පෘෂ්ඨවංශීන්

විකිපීඩියා, නිදහස් විශ්වකෝෂය වෙතින්
වෙත පනින්න: සංචලනය, සොයන්න

පෘෂ්ටවංශීන් යනු අස්ථි සැකිල්ලක් සහිත සත්ව විශේෂයන්ය.ගෙලේ සිට වලිගය දක්වා දික්වූ කොදුඇට පෙළක් මොවුන්ට පිහිටයි.පෘෂ්ඪවංශීන්ගේ මොළය ඝන හිස්කබලකින් ආවරණයවී ඇත.මොවුන්ගේ අස්ථි සැකිල්ළ කාටිරේජයන්ගෙන් නිර්මානයවන අතර පසුව කැල්සියම් එක්වී ශක්තිමත්වේ.මෙම අස්ථි තුල රුධිරය නිපදවන ඇට මිදුළු පිහිටා ඇත.මොවුන්ගේ මධ්‍යම ස්නාු පද්ධතිය කොඳුඇට පෙළ තුල පිහිටා ඇත.මෙම වංශයට අයත් සතුන් ලෙස මාළුවන් උභයජීවීන් සර්පයින් කුරුල්ලන් හා ක්ෂීරපායින් දැක්විය හැක.ලොව පුරා පෘෂ්ඨවංශීන් විශේෂ 65,000 පමණ වාසය කරති.මෙම වංශයට අයත් විශාලම සත්වයා නිල් තල්මසාය.මිනිසා අයත් වන්නේද මෙම සත්ව කාණ්ඩයටයි.

මොවුන්ගේ පරිණාමය පිළිබද විමසා බැලීමේදී පෘෂ්ඪවංශීන් එක් පොදු පූර්වජයකුගෙන් පරිණාමයවී ඇතිබව සත්ව විද්‍යාඥයන් මත පලකරයි.පෘෂ්ඨවංශිකයෙකුට අයත් පූර්වජයකුගේ ෆොසිලයක් කැරීබියන් දූපත් ආශ්‍රිතව සොයාගැනීමට විද්‍යාඥයන්ට හැකිවී ඇත.මෙම ෆොසිලය මාළුවකුට අයත් ෆොසිලයක් යැයි අනුමාන කෙරේ.පෘෂ්ඨවංශීන් අතරින් මුලින්ම ලොව පහලවූයේ මතස්‍යයන් බව සත්ව විද්‍යාඥයන් පෙන්වාදෙයි.මත්ස්‍යයන් ගෙන් උභයජීවීන්ද උභයජීවීන්ගෙන් පක්ෂීන් හා ක්ෂීරපායීන් පරිණාමය වන්නට ඇතැයි පැවසේ.

අර්ථ දැක්වීම[සංස්කරණය]

පහත දැක්වෙන ආකාරයේ ලක්ෂණ ඇති සතුන් පෘෂ්ඨවංශීන් ලෙස දැක්විය හැක

  • කාටිරේජයන් ගෙන් සැදුම්ලත් අස්ථි සැකිළ්ලක් ඇතැම් පෘෂ්ටවංශීන්ට හිමිව ඇති අතර එය නම්‍යශීලී වෙයි.එමගින් ඔවුන්ගේ සිරුර පහසුවෙන් නැමීමට හැකියාව ලැබී ඇත.විශේෂයෙන් මළුවන් වැනි ජලජ ජීවීන්ට මෙවන් කාටිරේජයන්ගෙන් සමන්විත නම්‍යශීලී සැකිළි හිමිවී ඇත..
  • මෙම අස්ථි සැකිළ්ල අභ්‍යන්තරයෙන් මධ්‍ය ස්නායු පද්ධතිය පිහිටා ඇත.එමගින් ඔවුන්ට බාහිර සංවේදනයන් පහසුවෙන් මොළයකරා ගැනයෑමට හැකියාව ලැබී ඇත.
  • පෘෂ්ඨවංශීන්ට ස්වසනය සදහා කරමල් හෝ පෙනහළු යුගලයක් පිහිටා ඇත
  • බොහෝ පෘශ්ඨවංශීන්ට වල්ගයක් පිහිටා ඇත.
  • පෘෂ්ඨවංශිකයන් කලල අවස්ථාවලදී මුඛයට පෙර ගුදය නිර්මාණය වේ.
  • පෘෂ්ඨවංශීන්ගේ සිරුර හරි මැදින් දික් අතට බෙදා වෙන්කල හැකි සේ පිහිටයි..[1]

