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    lesson content

    What is almost a but not quite?
    ::什么是几乎是但并非完全的?

    A of lancelet in the subphylum Cephalochordata. If you look closely, you can see the repeating units of muscle tissue , called myomeres, that run the length of the lancelet’s body. Why is it not a fish? Notice there are no fins and no true skeleton.
    ::螺旋状链条在子植物链中。如果你仔细看一看,你可以看到肌肉组织的重复单位,叫做米奥梅斯,它长于长于长于长于长于长于长于长于长于长于长于长于长于长于长于长于长于长于长于长于长于长于长于长于长于短于短于短于短于短于短于短于短于短于短于短于短于短于短于短的两米的肌肉组织。为什么它不是一条鱼呢?注意没有鳍,也没有真正的骨架。

    Lancelets
    ::射子

    Lancelets include about 25 species of that make up the subphylum Cephalochordata. Lancelets are also called amphioxus, which translates to “both ends pointed,” because of the shape of their elongated bodies, as shown in Figure .
    ::兰刀包括构成亚硫化支流的大约25种物种。 如图所示,兰刀也被称为“两端点 ” , 也就是“两端点 ” , 因为它们的长体形状(如图所示 ) 。

    Unlike many of the , the lancelets are capable of swimming, however, they spend most of their time buried in sandy, shallow regions of the ocean. Adult lancelets retain the pharyngeal slits , notochord , dorsal nerve cord, and post-anal tail , which are all characteristic of . Although lancelets have a brain-like swell at the end of the notochord in the head region, it is not very highly developed.
    ::与许多大湖不同的是,长矛可以游泳,但是,它们大部分时间都埋在浅浅的沙地海洋地区。 成年长矛保留了长矛、未切裂的、短发的神经线和尾巴。 虽然长矛在大湖地区末端有类似大脑的肿胀,但并不十分发达。

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    Lancelet (Cephalochordata). Unlike tunicates, lancelets retain all four defining chordate traits in the adult stage. Can you find them?

    Like most tunicates, lancelets are filter feeders with the pharynx situated in an atrial cavity where it functions to filter food particles from the currents that flow through the cavity. Water is taken in through the mouth and expelled through an opening called the atriopore . The mouth has several specializations including a filtering structure, called a wheel organ , that is made up of ridges covered with cilia that help draw in water. Lancelets have a closed with a heart-like, pumping organ located on the ventral side, and they reproduce sexually. Unlike other chordates, lancelets do not use the pharyngeal slits for respiration . Gas exchange occurs through the body wall. The sexes are separate, and is external. They undergo indirect involving a free-swimming larval stage. A significant feature of lancelets, which is shared with vertebrates but not tunicates, is the presence of myomeres. Myomeres are repeating units, or blocks, of muscle that partially overlap to form a row along the body of the animal, as shown in Figure . In addition to segmented muscle tissue, both cephalochordates and vertebrates have segmented bodies.
    ::象大多数土豆一样, 鼻孔是呼吸器过滤器的过滤器, 刺青器位于空腔中, 它的作用是从流经腔腔的流水中过滤食物颗粒。 水从口中吸进, 通过一个叫做小孔的开口排出。 嘴有几种专业, 包括过滤器结构, 叫做轮子器官, 由帮助抽水的硅覆盖的脊架组成。 兰切器有一个封闭的心脏相似的抽动器官, 位于通风侧, 并具有性繁殖功能。 不同于其他的花椰子, 葡萄圈不使用石脑切片进行呼吸。 气体交换发生在身体壁上。 性别是分开的, 外部的。 它们间接地涉及到一个自由旋转的幼虫阶段。 与脊椎共有, 但不提炼液的一个重要特征是存在我的蛋白质。 Myomeres正在重复部分重叠的肌肉单位, 或一些块块, 与动物脑部和脑部结构的一排成一行, 显示在图中。

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    A diagram showing the organization of myomeres. Myomeres are repeating muscle segments that run the length of a lancelet’s body.

    of lancelets is quite simple due to the small number of species. There is only one class of cephalochordates, called Leptocardii, that contains two families: Asymmetronidae and Branchiostomidae. Despite the small size of the cephalochordate subphylum, these animals hold an important place in the evolutionary history of vertebrates, a place that is still being characterized and defined by scientists today. This will be discussed in the next section.
    ::由于物种数量少,卵巢非常简单。只有一类叫做Leptocardi的切喉动物,由两个家族组成:Asymmetronidae和Brickiostomidae。尽管子植物体积小,这些动物在脊椎动物的进化史中占有重要位置,而今天科学家们仍在对它进行定性和界定。下一节将讨论这个问题。

    Summary
    ::摘要

    • Lancelets include about 25 species of marine animals that make up the subphylum Cephalochordata.
      ::花环包括大约25种海洋动物物种,组成了子植物链。
    • Adult lancelets retain the four characteristics common to all chordates.
      ::成人长袍保留所有长袍共有的四种特性。
    • Like most tunicates, lancelets are filter feeders.
      ::和大多数外衣一样,胸罩是过滤器的支线。

    Review
    ::回顾

    1. What are some major differences between adult tunicates and lancelets?
      ::成人外衣和长袍之间有什么重大区别?
    2. How do lancelets feed?
      ::护身符如何喂养?
    3. How do lancelets obtain oxygen?
      ::气囊如何获得氧气?
    4. What significant feature do lancelets share with vertebrates?
      ::长矛与脊椎动物有什么重要特征?