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There are soft constraints that separate chordates from other biological lineages, but are not part of the formal definition: All chordates are deuterostomes. This means that, during embryonic development, the anus forms before the mouth does. All chordates are based on a bilateral body plan. [15] All chordates are coelomates, and have a fluid ...
This, combined with an apparent lack of vertebral elements within the Myxini, suggested that the Myxini were descended from a more ancient lineage than the vertebrates, and that the skull developed before the vertebral column. The clade was thus composed of the Myxini and the vertebrates, and any extinct chordates with skulls.
The notochord is an elastic, rod-like structure found in chordates. In chordate vertebrates the notochord is an embryonic structure that disintegrates, as the vertebrae develop, to become the nucleus pulposus in the intervertebral discs of the vertebral column. In non-vertebrate chordates a notochord persists.
Despite their simple appearance and very different adult form, their close relationship to the vertebrates is certain. Both groups are chordates, as evidenced by the fact that during their mobile larval stage, tunicates possess a notochord, a hollow dorsal nerve cord, pharyngeal slits, post-anal tail, and an endostyle. They resemble a tadpole.
According to this theory, there is a deep agreement between the vertebrates and cephalochordates, and even all chordates. [ 75 ] [ 76 ] With the advent of molecular genetics lancelets are once again regarded as a model of vertebrate ancestors, and are used again as a model organism.
The term invertebrates does not describe a taxon in the same way that Arthropoda, Vertebrata or Manidae do. Each of those terms describes a valid taxon, phylum, subphylum or family. "Invertebrata" is a term of convenience, not a taxon; it has very little circumscriptional significance except within the Chordata.
Marine invertebrates are invertebrate animals that live in marine habitats, and make up most of the macroscopic life in the oceans. It is a polyphyletic blanket term that contains all marine animals except the marine vertebrates , including the non- vertebrate members of the phylum Chordata such as lancelets , sea squirts and salps .
Olfactores is a clade within the Chordata that comprises the Tunicata (Urochordata) and the Vertebrata [citation needed] (sometimes referred to as Craniata).Olfactores represent the overwhelming majority of the phylum Chordata, as the Cephalochordata are the only chordates not included in the clade.