扁形动物：再生与寄生的生活特征 Flatworms: Groups of regeneration or parasitism

大家可能听说过扁形动物或者扁虫。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)".

如果我们仔细观察，在我们身边，有一种神奇的动物，它们的名字叫做涡虫，属于扁形动物的一种。

Do you know? There is a kind of magical animal around us, their name is planarian, which belongs to a kind of flatworm.

涡虫是扁形动物研究的模式动物，它们有多神奇呢？主要有以下几个特点：

Planarians are model animals for flatworm research. How amazing are they? They have the following main characteristics:

它们的种类实在多，全世界大概有4500种，体长从一毫米到六十厘米不等，可以说是扁形动物中的百变王。

They have a super variety of species, about 4500 in the world, ranging from one millimeter to sixty centimeters in length, which can be said to be the ever-changing king of flatworms. 

它们大部分是自由生活的，也有少数是寄生的。它们可以在水里、土里、甚至其他动物的体内生存。

Most of them are free-living, and a few are parasitic. They can live in water, soil, or even inside other animals. 

它们有眼睛，但不是我们常见的眼睛，而是一对带色素的小眼，或者是触角上的许多小眼，甚至是遍布全身的小眼。它们用这些小眼来感受光线和方向。

They have eyes, but not the ones we usually see, but a pair of pigmented small eyes, or many small eyes on the antennae, or even small eyes all over the body. They use these small eyes to sense light and direction.

它们没有肛门，只有一个口，位于身体的中前偏下部。它们用口来吞食食物，也用口来排泄废物。有些涡虫还有一个简单的咽部，上面有纤毛，可以帮助它们把食物扫进口中。

They have no anus, only one mouth, located in the middle front and lower part of the body. They use their mouth to swallow food and excrete waste. Some planarians also have a simple pharynx with cilia on it, which can help them sweep food into their mouth.

它们的繁殖方式很特别，它们是雌雄同体，也就是说每个涡虫都有雌性和雄性的生殖器官。但是它们不是自己和自己交配，而是要找另一个涡虫进行"丁丁大战"。这是一种创伤式交配，就是两个涡虫用自己的阴茎互相刺伤对方，把精子注入对方体内。谁输了谁就要分化出卵细胞，并担任雌性角色。

Their reproduction method is very special. They are hermaphrodites, which means that every planarian has both female and male reproductive organs. But they don't mate with themselves, but look for another planarian to have a "penis fencing". This is a traumatic mating, that is, two planarians use their own penis to stab each other and inject sperm into each other's body. Whoever loses will differentiate into egg cells and take on the female role.

涡虫只是扁形动物中的一种，还有其他两种比较常见的扁形动物：吸虫和绦虫。

Planarians are just one kind of flatworms. There are two other common kinds of flatworms: flukes and tapeworms.

吸虫是一大类寄生性很强的扁形动物，它们的身体上有吸盘或钩子，可以紧紧地附着在宿主的体内。它们还有一个特殊的表皮结构，叫做合胞体，可以防止宿主的免疫系统攻击它们。

Flukes are a kind of flatworms with strong parasitism. They have suckers or hooks on their bodies that can attach tightly to the host's body. They also have a special epidermal structure called syncytium, which can prevent the host's immune system from attacking them.

在我们的生产生活实践中，吸虫分为两大类：复殖类和楯盘类。复殖吸虫有一个相对复杂的生活史，一般需要经过两个不同的宿主才能完成发育。第一个宿主通常是软体动物，如螺蛳、蚌类等；第二个宿主通常是脊椎动物，如鱼类、鸟类、哺乳动物等。复殖吸虫在不同的宿主中会变化出不同形态的幼虫和成虫。楯盘吸虫就比较简单了，只需要一个宿主就可以直接发育成成虫。

In practices, flukes are divided into two major categories: Digenea and Aspidocotylea. Digenean flukes have a complex life history and need two different hosts to complete their development. The first host is usually a mollusk, such as snails, clams, etc.; the second host is usually a vertebrate, such as fish, birds, mammals, etc. Digenean flukes change into different forms of larvae and adults in different hosts. Aspidocotylean flukes are simpler, they only need one host to develop directly into adults.

吸虫中最为臭名昭著的可能就是血吸虫类群了。血吸虫是一类能够寄生在人类循环系统中的吸虫，会导致血吸虫病。血吸虫病在我国曾经十分流行，但经过多年的防治工作，已经基本消灭了。

The most infamous fluke may be schistosome. Schistosoma is a kind of fluke that can parasitize in human blood vessels and cause schistosomiasis. Schistosomiasis was once very prevalent in China, but after years of prevention and control work, it has been basically eliminated.

