扁形动物：典型而原始的两侧对称 Platyhelminthes: Primitive Bilateral Animal

大家听说过扁形动物吧。它们是一些很奇怪的生物，有的长得像一张纸一样薄，没有骨头也没有关节。它们的名字就是来自希腊语的“扁平的蠕虫”。

Do you know what flatworms are? They are some very strange creatures, with bodies as thin as a sheet of paper, without bones or joints. Their name comes from the Greek words for "flat worm (πλατύ ἑλμινθ)".

扁形动物虽然简单，但也有两侧对称的身体结构，就是说它们的左右两边是一样的。但是它们没有像我们一样有血液循环和呼吸系统，它们只能靠自己的皮肤来吸收氧气和排出废物。这就是为什么它们要那么扁，因为这样可以增加它们的表面积，让它们更容易呼吸。

Flatworms are simple, but they also have a bilateral body structure, which means that their left and right sides are the same. But they do not have typical blood circulation and respiratory systems like us, they can only rely on their skin to absorb oxygen and excrete waste. That's why they are so flat, because it increases their surface area, making it easier for them to breathe.

扁形动物的消化系统也很特别，它们只有一个口，既用来吃，也用来排。所以它们不能一直吃个不停，要不然就会撑死或者堵住。

Flatworms' digestive system is also very special, they only have one mouth, which is used to eat and poop. They have no anus, and no complete digestive tract, only a blind sac-like gut. They cannot eat continuously, otherwise they will burst or get blocked.

扁形动物是一大类生物，但是它们之间的亲缘关系很复杂，不同的研究有不同的分类方法。

Flatworms are a large group of animals, but their phylogenetic relationships are very complex, and different studies have different classification methods.

最早的时候，人们只是根据它们的外形和生活习性，把它们分成三大类：涡虫、绦虫和吸虫。涡虫是自由生活的，绦虫和吸虫都是寄生在其他动物身上的。

In the beginning, people just classified them into three major groups based on their appearance and habits: turbellarians, tapeworms and flukes. Turbellarians are free-living, tapeworms and flukes are parasitic on other animals.

后来，人们又发现了一种叫做单殖类的扁形动物，它们也是寄生的，但是和绦虫和吸虫有些不同。有些人把它们归到吸虫里面，有些人把它们单独列为一类。

Later, people discovered a kind of flatworm called monogenean, which is also parasitic, but different from tapeworms and flukes. Some people put them into the fluke group, some people listed them as a separate group.

再后来，人们用更先进的技术，比如分子生物学，来研究扁形动物的亲缘关系。

Later on, people used more advanced techniques, such as molecular biology, to study the phylogenetic relationships of flatworms. 

动物学家们发现原来的分类方法都不太准确，因为有些类里面包含了不同的进化支系。

They found that the original classification methods were not very accurate, because some groups contained different evolutionary lineages. 

科学家们提出了一个新的分类方法，把涡虫分成一个亚门，把寄生的扁形动物分成另一个亚门，叫做新皮亚门。但是这个方法也不是完美的，因为单殖纲到底属不属于新皮亚门还有较大争议。

Scientists proposed a new classification method, dividing turbellarians into a subphylum (Turbellaria), and parasitic flatworms into another subphylum (Neodermata). But this method is not perfect either, because whether monogeneans belong to Neodermata or not is still controversial.

扁形动物是一类很奇妙的生物，它们的身体结构和功能都有很多特点，让我们来一起了解一下。

Flatworms are very amazing animals, their body structure and function have many features, let us learn about them together.

顾名思义，扁形动物的身体是扁平的，这样可以增加它们的表面积，让它们更容易通过皮肤呼吸和排泄。它们没有血液循环和呼吸系统，也没有骨骼和关节，所以它们的身体很柔软，可以随意变形。

First of all, flatworms' bodies are flat, which increases their surface area, making it easier for them to breathe and excrete through their skin. They have no blood circulation and respiratory systems, no skeleton and joints, so their bodies are very soft and can change shape at will.

其次，扁形动物的消化系统也很简单，它们只有一个口，既用来吃东西，也用来排出废物。即它们没有肛门，也没有完整的消化道，只有一个盲管式的肠腔。它们不能连续摄食，要不然就会撑死或者堵住。

Secondly, flatworms' digestive system is also very simple, they only have one mouth, which is used to eat and poop. They have no anus and no complete digestive tract, only a blind sac-like gut. They cannot eat continuously, otherwise they will burst or get blocked.

再次，扁形动物的神经系统也很原始，它们没有真正的大脑，只有一些神经节和神经索。它们的感觉器官也很简单，主要有眼点、触毛和平衡囊。它们可以感知光、触、化学和重力等刺激。

Thirdly, flatworms' nervous system is also very primitive, they have no real brain, only some nerve ganglia and nerve cords. Their sensory organs are also very simple, mainly consisting of eye spots, cilia and statocysts. They can perceive light, touch, chemical and gravity stimuli.

最后，扁形动物的生殖系统也很特殊，它们有两种生殖方式：无性生殖和有性生殖。无性生殖是通过分裂或者断裂再生来进行的，就是说它们可以把自己切成两半或者更多，然后每一部分都可以长成一个新的个体。

Finally, flatworms' reproductive system is also very special, they have two ways of reproduction: asexual reproduction and sexual reproduction. Asexual reproduction is done by splitting or regeneration by breaking off parts of their bodies. That means they can cut themselves into two or more pieces and then each piece can grow into a new individual. 

有性生殖是通过交配或者自交来进行的，就是说它们可以和别的个体或者自己交换精子和卵子来产生后代。扁形动物大多数是雌雄同体的，就是说一个个体既有雌性器官又有雄性器官。

Sexual reproduction is done by mating or self-fertilization. That means they can exchange sperm and eggs with other individuals or themselves to produce offspring. Most flatworms are hermaphrodites. That means one individual has both female and male organs.

扁形动物虽然简单，但也有着丰富的生活方式和生存策略。它们是自然界的奇妙创造，值得我们去认识和了解。

Flatworms are simple but also have rich lifestyles and survival strategies. They are wonderful creations of nature that deserve our recognition and understanding.

但是你知道吗，扁形动物并不都是自由生活的，有些扁形动物是寄生在其他动物身上的，它们有着不同的生活习性和适应方式。那么，寄生的扁形动物和自由生活的扁形动物有什么区别呢？它们是怎么生活的呢？它们对我们人类有什么影响呢？

But do you know that not all flatworms are free-living? Some flatworms are parasitic on other animals. They have different habits and adaptations. So what are the differences between parasitic flatworms and free-living flatworms? How did they live? What impact do they have on humans?

《送瘟神》诗词 via 中国科学院网站

如果你想了解这些问题的答案，就请阅读我们扁形动物的下一篇文章吧！我们将为你揭开扁形动物的另一面，让我们更加深入地了解这些奇妙的生物。

If you want to know the answers to these questions, please follow our next article! We will reveal another side of flatworms for you and let you understand these amazing animals more deeply. Stay tuned!