Introduction

• Fishes are cold-blooded aquatic vertebrates that breathe using pharyngeal gills and propelling and balancing themselves using fins which are supported by dermal fin rays.
• The smallest fish in Nepal is Hara hara(2.5 cm) and the largest fish is Bagariusyarrrelli.
• The largest fish in the world is Rhinocodontypus(70 feet) and the smallest is paedocyprisprogenetica(7.9mm).
• Some fish live exclusively in freshwater, 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 a head shield and the trunk and tail were much like those of a modern fish. On each side of the body were paired appendages.
• The head of the cephalalgia was covered with a head shield and the body contained dorsal, pectoral, and caudal fins, the forward part of the trunk was covered with transverse band-like scales that passed into a 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 related to the ostracoderms.
• It includes Petromyzone, lamprey, and huge fishes, these include long cylindrical bodies with the presence of large dorsal and caudal fins.