Phylum-Chordata Includes The Animals Which Are Characterised By The Presence Of A Notochord, A Dorsal Tubular Nerve Cord, Paired Pharyngeal Gills Slits And Post-Anal Tail At Some Stages in Their Life. This Phylum Is Divided Into Two Groups, i.e. Acraniates And Craniates. Acraniata Includes Urochordates And Cephalochordates While Craniata Comprises Of All Vertebrates or Advanced Chordates. Chordates Are Bilaterally Symmetrical, Triploblastic, Coelomate Animals With Organ Level Of Organisation.

        Phylum-Hemichordata Are Worm-Like Marine Animals Which Are Commonly Called Half Chordates. This Is Because They Lack Two Major Characteristics Of Chordates, i.e. Notochord And Post-Anal Tail. There Body Is Cylindrical And Is Composed Of An Anterior Proboscis, A Collar And A Long Trunk. They Have Open Type Circulatory System Including A Dorsal Heart. Respiration Is By Gills And Excretory Organ Is Proboscis Gland. Sexes Are Separate, Fertilisation Is External And Development Is Indirect With A Free-Swimming Tornaria Larva, e.g. Balanoglossus (Acorn or Tongue Worm), Saccoglossus, etc.

        Subphylum-Urochordates And Subphylum-Cephalochordates Along With Phylum-Hemichordata Are Often Referred To As Protochordates or Primitive Chordates. These Are Considered As The Connecting Link Between True Invertebrates And Vertebrates.

        Urochordates Are Also Called Tunicates, As There Body Is Covered With Test or Tunic. They Have Notochord, Which Is Present In Larval Stage And Restricted To Tail And Show Retrogressive Metamorphosis. These Are Herdmania, Ascidia, etc. While Cephalochordates Have Notochord Persistent Throughout Life And Extend, Anteriorly Upto The Tip Of The Snout, e.g. Amphioxous.

        Subphylum-Vertebrata Posses Notochord During Their Embryonic Period Which Is Replaced By Cartilaginous or Bony Vertebral Column In Adult. Animals Have Ventral Muscular Heart (With Two, Three or Four Chambers), Kidneys (Excretory Organs) And Paired Appendages (Fins/Limbs). Vertebrata Is Further Divided Into Two Groups, i.e. Agnatha (Without Jaws) And Gnathostomata (With True Jaws).

        Agnatha Comprises Of An Extinct Class-Ostracodermi (e.g. Pterspis) And A Living Class-Cyclostomata (e.g. Lamprey And Hagfish). Cyclostomes Are Marine Fishes, Which Migrates To Freshwater For Spawning. These Are Ectoparasites On Some Fishes And Have Gill Slits For Respiration. They Have Sucking Type Circular Mouth Without Jaws. Their Endoskeleton Is Cartilaginous.

        Subclass-Pisces Contains True Fishes, Which Are Divided Into Class-Chondrichthyes (Cartilaginous Fishes), Class-Osteichthyes (Bony Fishes) And An Extinct Class Placodermi.

        Class Chondrichthyes Are Fishes With Ventral Mouth, Heterocercal Tail, Placoid Scales, 5-7 Pair Of Naked Gills, Cloacal Apertures Present, Exhibit Sexual Dimorphism, e.g. Scoliodon (Dogfish And Shark), Torpedo (Electric Ray), Pristis (Sawfish), Etc.

        Class-Osteichthyes Are Fishes With Terminal Mouth, Homocercal Tail With Ctenoid, Ganoid Or Cycloid Scales, Bladders With Four Pairs Of Gills Covered With Operculum. Anus And Genital Apertures Separate, Sexual Dimorphism Is Rare, e.g. Labeo (Rohu), Catla, Hippocampus (Sea Horse), Exocoetus (Flying Fish), Anabas (Climbing Perch), Etc.

        Super-Class-Tetrapoda Are Animals With Limbs. They Respire By Gills, Lungs And Skin. These Are Further Categorised Into Four Classes, i.e. Amphibia, Reptilia, Aves And Mammalia.

        Class-Amphibia Are The First Animals To Attempt Transition From Aquatic To Terrestrial Like. Most Of Them Have Two Pairs Of Limbs. Their Body Is Divisible Into Head And Trunk, While Tail Maybe Present In Some. They Have Dicondylic Skull, Smooth, Glandular Skin. Eyes With Eyelids And A Tympanum Represent Ear. These Are Cold-Blooded Animals And Perform Respiration By Gills, Lungs And Through Skin. Their Heart Is Three-Chambered. Alimentary Canal And Urinogenital Tracts Open Into A Common Chamber Called Cloaca, Which Opens To The Exterior. They Are Oviparous And With Indirect Development, e.g. Bufo (Toad), Rana (Frog), Salamandra, Ichthyophis (Limbless Amphibia), etc.

        Class-Reptilia Includes Mostly Terrestrial Animals And Their Body Is Covered By Dry And Cornified Skin, Epidermal Scales Or Scutes. Most Have Two Pairs Of External Limbs, Perform Creeping And Crawling Type Of Locomotion. Heart Is Three-Chambered But It Is Four Chambered In Crocodiles. Reptiles Are Poikilotherms. There Sexes Are Separate And They Show Internal Fertilisation. They Are Oviparous With Direct development, e.g. Chelone (Turtle), Testudo (Tortoise), Chameleon (Tree Lizard), Hemidactylus (Wall Lizard), Crocodiles, Snakes, etc.

        Class-Aves Includes Birds Having A Unique Feature, i.e. Having Feathers On Their Body. They Are Warm-Blooded Animals With Their Forelimbs Modified Into Wings And Hindlimbs Have Scales, Modified For Walking, Swimming Or Clasping The Tree Branches. They Possess Horny Beak. Bones Are Pneumatic, i.e. Hallow Having Air Cavities. Bone Marrow Is Absent. Skin Is Dry Without Glands Except Oil Gland Or Green Glands Which Are Present At The Base Of The Tail. There Heart Is Completely Four-Chambered. Birds Exhibit Double Respiration By Lungs. They Also Produce Different Sounds Through Syrinx (Sound Box) Present In  Them. Reproduction In Them Is Similar To That Of Reptiles, e.g. Corvus (Crow), Columba (Pigeon), Neophron (Vulture), etc.

        Class Mammalia Are The Most Advanced Group Of Chordates Having Unique Features Like Mammary Glands, Hairs On Their Body, External Pinna, Jaw With Teeth And Vivipary. They Are Warm-Blooded Land Vertebrates. They Perform Respiration By Lungs. Heart Is Completely Four-Chambered. They Have Quadrated Limbs But Few Species Like Whales And Dolphins Are Without Limbs. Sexes Are Separate And Fertilisation Is Internal. They Are Viviparous In Kangaroo, Camel, Monkey, Blue Whale, Human, etc.   

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