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A chordate (/ ˈkɔːrdeɪt / KOR-dayt) is a deuterostomic bilaterial animal belonging to the phylum Chordata (/ kɔːrˈdeɪtə / 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.
Bilateria (/ ˌ b aɪ l ə ˈ t ɪər i ə / BY-lə-TEER-ee-ə) [2] is a large clade or infrakingdom of animals called bilaterians (/ ˌ b aɪ l ə ˈ t ɪər i ə n / BY-lə-TEER-ee-ən), [3] characterized by bilateral symmetry (i.e. having a left and a right side that are mirror images of each other) during embryonic development.
This ancestral organism adopted an attached mode of life with suspension feeding, and developed radial symmetry. Even so, the larvae of all echinoderms are bilaterally symmetrical, and all develop radial symmetry at metamorphosis. Like their ancestor, the starfish and crinoids still attach themselves to the seabed while changing to their adult ...
Cephalization is a characteristic feature of the bilaterians, a large group containing the majority of animal phyla. [2] These have the ability to move, using muscles, and a body plan with a front end that encounters stimuli first as the animal moves forwards, and accordingly has evolved to contain many of the body's sense organs, able to detect light, chemicals, and gravity.
The acorn worms or Enteropneusta are a hemichordate class of invertebrates consisting of one order of the same name. [2] The closest non-hemichordate relatives of the Enteropneusta are the echinoderms. [3] There are 111 known species of acorn worm in the world, [4] the main species for research being Saccoglossus kowalevskii.
[73] [74] More recently, the fundamental symmetric and twisted development of vertebrates is the topic of the axial twist theory. According to this theory, there is a deep agreement between the vertebrates and cephalochordates, and even all chordates. [75] [76]
Hemichordate. Acorn worm, a hemichordate. Hemichordata (/ ˌhɛmɪkɔːrˈdeɪtə / HEM-ih-kor-DAY-tə) is a phylum which consists of triploblastic, eucoelomate, and bilaterally symmetrical marine deuterostome animals, generally considered the sister group of the echinoderms.
Bilateral symmetry creates an anterior “front” end and posterior “back” end in living organisms. The anterior end of bilateral organisms often undergoes cephalization and becomes a “head”, hosting a cluster of sensory organs, the mouth, or both.