උප විශේෂ[සංස්කරණය]

කශේරුකයෝ[සංස්කරණය]

හැග්ෆිෂ් නම් ආඳා මත්ස්‍යයා

පෘෂ්ඨවංශිකයින් ප්‍රධාන උප විශේෂ තුනකට බෙදෙයි.මේ අතරින් කශේරුකයන් මෙම ප්‍රභේද තුකෙන් එකකට අයත්වේ.මෙම ප්‍රභේද තුනටම අයත් සතුන්ට හිස්කබලක් පිහිටයි.එහෙත් ඇතැම් පෘෂ්ඪවංශිකයන්ට හොදින් වැඩුන කශේරුකාවක් පිහිටා නැත.එහෙත් ඔවුන්ට අසම්පූර්ණව වැඩුන කශේරුකාවක් පිහිටයි.උදාහරණ ලෙස හැංෆිෂ් නමින් හදුන්වන අාඳා මත්ස්‍යයන් විශේෂය දැක්විය හැක.

Most are vertebrates, in which the notochord is replaced by the vertebral column.[2] These consist of a series of bony or cartilaginous cylindrical vertebrae, generally with neural arches that protect the spinal cord, and with projections that link the vertebrae. However hagfish have incomplete braincases and no vertebrae, and are therefore not regarded as vertebrates,[3] but as members of the craniates, the group from which vertebrates are thought to have evolved.[4] However the cladistic exclusion of hagfish from the vertebrates is controversial, as they may be degenerate vertebrates who have lost their vertebral columns.[5]

The position of lampreys is ambiguous. They have complete braincases and rudimentary vertebrae, and therefore may be regarded as vertebrates and true fish.[6] However, molecular phylogenetics, which uses biochemical features to classify organisms, has produced both results that group them with vertebrates and others that group them with hagfish.[7] If lampreys are more closely related to the hagfish than the other vertebrates, this would suggest that they form a clade, which has been named the Cyclostomata.[8]

Tunicata (tunicates, or urochordates)[සංස්කරණය]

Comparison of three invertebrate chordates

A. Lancelet, B. Larval tunicate, C. Adult tunicate
--------------------------------------------------------
1. Notochord, 2. Nerve chord, 3. Buccal cirri, 4. Pharynx, 5. Gill slit, 6. Gonad, 7. Gut, 8. V-shaped muscles, 9. Anus, 10. Inhalant syphon, 11. Exhalant syphon, 12. Heart, 13. Stomach, 14. Esophagus, 15. Intestines, 16. Tail, 17. Atrium, 18. Tunic
Tunicates: sea squirts

Most tunicates appear as adults in two major forms, both of which are soft-bodied filter-feeders that lack the standard features of chordates: "sea squirts" are sessile and consist mainly of water pumps and filter-feeding apparatus; salps float in mid-water, feeding on plankton, and have a two-generation cycle in which one generation is solitary and the next forms chain-like colonies.[9] However, all tunicate larvae have the standard chordate features, including long, tadpole-like tails; they also have rudimentary brains, light sensors and tilt sensors. The third main group of tunicates, Appendicularia (also known as Larvacea) retain tadpole-like shapes and active swimming all their lives, and were for a long time regarded as larvae of sea squirts or salps.[10] The etymology of the term Urochorda(ta) (Balfour 1881) is from the ancient Greek οὐρά (oura, "tail") + Latin chorda ("cord"), because the notochord is only found in the tail.[11] The term Tunicata (Lamarck 1816) is recognised as having precedence and is now more commonly used.[12]

Cephalochordata: Lancelets[සංස්කරණය]

