椎动物，有脊骨；而所有的腔肠动物都是无脊椎动物。水母没有骨骼，也没有大脑或是中枢神经系统。而且，与鱼不一样的是，水母在不同的生命阶段，有的阶段进行有性繁殖，有的阶段进行无性繁殖。

Typically, a jellyfish goes through four main phases of development: planula, polyp, ephyra, and medusa. A jellyfish looks very different when it's in the earlier stages of its life cycle: it's the later, medusa form that we think of when we call a typical jellyfish to mind. A jellyfish begins as a planula, a larval stage, having a flattened, solid body. The next phase is a polyp, which can reproduce asexually, a process known as budding. An ephyra (a small medusa) is also produced asexually, from a polyp. The ephyra then matures into the medusa stage, which can reproduce sexually, from an egg of one individual and a sperm of another.

一般来说，水母会经历四个主要的生命发展阶段：浮浪幼体阶段、水螅阶段、蝶状幼体阶段和成体阶段。水母在生命的早期阶段，看起来与我们说到水母时所想到的成体水母，完全不一样。在浮浪幼体阶段时，水母尚处于幼体阶段，身体扁平而坚实。下一个阶段是水螅阶段，这时水母可以进行无性繁殖，这个过程被称为budding（萌芽阶段）。碟状幼体阶段的水母也进行无性繁殖，直接从浮浪幼体发育而来。碟状幼体成熟之后，就成了成体水母，此时进行的是有性繁殖，即来自不同水母的卵子和精子相结合，繁殖后代。

The confusion between fish and jellyfish probably comes from the name that they share and the fact that they both live in water, but that, essentially, is where the similarities end.

水母到底是否是鱼类的困惑主要源自水母的名字jellyfish，以及水母也生活在水里的习性，但是实际上，水母和鱼的相似之处也仅限于此。