Introduction

• Fishes are cold-blooded aquatic vertebrates which breathe by means of pharyngeal gills and propelling and balancing themselves by means of fins which are supported by dermal fin rays.
• The smallest fish of Nepal is Hara hara(2.5 cm) and largest fish is Bagariusyarrrelli.
• The largest fish of the world is Rhinocodontypus(70 feet) and smallest is paedocyprisprogenetica(7.9mm).
• Some fish’s lives exclusively in fresh water, large numbers of species are found in marine but few species prefer in brackish water.
• This includes skeletal fish (Osteichthyes), cartilaginous fish (Chondrichthyes), and jawless fish (Agnatha).
• The modern fish relates to Agnatha and Gnathostomata are division of vertebrates.
• The head was covered with head shield and trunk and tail were much like those of a modern fish. On each side of the body were paired appendages.
• The head of cephalaspis was covered with head shield and the body contain dorsal, pectoral and caudal fins, the forward part of trunk was covered with transverse band like scales which pass into more typical scale to the tail.
• The endoskeleton was completely ossified as modern fishes. A striking peculiarity was of the electric organs as modern fishes.
• Cyclostomes are free swimming, jawless, limbless vertebrates are relating to the ostracoderms.
• It includes Petromyzone, lamprey and hage fishes, these include long cylindrical body with presence of large dorsal and caudal fin.