- They are the first jawed vertebrates and the study of fish is called Ichthyology.
- They are aquatic (may be fresh water or marine).
- Fish are cold blooded animals (poikilothermic).
- Their body is spindle shaped or streamlined to enable them to move rapidly in water.
- Body is covered with water proof dermal scales; which may be of placoid, cycloid, ctenoid or ganoid type.
- Fins are the locomotary organs. Pectoral and pelvic fins are paired whereas dorsal and caudal fins are unpaired.
- Eyes are without eyelids. External and middle ears are absent; only internal ear is present.
- They consist of 4-7 pairs of gills for respiration which may be naked or covered with an operculum.
- Heart is two chambered, known as a venous heart. Heart consists of only the impure blood.
- Circulation of blood is uncircuit with the presence of renal and hepatic portal systems.
- RBCs are nucleated.
- Endoskeleton is made up of either cartilage or true bones.
- Vertebrae in fish are amphicoelus, in which centrum is concave at both the surfaces.
- Skull is monocondylice with only one occipital condyle.
- 10 pairs of cranial nerves are present.
- They have a lateral line sense organ system consisting of rheoreceptors.
- They may be ammonotelic or ureotelic with mesonephric kidneys.
- Fish are unisexual. Fertilization may be internal or external.
- Eggs are of mesolecithal or megalecithal type.