There are about 45 . 000 species of chordates , occupying marine , fresh and brackish water and terrestrial habitats 有大约45 . 000种脊索动物，它们占据着海、淡水或淡咸水和陆地的生态环境。
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Amphioxus or lancelet , a cephalochordate , is the extant invertebrate chordate considered to be the closest relationship to the vertebrates 文昌鱼被认为是和脊椎动物亲缘关系最近的动物。
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Chordates are bilaterally symmetrical , triploblastic , segmented coelomates which demonstrate a deuterostomic pattern of early embryonic development 脊索动物两侧对称、三胚层，分节的体腔，在胚胎发育的早期过程中，显示为后口类型。
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The sea urchin represents the first sequenced genome from the echinoderms , which are the closest known relatives of the chordates , the group that includes vertebrates , animals with spinal columns 海胆是首个被排序的棘皮动物，这是与离脊索动物（也包括有脊椎动物）最近的亲属。
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Phylogenetic analysis based on the amino acids of the ubiquitin / ribosomal fused proteins of different animals well reflected the established phylogeny of the chosen organisms , and amphioxus was branched off from chordates and formed a separate cluster with an intermediate placement between invertebrates and vertebrates in the tree 通过对ubiquitin的系统进化分析，发现文昌鱼ubf80除了具有脊椎动物和无脊椎动物的特征外，还具有自己的一些独特的特征，从分子水平上支持了文昌鱼是脊索动物进化过程中的一侧枝的观点。
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However , amphioxus ubf80 also showed some unique characters such as its amino acid residues at sites 39 , 54 , 63 and 74 in the cep . it appears that amphioxus not only represents an organism tr ansitional from invertebrates to vertebrates in phylogeny but also a possible diverged form radiated from the chordate ancestor at early chordate evolution . the possibility that some mutations in ub genes have taken place in different lines early at the divergence of plants and animals is also discussed 发现文昌鱼a二phi尸rxv基因是具有线粒体和过氧化物酶体信号特征( mitochondrialandperoxisomalsortingsignals )的抗氧化物酶基因;通过序列比对，我们把尸rxv归为l一cys和2一cys之外的3一cys类，从而把尸rx基因家族的两种分类方法统一了起来。
