General characteristics of fish

• Aquatic, cold-blooded vertebrates.
• Body of fish is generally fusiform and streamlined but in globiform, puffers the body is globe shape and in eels is of serpentine form. Inspite of many variations in shape the ground plan of body is bilateral symmetrical with prominent lateral line system.
• The body of the fish generally covered by tough skin armoured by variety of scales with anterior cephalization.
• The appendages of the fish comprise of the fins, which are generally paired (pectoral and pelvic fins) unpaired dorsal, anal and caudal fins. All supported by dermal fin rays.
• Mouth 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 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.
• Skeletal of the fishes are in form of notochord, connective tissues, bones, cartilage & non-bony scales.
• The digestive tract of fish is well developed (with mouth, oral cavity, pharynx, oesophagus, stomach, intestine & anus).
• Heart is generally valved pump with one auricle & one ventricle, which is of 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.
• Brain is well developed with ten pairs of cranial nerves.
• Middle ear is completely absent but 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 carps fertilization is internal).