1. Fishes are cold blooded vertebrates which are more completely adapted to aquatic life than any other vertebrates.
2. The body is streamlined to facilitate the locomotion through water.
3. The skin is covered with scales, dermal denticles.
4. Tail is muscular and is used for propulsion
5. Two pair of limbs are modified into fins provided with fin-rays. The main organ of locomotion is tail.
6. There are also median, and anal fins. They regulate swimming.
7. Endoskeleton may be cartilaginous or bony
8. The typical vertebra is amphicoelous or biconcave.
9. The heart is two-chambered and contains only venous or deoxygenated blood.
10. The respiration is done by gills associated with pharyngeal gill slits which are persistent throughout life.
11. The olfactory lobes and cerebellum are generally well developed.
12. The nostrils do not open into the pharynx because they are not having a respiratory function. There is neither external nor the middle ear but only the inner ear is present.
13. Sexes are separate. Fertilization is internal or external. Oviparous or Viviparous, The development is without metamorphosis.
14. The presence of lateral line sense organs and streamlined body are the special characters.
15. Accessory respiratory organs may be present.
16. Embryonic membranes in the embryo stage are absent.