Cephalochordate: Lancelet

Cephalochordates are small, "vaguely fish-shaped" animals that lack brains, clearly defined heads and specialized sense organs.[13] These burrowing filter-feeders compose the earliest-branching chordate sub-phylum.[14][15][16]

Origins[සංස්කරණය]

The majority of animals more complex than jellyfish and other Cnidarians are split into two groups, the protostomes and deuterostomes, the latter of which contains chordates.[17] It seems very likely the 555 million-year-old Kimberella was a member of the protostomes.[18][19] If so, this means the protostome and deuterostome lineages must have split some time before Kimberella appeared—at least 558 million years agoසැකිල්ල:Ma, and hence well before the start of the Cambrian 541 million years agoසැකිල්ල:Ma. The Ediacaran fossil Ernietta, from about 549 to 543 million years agoසැකිල්ල:Ma, may represent a deuterostome animal.[20]

Haikouichthys, from about 518 million years ago in China, may be the earliest known fish.

Fossils of one major deuterostome group, the echinoderms (whose modern members include starfish, sea urchins and crinoids), are quite common from the start of the Cambrian, 542 million years agoසැකිල්ල:Ma.[21] The Mid Cambrian fossil Rhabdotubus johanssoni has been interpreted as a pterobranch hemichordate.[22] Opinions differ about whether the Chengjiang fauna fossil Yunnanozoon, from the earlier Cambrian, was a hemichordate or chordate.[23][24] Another fossil, Haikouella lanceolata, also from the Chengjiang fauna, is interpreted as a chordate and possibly a craniate, as it shows signs of a heart, arteries, gill filaments, a tail, a neural chord with a brain at the front end, and possibly eyes—although it also had short tentacles round its mouth. Haikouichthys and Myllokunmingia, also from the Chengjiang fauna, are regarded as fish.[25][26] Pikaia, discovered much earlier (1911) but from the Mid Cambrian Burgess Shale (505 Ma), is also regarded as a primitive chordate.[27] On the other hand, fossils of early chordates are very rare, since invertebrate chordates have no bones or teeth, and only one has been reported for the rest of the Cambrian.[28]

A consensus family tree of the chordates

The evolutionary relationships between the chordate groups and between chordates as a whole and their closest deuterostome relatives have been debated since 1890. Studies based on anatomical, embryological, and paleontological data have produced different "family trees". Some closely linked chordates and hemichordates, but that idea is now rejected. Combining such analyses with data from a small set of ribosome RNA genes eliminated some older ideas, but opened up the possibility that tunicates (urochordates) are "basal deuterostomes", surviving members of the group from which echinoderms, hemichordates and chordates evolved.[29] Some researchers believe that, within the chordates, craniates are most closely related to cephalochordates, but there are also reasons for regarding tunicates (urochordates) as craniates' closest relatives.[30]

Since early chordates have left a poor fossil record, attempts have been made to calculate the key dates in their evolution by molecular phylogenetics techniques—by analyzing biochemical differences, mainly in RNA. One such study suggested that deuterostomes arose before 900 million years agoසැකිල්ල:Ma and the earliest chordates around 896 million years agoසැකිල්ල:Ma. However, molecular estimates of dates often disagree with each other and with the fossil record, and their assumption that the molecular clock runs at a known constant rate has been challenged.[31][32]

වර්ගීකරණය[සංස්කරණය]

Taxonomy[සංස්කරණය]

A skeleton of the blue whale, the world's largest animal, outside the Long Marine Laboratory at the University of California, Santa Cruz
A peregrine falcon, the world's fastest animal

Traditionally, Cephalochordata and Craniata were grouped into the proposed clade "Euchordata", which would have been the sister group to Tunicata/Urochordata. More recently, Cephalochordata has been thought of as a sister group to the "Olfactores", which includes the craniates and tunicates. The matter is not yet settled.

The following schema is from the third edition of Vertebrate Palaeontology.[33] The invertebrate chordate classes are from Fishes of the World.[34] While it is structured so as to reflect evolutionary relationships (similar to a cladogram), it also retains the traditional ranks used in Linnaean taxonomy.

