• Paired fins and gills present.
  • Skin with dermal scales.




  • Primitive jawed fishes with transverse articulation between skull and shoulder girdle.
  • Full-sized functional gill arch preceded by hyoid arch.
    Example :- Extinct jawed fishes such as Climatius, Acanthodes,
    Coccosteus, Pterichthyodes.

CLASS 2 :- CHONDRICHTHYES (Elasmobranchi)

Natural history

Skeleton is made of cartilage. Most sharks and rays are marine, but a few live in tropical rivers above salt water. Sharks live in open water, and rays on the bottom. The sharks are predaceous, active swimmers and feed on small fishes. Sharks and rays are used as human food in many countries. Shark liver contains much oil rich in vitamin A. Sharks are also a nuisance to fishermen, because they tear nets and steal captured fishes. Large sharks may capsize small boats and injure or kill fishermen. Living fishes probably exceed 20,000 species, an abundance and diversity unequalled among all other vertebrates. They originated 530 million years ago allowing vast span of time for evolutionary divergence and for origin and extinction of major phyletic lines.

  • Cartilaginous fishes (Skeleton of cartilage).
  • Scales placoid. Ist gill slit a spiracle.
  • Mouth and 2 nostrils ventral.
  • Males with claspers.
  • Ova few, large, with much yolk. Cleavage meroblastic.

Sub-class A :- Selachii

  1. Sharks. Spiracle behind each eye.
  2. Multiple gill slits on either side protected by individual skin flaps.

Order 1. Squaliformes or Pleurotremata

    • Body typically spindle-shaped.
    • 5 to 7 pairs of lateral gill slits.
    • 2 dorsal and I caudal fin. Pectoral fins moderate.
      Example :- Sharks. Scoliodon, Mustelus, Sphyrna, Stegostoma, Scyllium, Rhineodon, Sauatina and Chiloscyllium.

Order 1. Squaliformes or Pleurotremata

  • Sub-class B. Brachyodonti

Order 1. Eubrachydonti

    • Example :- Extinct fishes.

Order 2. Holocephali

    • Gill slits covered by fleshy operculum and a single gill opening.
    • Tail whip like, spiracles, cloaca and scales absent.
      Example :- Hydralogus (= Chimaera)


Natural history

Bony fishes occur from polar seas to equator, from surface to depths in waters more than 12,000 feet deep. They live variously in open water, sandy, rocky or muddy bottoms, in crannies of reefs, in saline bays, estuaries, fresh alkaline rivers, lakes and cave water, and hot and cold springs. Most fishes migrate from salt water to fresh-water (anadromous) and fresh-water to salt water (catadromous) for spawning. They are important human protein food. The class Osteichthyes comprises of bony fishes, which are famous for their bony skeleton, scales, plates, air bladder and advanced brains. Various fishes are found in different types of water such as fresh-water, brackish water, salt water, warm water and cold water

  • Skeleton chiefly of bone (Cartilage in sturgeon)
  • Skin with many mucous glands with embedded bony dermal scales (Cycloid, ctenoid or ganoid).

Sub-class A. Actinopterygii

  • Ray-finned fishes. Paired fins thin, broad, without fleshy basal lobes and supported by dermal fin rays.
  • Nostril not connected to mouth cavity except in stargazer.

Superorder I. Chondrostei

  • Primitive ray-finned fish or cartilaginous ganoids.
  • Tail fin heterocercal.

Order 1. Polypteriformes

    1. Body slender. Scales thick ganoid and rhombus.
    2. 8 or more dorsal fins, each preceded by a spine. Caudal fin arrow shaped.
      Example :- Polypterus

Order 2. Acipenseriformes

    1. Snout long or paddle shaped.
    2. 8 rows of bony ganoid scutes or body naked with few vestigeal scales.
      Example :- Acipenser, Polyodon.

Superorder II. Holostei

  • Intermediate forms between Chondrostean and teleost fishes.
  • Scales ganoid. Ray finned fishes.

Order 1. Amiiformes

    1. Snout normal. Dorsal fin long. Tail fin short.
    2. Scales thin, Overlapping and cycloid.
      Example :- Amia (bow fin).

