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Hemichordates ("half chordates") have some features similar to those of chordates: branchial openings that open into the pharynx and look rather like gill slits; stomochords, similar in composition to notochords, but running in a circle round the "collar", which is ahead of the mouth; and a dorsal nerve cord—but also a smaller ventral nerve cord.
A tunicate is an exclusively marine invertebrate animal, a member of the subphylum Tunicata (/ ˌ tj uː n ɪ ˈ k eɪ t ə / TEW-nih-KAY-tə).This grouping is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords (including vertebrates).
They feed both at the ocean's surface and below it, and even feed on each other. Marine birds can be highly pelagic, coastal, or in some cases spend a part of the year away from the sea entirely. Some marine birds plummet from heights, plunging through the water leaving vapour-like trails, similar to that of fighter planes. [35]
As their habitats are mostly defined by ocean currents, [1] many species have a cosmopolitan distribution, with some like Oikopleura dioica being found in all of the world's oceans. [27] Larvaceans have been reported as far as the Southern Ocean , where they are estimated to comprise 10.5 million tonnes of wet biomass.
Acorn worm on the ocean floor Most acorn worms range from 9 to 45 centimetres (3.5 to 17.7 in) in length, with the largest species, Balanoglossus gigas , reaching 1.5 metres (5 ft) or more. The body is made up of three main parts: an acorn-shaped proboscis, a short fleshy collar that lies behind it, and a long, worm-like trunk.
The five synapomorphies are pharyngeal slits, a dorsal nerve cord, notochord, endostyle, and the post-anal-tail which is depicted and labeled well on the chordates page. This image is helpful to visualize the regions where the five synapomorphies existed in chordates and what they looked like.
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[citation needed] Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone. The phylum contains about 7,000 living species, [54] making it the second-largest grouping of deuterostomes (a superphylum), after the chordates (which include the vertebrates, such as birds, fishes, mammals, and reptiles).