Fishes are first evolved in silurian period of palaeozoic era. They increased their number in Devonian period. Devonian Period Is Called Age of Fishes. 
Fishes are first jawed vertebrates. They live in water. They are cold blooded vertebrates.
Fishes-Characters:
1. Fishes are aquatic; cold blooded vertebrates.
2. Their body is divisible into head, trunk and tail. Neck is absent.
3. It has a spindle shaped body. It is helpful in swimming.
4. The body is covered by scales. They are placoid scales, cycloid scales, ctenoid scales, ganoid scales etc.
5. Respiration is by gills. Gills are the extensions of the pharynx. In the elasmobranches fishes, the gills will open separate. In bony fishes the gill slits are covered by operculum.
6. On the head a pair of nostrils are present internal nostril are absent. In Dipnoi Internal nostrils are present
7. On the head a pair of eyes is present.
8. On the lateral sides of the body LATERAL LINE SENSE organs are present .They detect the pressure changes of water.
9. The body shows paired and unpaired fins. Pelvic and pectoral fins are paired. Dorsal and ventral fins are unpaired. They maintain balance in water. They are useful for locomotion. 
10. The digestive system is well developed. In the intestine of shark scroll valve Is present. The nervous system contains brain and spinal cord. Brain is small & it will not occupy the entire cranial cavity.
11. 10 pairs of cranial nerves are present.
12. Kidneys are mesonephrlc.
13. Urinary bladder is absent
14. The skeleton of some fishes is made by cartilage. They are called cartilage fishes. In some fishes the skeleton is made by bone. They are called bony fishes.
15. In some fresh water fishes accessory respiratory organs are present. They will take up aerial respiration.
16. Sexes are separate. In male sharks claspers are present.
17 Many fishes are oviparous. Some fishes are viviparous.
18. In the development of fishes amnion is not developed. Hence these animals are called anamniotes
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