Order 2. Lepidostiformes

    1. Snout and body elongated.
    2. Scales rhomboidal, ganoid, in oblique rows.
      Example :- Lepidosteus.

Superorder III. Teleostei

  • Light skeleton. Density of bones reduced. Ganoid scales reduced or absent.
  • Swim bladder hydrostatic.
  • Modem ray-finned fishes

Order 1. Clupeiformes

    1. Silvery compressed marine fishes. Scales cycloid.
    2. Air bladder connects to pharynx. Tail fin homocercal. Pelvic fin abdominal.
      Example :- Clupea, Salmo, Tarpon, Sardinops and Esox.

Order 2. Ostariophysi or Cypriniformes

    1. Air bladder communicates to pharynx.
    2. Weberian ossicles between air bladder and internal ear.
      Example :- Characius, Electric eels, Suckers, Minnows, Catfishes, Lobeo, Mystus, Notopterus, Electrophorus, Cyprinus, Barbus, Arius,
      Clarius, Wallago, Catla, Heteropneustes

Order 3. Ophiocephaliformes

    1.  Head depressed with plate like scales.
    2.  Air bladder long without duct
    3. Accessory respiratory organs present.
      Example :- Ophiocephalus (Channa)

Order 4. Anquilliformes

    1. Eels. Body long and slender.
    2. Dorsal, caudal and anal fins continuous. Pelvic fins absent.
      Example :- Anguilla, Muraena and Amphipnous.

Order 5. Beloniformes or Synentognathi

    1. Pectoral fins large and high on body.
    2. Scales cycloid.
      Example :- Flying fishes. Exocoetus, Belone, Cypselurus, Scomberesox

Order 6. Syngnathiformes or Solenichthyes

    1. Snout tubular with suctorial mouth. Swim bladder closed.
    2. Bony rings or protective scales over body.
      Example :- Hippocampus, Hemirhamphus, Xenantodon, Syngnathus, Fistularia.

Order 7. Perciformes or Percomorphi

    1. Trout perches. Dorsal and anal fins preceded by 1 or 4 spines. Pectoral fin well up on sides.
    2. Weberian apparatus absent. Air bladder without duct.
      Example :- Anabas (Climbing perch), Perea, Lates, Huga, Lepomis, Scomber.

Order 8. Pleuronectiformes

    1. Asymmetrical head with both eyes on one side.
    2. Body strongly compressed. Dorsal and anal fins fringing the body.
      Example :- Pleuronectes, Hippoglossus, Achirus, Synaptura, Solea.

Order 9. Tetradontiformes (= Plectognathi)

    1. Body shape variable. Some globose.
    2. Strong jaws with sharp teeth. Scales generally spiny.
      Example :- Ostracion, Diodon, Tetradon, Molamola, Fieraster.

Order 10. Echeiniformes

    1. Dorsal fm modified into flat, oval, adhesive disc on upper surface of head.
    2. Scales cycloid. Air bladder absent.
      Example :- Echeneis or Remora

Order 11. Lophiiformes

    1. Dorsal fin like flexible spines.
    2. First dorsal spine located on head with tip modified into lure. Mouth wide.
      Example :- Lophius, Antennarius

Sub-class B. Sarcopterygii (= Choanichthyes)



The lung fishes deviated early from the osteolepids with whom earlier lung fishes resembled in having’ heavy scales, heterocercdl tail, lobed fins and well ossified skull. The modern lung fishes have changed from their ancestors and are represented today by Protopterus, Lepidosiren and Neoceratodus. Their jaws are short, teeth form crushing plates. internal nares present, skeletal system reduced, tail diphycercal and one or two lungs (air bladders) functional. The three genera are distributed discontinuously. This is the ancient group which can be regarded as Devonian offshoot from the crossopterygian stem. They first appeared in mid Devonian, flourished moderately in the Permian and Triassic and then became rate. These show combination of primitive and advanced characters.

  • Paired fins leg like or lobed with a fleshy bony central axis covered by scales.
  • Dorsal fin 2, caudal fin heterocercal with an epichordal lobe.
  •  Olfactory sacs usually connected to mouth cavity by internal nostrils or choannae.

Order 1. Crossopterygii

    1.  Lobed finned fishes.
    2. Premaxillae and maxillae present. Teeth well developed and conical.
      Example :- Latimeria

Order 2. Dipnoi

    1. Lung fishes. Air bladder paired lung like.
    2. Median fins continuous to form diphycercal tail.
      Example :- Lepidosiren, Protopterus, Neoceratodus

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