  • Phylum Chordata
    • †Vetulicolia
    • Yunnanozoon ?
    • Subphylum Tunicata (Urochordata) – (tunicates; 3,000 species)
      • Class Ascidiacea (sea squirts)
      • Class Thaliacea (salps)
      • Class Appendicularia (larvaceans)
      • Class Sorberacea
    • Subphylum Cephalochordata (Acraniata) – (lancelets; 30 species)
      • Class Leptocardii (lancelets)
    • Subphylum Vertebrata (Craniata) (vertebrates – animals with backbones; 57,674 species)
      • Infraphylum incertae sedis Cyclostomata
        • Superclass 'Agnatha' paraphyletic (jawless vertebrates; 100+ species)
          • Class Myxini (hagfish; 65 species)
          • Class Petromyzontida or Hyperoartia (lampreys)
          • Class †Conodonta
          • Class †Pteraspidomorphi
          • Class †Thelodonti
          • Class †Anaspida
          • Class †Cephalaspidomorphi
      • Infraphylum Gnathostomata (jawed vertebrates)
        • Class †Placodermi (Paleozoic armoured forms; paraphyletic in relation to all other gnathostomes)
        • Class Chondrichthyes (cartilaginous fish; 900+ species)
        • Class †Acanthodii (Paleozoic "spiny sharks"; paraphyletic in relation to Chondrichthyes)
        • Superclass Osteichthyes (bony fish; 30,000+ species)
          • Class Actinopterygii (ray-finned fish; about 30,000 species)
          • Class Sarcopterygii (lobe-finned fish: 8 species)
        • Superclass Tetrapoda (four-limbed vertebrates; 28,000+ species) (The classification below follows Benton 2004, and uses a synthesis of rank-based Linnaean taxonomy and also reflects evolutionary relationships. Benton included the Superclass Tetrapoda in the Subclass Sarcopterygii in order to reflect the direct descent of tetrapods from lobe-finned fish, despite the former being assigned a higher taxonomic rank.)[35]
          • Class Amphibia (amphibians; 7,000+ species)
          • Class Sauropsida (reptiles (including birds); 9,000+ species)
          • Class Synapsida (mammals; 5,700+ species)

Phylogeny[සංස්කරණය]

Closest nonchordate relatives[සංස්කරණය]

Hemichordates[සංස්කරණය]

Hemichordates ("half (½) 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.

There are two living groups of hemichordates. The solitary enteropneusts, commonly known as "acorn worms", have long proboscises and worm-like bodies with up to 200 branchial slits, are up to 2.5 metres (8.2 ft) long, and burrow though seafloor sediments. Pterobranchs are colonial animals, often less than 1 millimetre (0.039 in) long individually, whose dwellings are interconnected. Each filter feeds by means of a pair of branched tentacles, and has a short, shield-shaped proboscis. The extinct graptolites, colonial animals whose fossils look like tiny hacksaw blades, lived in tubes similar to those of pterobranchs.[36]

ශල්‍යවර්මියෝ[සංස්කරණය]

මොවුන් අනෙකුත් පෘෂ්ඨවංශීන්ට වඩා තරමක් වෙනස්ය.සමමිතික හැඩයෙන් යුක්ත වන අතර ශරීර ස්වභාවය ඇතැම්විට රෝදයක හැඩයගනී.මොවුන්ට නලාකාර පාද පිහිටා ඇත.බාහිරින් පිහිටි ඝණ කවචයක් පිහිටයි.මොවුන් ගස් ගල් වැනි ආධාරකයක් මත වාසය කරති.මොවුන්ගේ සංචරණ වේගය ඉතා මන්දගාමීය.තම අඩු වලින් අසල පවතින ආහාර ඩැහැගැනීම සිදුකරති.මෙවන් ශල්‍යචර්මීන්ට උදාහරණ ලෙස තාරකා මාළුවා වැනි සතුන් දැක්විය හැක.[37]

See also[සංස්කරණය]

  • Chordate genomics
  • List of chordate orders

References[සංස්කරණය]

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"https://si.wikipedia.org/w/index.php?title=පෘෂ්ඨවංශීන්&oldid=398671" වෙතින් සම්ප්‍රවේශනය කෙරිණි