Classification of Chordata



(Primitive chordates without head and vertebral column)

Subphylum HEMICHORDATA, Balanoglossus,Cephalodiscus, Rhabdopleura, primitive and doubtful chordates, now classified under non-chordates after echinoderms.

Subphylum UROCHORDATA, Herdmania, Salpa, Doliolum, Pyrosoma, Oikopleura sedentary or planktonic tunicates in which chordate characters manifest in the larval stage.

Subphylum CEPHALOCHORDATA, Amphioxus, Asymmetron, typical chordates having chordate characters in the larval as well as adult stage.


(Chordates with skull, with 54,000 species of true chordates)

Subphylum VERTEBRATA, chordates with head, brain and vertebral column.

Superclass AGNATHA, 90 species of paraphyletic group of jawless fishes, which were also the first vertebrates. Living forms are elongated, scaleless, slimy parasites and scavengers that include lampreys and hagfishes. They have no paired fins.

Class OSTRACODERMI, extinct shelled jawless fishes of Ordovician period. Cephalaspsis.

Class CYCLOSTOMATA, jawless fishes of today, without scales and paired fins.

Order Myxinoidea: the hagfishes, 40 species. Myxine, Bdellostoma, Eptatretus.

Order Petromyzontia: lampreys, 41 species, parasitic on other fishes. Petromyzon.

Superclass GNATHOSTOMATA, vertebrates with jaws that are modified gill arches and paired appendages. They include cartilaginous fishes, bony fishes and tetrapods.


Class PLACODERMI, extinct group of spiny sharks. Climatius.

Class CHONDRICHTHYES – cartilaginous fishes that have cartilaginous skeleton, ventral mouth, placoid scales, heterocercal tail fin and 5 pairs of gill slits.

Subclass Elasmobranchii— 850 species of sharks, rays and skates.

Subclass Holocephali— 30 species of ratfish (Chimaeras).

Class OSTEICHTHYES – 20,000 species, bony fishes. Skeleton contains bone, four pairs of gills, covered with operculum. Possess swim bladder or lung.

Subclass Actinopterygii, ray-finned fishes.

Superorder Chondrostei, 25 species of sturgeons, bichirs and paddlefish.

Superorder Holostei, which includes Lepistosteus 7 species and Amia 1 species.

Superorder Teleostei, includes 20,000 species of bony fishes, such as tarpon, herring, perch, etc.

Subclass Crossopterygii, includes 2 species of coelacanth (Latimeria).

Class CHOANICHTHYS (=DIPNOI), has 6 species of lungfishes under three genera, namely, Protopterus, Lepidosiren and Neoceratodus.



Order Gymnophiona (Apoda) —165 species of burrowing caecilians, elongated bodies, limbless, dermal scales embedded in annular folds of skin.

Order Caudata (Urodela) —425 species of salamanders, tailed amphibians, usually with two pairs of limbs.

Order Anura (Salientia) —4300 species of frogs and toads, tail-less amphibians, long hind limbs for jumping, head and trunk fused. They have sound producing and hearing organs.

Class REPTILIA, 7800 species, turtles, crocodiles, lizards, snakes, etc.

They have internal fertilization and produce large cleioid eggs with leathery shells and are ectotherms. Body covered with epidermal scales, vertebrae procoelous.

Order Chelonia, 300 species of turtles and tortoises having bony shell on the body.

Superorder Lepidosauria, lizard-like with acrodont or pleurodont dentition.

Order Rhynchocephalia (=Sphenodontia), 2 species of tuatara (Sphenodon) in New Zealand. They have acrodont teeth, amphicoelous vertebrae and a parietal eye.

Order Squamata, with pleurodont teeth, procoelous vertebrae, without third eye.

Suborder Lacertilia, includes 4000 species of lizards.

Suborder Ophidia, includes 2700 species of snakes.

Superorder Archosauria, includes modern crocodiles and extinct dinosaurs.

Order Crocodilia, 23 species of alligators, crocodiles and gavials

 Class AVES, 9100 species.

Birds being feathered bipeds have internal fertilization and lay hard-shelled eggs and are endotherms. Nearly every anatomical feature is related to ability to fly. They are the only animals with feathers that are modified from reptilian scales.

 Class MAMMALIA, 4,500 species.

Mammals evolved in the late Triassic, the time dinosaurs first appeared and diversified greatly following the extinction of dinosaurs during the Coenozoic. Characteristics include hairs for protection and from heat loss; mammary glands; heterodont teeth; endothermy; 4 chambered heart etc.

Subclass PROTOTHERIA, Order Monotremata, egg laying mammals having 6 species in Australia and New Zealand. No teeth and true mammary glands.

Subclass METATHERIA, Order Marsupialia, 275 species of marsupials that have brief gestation period after which the embryo develops in a pouch. They have prolonged lactation and parental care. Marsupials include: opossum, kangaroo, koala, Tasmanian devil, wombat, etc.

Subclass EUTHERIA, 16 orders which include 4700 species of placental mammals that are truly viviparous, with a placenta for gas and nutrient exchange between the mother and foetus. They also have true mammary glands.