单殖吸虫身体较小，体长0.15～20mm，个别可达3cm，根据其吸盘（固着器）多少分为単后吸盘目和多后吸盘目。淡水种类大多数体长在0-5cm以下，如指环虫类超过1mm者不多见。身体形状不一，有指状、尖细叶片状、椭圆、圆盘状或圆柱状等外表形态；一般淡水品种其体形较单纯，海水种类形态较为多样性变化。体表通常无棘，但有时具有乳状突起，或某些种类的体侧、背面具刺或体表有皱褶。

Monogeneans are small, with a body length of 0.15-20mm, and some can reach 3cm. They are divided into Monopisthocotylea and Polypisthocotylea according to the number of their suckers (attachment organs). Most freshwater species have a body length of less than 0.5cm, such as Dactylagyrus spp., which are rarely more than 1mm. The body shape varies, with finger-like, pointed thin leaf-like, elliptical, disc-shaped or cylindrical forms. Generally, freshwater species have simpler body shapes, while marine species have more diverse morphological changes. The body surface is usually spineless, but sometimes has papillae, or some species have spines on the sides or back of the body or folds on the body surface.

绦虫也是一种寄生性很强的扁形动物，它们的外形粗看和我们常见的蚯蚓很像，都是由许多节段组成的。但是绦虫和蚯蚓不同的是，它们没有消化系统，也没有神经系统。它们主要是依靠一个头节，上面有吸盘或钩子，可以固定在宿主的肠道中。它们靠吸收宿主的营养来生存。

Tapeworms are also a kind of flatworms with abilities of parasitism. They are similar to the common earthworms, which are composed of many segments. But unlike earthworms, they have no digestive system, no nervous system. They have only one head segment, which has suckers or hooks that can fix them in the host's intestine. They rely on absorbing the host's nutrition to survive.

我们常见的绦虫可以分为两大类：单节绦虫亚纲和多节绦虫亚纲。单节绦虫亚纲的绦虫只有一个节片，不需要中间宿主，直接在一个宿主中发育成成虫。多节绦虫亚纲的绦虫有许多节片，每个节片都有自己的生殖器官，可以进行繁殖。多节绦虫亚纲的绦虫需要两个或以上的宿主才能完成发育。第一个宿主通常是无脊椎动物，如昆虫、甲壳类等；第二个宿主通常是脊椎动物，如鱼类、鸟类、哺乳动物等。

Tapeworms are divided into two major classes: Cestodaria and Eucestoda. Cestodan tapeworms have only one segment and do not need intermediate hosts. They develop directly into adults in one host. Eucestodan tapeworms have many segments, each with its own reproductive organs, which can reproduce by themselves. Eucestodan tapeworms need two or more hosts to complete their development. The first host is usually an invertebrate, such as insects, crustaceans, etc.; the second host is usually a vertebrate, such as fish, birds, mammals, etc.

绦虫中最出名的可能就是牛肉绦虫和猪肉绦虫了。这两种绦虫都能够寄生在人类的肠道中，会导致绦虫病。如果人类吃了带有牛肉绦虫或猪肉绦虫囊蚴的生牛肉或生猪肉，就会感染这两种绦虫。这两种绦虫在人体内可以长到几米甚至十几米的长度，会消耗人体的营养，引起消瘦、贫血、腹泻等症状。

The most infamous tapeworms may be beef tapeworm and pork tapeworm. These two tapeworms can parasitize in human intestines and cause taeniasis. If humans eat raw beef or pork with beef tapeworm or pork tapeworm cysticerci, they will be infected with these two tapeworms. These two tapeworms can grow to several meters or even tens of meters in length in the human body, consume human nutrition, and cause symptoms such as emaciation, anemia, diarrhea, etc.

扁形动物是一种非常重要的动物类群，它们是最早出现两侧对称体型的动物，也是最早出现真正的器官和组织的动物。涡虫具有强大的再生能力，是干细胞研究的重要材料。吸虫和绦虫则是人畜共患病的重要致病源，需要我们加强防治和监测。

Flatworms are a very important group of animals. They are the earliest animals to have bilateral symmetry and the earliest animals to have true organs and tissues. Planarians have a strong regenerative ability and are an important material for stem cell research. Flukes and tapeworms are important pathogens of zoonotic diseases, which require us to strengthen prevention and monitoring.

扁形动物的世界是多姿多彩的，也是充满奥秘和挑战的。如果你想了解更多关于扁形动物的知识，请关注"开放动物学"，我们将为你带来更多有趣有料的动物内容。

The world of flatworms is colorful and full of mystery and challenge. If you want to know more about flatworms, please follow our public account "NakedZoology", we will bring you more interesting and informative animal content.

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