Question

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

Answer 1

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.