1.2. GENERAL CHARACTERISTICS OF FISH

characteristics 
All the fishes are built on a fundamental plan and possess following characters in general.
1. Fishes are aquatic, cold-blooded (poikilothermic) vertebrates.
2. The body of fish is generally fusiform and streamlined, but in puffers (e.g. Tetraodon spp.) the body is globe shaped and in eels (e.g. Anguilla spp.) is of serpentine form. In spite of many variations in shape, the ground plan of the body is bilaterally symmetrical with prominent lateral line system.
3. The body of the fish is generally covered by tough skin armored by variety of scales.
4. The appendages of the fish comprised of the fins, which are generally paired (pectoral and pelvic fins) and unpaired (dorsal, anal and caudal fins). All fins are supported by dermal fin rays. These fins constitute the main locomotory organ.
5. Mouth is generally situated anteriorly in the head, and the anus is in the second half of the overall length of the body just in front of the anal fin.
6. Respiratory organs are generally in the form of gills and other accessory respiratory organs.
7. Nostrils are paired and do not open into the pharynx, except in lung-fishes and lobed-fishes.
8. Skeleton is in the form of notochord, connective tissues, bones, cartilages, and non-bony scales.
9. The digestive tract is well developed (with mouth, oral cavity, pharynx, oesophagus, stomach, intestine, and anus).
10. Heart is generally two-chambered, pump with one auricle and one ventricle. The circulation is a single circuit, i.e. heart-gills-body-heart.
11. The kidneys are paired, longitudinal structures that lie above the body cavity close to the vertebral column.
12. Brain is well developed with ten pairs of cranial nerves.
13. External ear is completely absent but internal ear with well-developed semi-circular canals.
14. Sexes are separate. Most are oviparous and some are viviparous or ovoviviparous.
15. Fertilization is generally external, i.e. the male releases sperm over eggs as they escape from the body of the female.
16. Embryonic membranes are absent.
17. Eggs are large with much yolk.
18. Development is direct, i.e. without any metamorphosis.