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The calcichordate theory is not widely accepted as a viable theory on the origins of the chordates. [7] Many cite its overall unparsimonious nature as unnecessary, however there are many specific points that can be raised. All in all, the carpoids are much closer to echinoderms than to chordates.
A chordate (/ ˈ k ɔːr d eɪ t / KOR-dayt) is a deuterostomal bilaterian animal belonging to the phylum Chordata (/ k ɔːr ˈ d eɪ t ə / kor-DAY-tə).All chordates possess, at some point during their larval or adult stages, five distinctive physical characteristics (synapomorphies) that distinguish them from other taxa.
The Cambrian chordates are characterised by the presence of segmented muscle blocks called myomeres and notochord, the two defining features of chordates. Before the full understanding of Cambrian fossils, chordates as members the most advanced phylum were believed to appear on Earth much later than the Cambrian.
Another feature present in both the Hemichordata and Chordata is pharyngotremy — the presence of spiracles or gill slits into the pharynx, which is also found in some primitive fossil echinoderms . [17] [18] A hollow nerve cord is found in all chordates, including tunicates (in the larval stage). Some hemichordates also have a tubular nerve cord.
Some biologists have proposed that the Hemichordates (specifically the Enteropneusta) may represent an intermediate body plan in the evolution of the "inverted" state of the chordates. [10] Though they are considered deuterostomes, the dorsoventral axis of hemichordates retains features of both protostomes and chordates.
Idealised vertebrate body plan, showing key characteristics Vertebrates (and other chordates) belong to the Bilateria , a group of animals with mirror symmetrical bodies. [ 6 ] They move, typically by swimming, using muscles along the back, supported by a strong but flexible skeletal structure, the spine or vertebral column . [ 7 ]
According to the recapitulation theory, such structures should invariably be present in frogs because they were assumed to be at a lower level in the evolutionary tree. Von Baer concluded that while structures like the notochord are recapitulated during embryogenesis, whole organisms are not. [ 7 ]
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