Salient features :
- Aquatic (both marine and fresh water), solitary free swimming, free living or ectoparasite or commensal.
- The body is streamlined (Boat shaped) and divisible into head, trunk and tail.
- Presence of Paired fins (Pectoral fins and Pelvic fins) and Unpaired fins (Caudal fins, dorsol fin, ventral fin). The fins are supported by fin rays (endoskeleton)
- Body is covered by mesodermal scales. They are placoid / ctenoid / cycloid / rhomboid type.
- They are poikilothermic.
- Presence of 5 to 7 pairs of gill slits. Some have Operculum or gill cover.
- Respiration is by gills
- Closed blood vascular system and RBC’s are nucleated and biconvex
- Presence of renal portal system.
- The heart is two – chambered (one auricle and one ventricle) and it is called as venous heart, as it contains only de oxygenated blood.
- Presence of sinus venosus in the heart.
- Presence of six pairs of aortic arches.
- Mostly ammonotelic but some excrete urea and TMAO (trimethyl amine oxide).
- Kidney are mesonephric and urinary bladder is absent.
- Presence of only internal ear bearing three semicircular canals
- Presence of ten pairs of cranial nerves.
- Uni sexual, fertilization is external or internal, direct development.
- Mostly Oviparous, Eggs are polylecithal
- Holoblastic and unequal cleavage.
- They are anamniotes i.e without foetal membranes.
- Some fishes exhibit seasonal migration which is of two types –
(a) Catadromous – Fresh water fishes which migrate to sea for reproduction”
Example : Anguilla.
(b) Anadromous – Marine fishes which migrate to fresh water for reproduction.
Example – Salmon, Hilsa, Trout etc
