Name the two major groups of fishes based on the composition of endoskeleton.

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Hint: Fishes are aquatic. Paired and unpaired fins are present. Respiration occurs typically by gills. Each eye has a well-developed nictitating membrane, eyelids are absent. Lateral line sense organs are present. Heart is two chambered (S-shaped, venous heart) with sinus venosus and conus arteriosus.

Complete step-by-step answer:
Based on the endoskeleton fishes are of two types namely:
A. Chondrichthyes:
> They are marine animals with streamlined bodies.
> Endoskeleton is made up of cartilage.
> Notochord is persistent throughout life.
> Their jaws are powerful. Mouth is located ventrally.
> Teeth are modified placoid scales which are backwardly directed.
> Kidneys are mesonephric.
> Skin is tough, containing minute placoid scales.
> Gill slits are isolated and without operculum (gill cover). Except in the Chimaeras.
> Air bladder or swim bladder is absent, so they have to swim constantly to avoid sinking.
> Heart has two chamber one auricle and one ventricle).
> Organisms belong to this class are cold-blooded (poikilothermous) i.e. they lose the capacity to regulate their body temperature.
> Organisms are predaceous, some of them have electric organs (e.g. Torpedo) and some possess poison sting (e.g. Trygon).
B. Osteichthyes :-
> It comprises both marine and freshwater fishes with streamlined bodies.
> Endoskeleton is made up of bone
> Notochord is persistent throughout. Life 4Mouth is located terminally.
> Teeth are present in the jaws.
> Skin is covered with cycloid/ctenoid scales.
> They have four pairs of gills which are protected by an operculum on each side.
> Air bladder is present which regulates buoyancy.
> Heart is two-chambered (one auricle and one ventricle). Lung fishes have a three chambered
> Caudal fins are usually homocercal.
> Kidneys are mesonephric. Ammonia is chief nitrogenous waste. Cloaca is absent
> There are 10 pairs of cranial nerves present.

Note: Fish belong to kingdom vertebrata. The notochord in vertebrates is replaced by the vertebral column. It consists of five sub phylum fishes, amphibians, reptiles , birds and mammals. Fishes have two chambered hearts to possess double circulation.