General characteristics of fish

• Aquatic, cold-blooded vertebrates.
• The body of fish is generally fusiform and streamlined but in globiform, puffers the body is globe shape and in eels is of serpentine form. Despite many variations in shape, the ground plan of the bodybilaterallyeral symmetrical with a prominent lateral line system.
• The body of the fish is generally covered by tough skin armored by a variety of scales with anterior cephalization.
• The appendages of the fish comprise the fins, which are generally paired (pectoral and pelvic fins) unpaired dorsal, anal and caudal fins. All are supported by dermal fin rays.
• The mouth is generally situated anteriorly in the head and the anus is in the second half of the overall length of the individual behind the bases of the pelvic fins & just in front of the anal fin.
• Respiratory organs are generally in the form of gills and other accessory respiratory organs.
• Nostrils are paired and do not open into the pharynx, except in lung-fishes and lobed fishes.
• The skeletal of the fishes are in the form of notochord, connective tissues, bones, cartilage & non-bony scales.
• The digestive tract of fish is well developed (with the mouth, oral cavity, pharynx, esophagus, stomach, intestine & anus).
• The heart is generally a valved pump with one auricle & one ventricle, which is of the venous type that forces the blood forward towards the gills for aeration.
• The kidneys of fish are paired, longitudinal structures that lie above the body cavity.
• The brain is well developed with ten pairs of cranial nerves.
• The middle ear is completely absent but the internal ear with well-developed semi-circular canals.
• Sexes are separate (male with claspers).
• Some are viviparous and many are oviparous.
• Generally, fertilization is external (In some cases fertilization is internal).