海绵，演化的侧生动物 Sponge, the evolutionary Parazoa

朋友们还记得前两次推送中的海绵动物吗？它们是一类生活在水中的多孔动物，有着奇特的形态和功能。它们不仅是我们洗澡时用的海绵的原料，还是许多药物和化妆品的来源。但是你可能不知道，在经典动物学中，海绵动物还有一个特殊的名字——侧生动物。为什么它们会被叫做侧生动物呢？今天我们就来再揭这个谜团。

Do you still know about sponges? They are a kind of porous animals that live in water and have peculiar shapes and functions. They are not only the raw material for the sponges we use when bathing, but also the source of many drugs and cosmetics. But you may not know that sponges have a special name - parazoans in the classic zoology. Why are they called parazoans? Today I will reveal this mystery to you.

海绵的化石已经在5.8亿年前的岩石中发现，使它们成为地球上较早的动物之一。

Fossils of these sponges have been discovered in rocks dating back to 580 million years ago, making them some of the earliest animals on Earth. 

海绵是动物界独特的一群，属于多孔动物门，西文的意思就是“有孔的动物”。它们是演化树上最早分支的原始动物，是地球上其他所有动物（相对栉水母动物还有争议，但争议不大）的姊妹群。

Sponges are unique members of the animal kingdom, belonging to the phylum Porifera, which means "pore bearer." They are a primitive group of animals that branched off the evolutionary tree before all other animals, making them the sister group of every other animal on Earth.

因此，海绵动物类群也被称为侧生动物类群，因为它们可以被划为演化树的侧枝。侧生动物是动物界演化树上最底层的一个引人入胜的动物群体。它们与其较为先进的对应物——真后生动物相区别，后者已经发展出了完全的组织结构。

Thus, Porifera is also called as Parazoa, which is a fascinating group of animals that occupies the bottommost position on the animal kingdom's evolutionary tree. They are distinct from their more advanced counterparts, the Eumetazoa, which have developed proper tissues. 

除了演化位置外，侧生动物还保留了其他原始的特点，它们缺乏明确分化的组织，也可以反过来说，它们只出现了部分分化的组织。

Parazoans are primitive organisms that lack well-defined tissues or have tissues that are only partially differentiated.

多孔动物门是目前唯一公认可以分为侧生动物群体的动物门。海绵是迷人的生物，它们不具有肌肉、神经或内部器官。它们通常更像是细胞群体而不是多细胞生物。

The Porifera is the only phylum of animals that belongs to the Parazoa group. Porifera are fascinating creatures that do not possess muscles, nerves, or internal organs. They often resemble a colony of cells rather than a multicellular organism.

有时，侧生动物类别包括其他动物群，如已灭绝的古杯动物，有时被归类为其中一个单独的门。在其他情况下，简单、扁平、有纤毛的扁盘动物也会根据不同的研究，而时不时地被包括进来。

Sometimes, the Parazoa category includes other groups of animals such as the Archaeocyatha, which are extinct sponges that are sometimes classified as a separate phylum. In other cases, Placozoa, a group of simple, flattened, and ciliated animals, is also included, depending on the authors.

近年，一大类古老的海绵状古杯动物，它们曾被认为是一个独立的门类，现在也被部分研究者认为属于海绵动物类群。

Even a group of ancient sponge-like creatures called Archaeocyathids, once thought to be a separate phylum, are now considered to be sponges by some researchers.

通过上面的介绍，我们又了解了海绵动物为什么叫侧生动物。海绵动物的演化历史告诉我们，即使看起来低调无趣的东西，也可能隐藏着惊人的秘密。让我们一起用好奇心去探索这个神奇而美丽的世界吧！

Through the above introduction, you should have understood why sponges are called parazoans. Sponges' history tells us that even things that look simple and boring may hide amazing secrets. Let's explore this magical and beautiful world together with curiosity!