On the basis of presence of notochord and cranium (brain box) phylum chordata can be divided into two groups-Acraniata (Protochordata) and Craniata (Euchordata).
Group A:
Acrania (Protochordata) (Gr., a, absent; kranion, head) or(Gr. protos, first; chorde, cord).
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(i) All marine, small, primitive or lower chordates.
(ii) Head, Cranium, vertebral column, jaws and brain are absent.
(iii) About 2000 species.
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Subphylum 1-
Hemichordata (Gr; hemi, half; chorda, cord)
(i) Body divided into 3 regions- proboscis collar and trunk.
(ii) Notochord doubtful, short, confined to proboscis and nonhomologous with that of chordates.
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Class 1:
Enteropneusta (Gr, enteron, gut; pneustos, breathers).
(i) Body large sized and worm like.
(ii) Gill slits numerous. Intestine straight.
Examples:
Balanoglossus, Saccoglossus.
Class 2:
Pterobranchia (Gr., pteron, feathers; branchion, gill)
(i) Body small sized and compact.
(ii) Gill slits one pair or none.
(iii) Intestine U-shaped.
(iv) Pterobranks, 20 species.
Examples:
Cephalodiscus, Rhabdopleura etc.
Subphylum 2:
Urochordata or Tunicata (Gr. Oura, a tail; L., chorda, cord)
Notochord and nerve cord only in tadpole like larva.
Adult sac-like, enclosed in a protective tunic.
Class 1:
Ascidiaceae
(i) Sessile tunicates with scattered muscles in tunic.
(ii) Solitory colonial or compound.
(iii) Gill clefts numerous.
(iv) Commonly known as sea squirts 1,200 species.
Examples:
Herdmania, Ciona, molgula etc.
Class 2:
Thaliaceae
(i) Free swimming or pelagic tunicates with circular muscles in tunic.
(ii) Sometimes colonial salps or chain tunicates 30 species.
Examples:
Salpa, Doliolum etc.
Class 3:
Larvacea
(i) Tiny transparent, free floating.
(ii) Adults retain many larval features including tail.
(iii) Only two gill slits 30 species.
Examples:
Oikopleura.
Subphylum 3:
Cephalochordata (Gr. kephale, head; L…Chorda, cord)
(i) Nerve cord and notochord present throughout life along entire length of the body.
Class 1:
Leptocardii
(i) Body fish like segmented with distinct myotomes and numerous gill-slits.
(ii) Free swimming and burrowing.
(iii) Lancelets 30 species.
Examples:
Branchiostoma, Asymmetron.
Group B:
Craniata (Euchordata)
(i) Aquatic or terrestrial, usually large sized.
(ii) Higher chordates with distinct head, a vertebral column, jaws and brain with in brain box or cranium.
Subphylum 4:
Vertebrata (L. Vertebratus, backbone)
(i) Notochord is supplemented or replaced by a vertebral column or backbone.
(ii) Vertebral column consists of several overlapping vertebrae.
(iii) Body divisible into head, neck and tail.
(iv) Usually dioecious vertebrates.
(v) Having 46,500 species.
(i) It includes several extinct orders of ancient primitive heavily armoured first vertebrates.
Division 1:
Agnatha (Gr, a, not; gnathes, jaw)
Class 1:
Ostracodermi ( Gr, Ostrakon, Shell; derma,skin).
(i) It includes several extinct orders of ancient primitive heavily armoured first vertebrates.
Class 2:
Cyclostomata (Gr. Cyclos, circular; stoma, mouth)
(i) Body eel-shaped without scales, jaws and lateral fins.
(ii) Mouth rounded and suctorial. Gills 5-16 pairs.
(iii) Parasites and scavengers; 45 species.
Example:
Lampreys (petromyzon) and hagfishes (Myxine).
Division II:
Gnathostomata (Gr, Gnathos, jaws; stoma, mouth).
It is divided into two superclasses-
Super class 1:
Pisces (L, piscis, fish)
(i) They are fish like aquatic forms.
(ii) Having paired and unpaired fins, gills and scaly skin.
Class 3:
Placodermi
(i) Several extinct orders of primitive fishes.
(ii) Placoderms.
Examples:
Climatius, Dinichthys.
Class 4:
Chondrichthyes (Gr, chondros, cartilage, ichthyes, fish)
(i) Mostly marine.
(ii) Cartilaginous endoskeleton.
(iii) Skin with placoid scales.
(iv) Operculum absent.
(v) Pelvic claspers in male are found.
(vi) About 600 species.
Examples:
Scoliodon, chimaera etc.
Class 5:
Osteichthyes (Gr, Osteon, bone; ichthyes, fish)
(i) Freshwater and marine. Bony fishes.
(ii) Endoskeleton mostly bony.
(iii) Skin having various types of scales except placoids.
(iv) Operculum present.
(v) About 20,000 species.
Examples:
Labeo (rohu), Protopterus, Hippocampus etc.
Super Class 2:
Tetrapoda (Gr, tetra, four, podos, foot)
(i) Land vertebrates with two pairs of pentadactyle limbs.
Class 6:
Amphibia (Gr, amphi, both; bios, life)
(i) Larval stage usually aquatic and breathes by gills.
(ii) Skin moist, glandular and with no external scales.
(iii) Heart 3 chambered.
(iv) Approximately 2,500 species.
Examples:
Rana, Bufo, Hyla etc.
Class 7:
Reptilia (L, reptilis, creeping)
(i) Terrestrial tretrapods.
(ii) Skin dry covered by ectodermal horny scales or bony plates.
(iii) Heart incompletely 4 chambered.
(iv) Cold blooded respiration by lungs.
(v) About 7,000 species.
Examples:
Hemidactylus, Uromastix, Naja, Sphenodon etc.
Class 8:
Aves (L, avis, bird).
(i) Typically flying vertebrates.
(ii) Body is covered by feathers.
(iii) Forelimbs modified into wings.
(iv) Heart 4 chambered.
(v) Birds. About 9,000 species.
Examples:
Stuthio, Columba, Grallus etc.
Class 9:
Mammalia (L. mamma, breast)
(i) Body is covered by hairs.
(ii) Skin glandular.
(iii) Females with mammary glands which secrete milk for the young’s.
(iv) Heart 4 Chambered.
(v) Warm blooded, air breathing vertebrates.
(vi) About- 4,500 species.
Examples:
Echidna, Macropus, Rattus, Homo etc.