List out eight characteristics that helps distinguish a fish from the other vertebrates.

Explanation

General characters :
1. Fishes are completely aquatic poikilothermic (cold blooded) animals.
2. Body of a fish is usually streamlined and differentiated into head, trunk and tail.
3. Exoskeleton consists of mesodermal scales or bony plates. A few are scaleless.
4. Endoskeleton may be cartilaginous or bony. Skull is monocondylic. Vertebrae are amphicoelous. Centrum is concave at both anterior and posterior faces.
5. Locomotion is assisted by unpaired (median and caudal) fins along with paired (pectoral and pelvic) fins.
6. Mouth is ventral or terminal. Teeth are usually acrodont, homodont and polyphodont.
7. Exchange of respiratory gases is performed by the gills. Heart is two chambered.
8. Kidneys are mesonephric. Fishes are mostly ammonotelic and some are ureotelic. (cartilaginous fishes).
9. Cranial nerves are 10 pairs, Meninx primitiva is the only .meninx. enveloping the central nervous system.
10. Internal ear consists of three semicircular canals. Lateral-line sensory system (to detect movement and vibration in the surrounding water) is well-developed. Eyes are without eyelids and each eye ball is protected by a nictitating membrane.
11. Sexes are separate. Fertilization is internal or external. Development may be direct or indirect.