Introduction

• Fish are cold-blooded aquatic vertebrates that breathe through pharyngeal gills and propel and balance themselves using fins supported by dermal fin rays.

General characteristics of fishes

• Are aquatic, cold-blooded vertebrates.
• The body is generally fusiform and streamlined but in globiformes ( eg Pufferfish), the body is globe shape and in eels is of serpentine form. The ground plan of the body is bilaterally symmetrical with a prominent lateral line system despite variations in shape.
• The body is covered by tough skins armored by a variety of scales.
• Appendages of fish comprise the fins which are generally paired ( Pectoral and pelvic fins) and unpaired ( dorsal, anal, and caudal fins). ADermal rays support all fins These fins are the main locomotory organ.
• The mouth is generally situated anteriorly in the head and the anus is in the second half of the overall length of the individual behind the bases of the pelvic fins and just in front of the anal fin.
• Jaws are hinged.
• Respiratory organs are generally in the form of gills and other accessory respiratory organs.
• Nostrils are paired but don’t open into the pharynx, except in lung-fishes and lobed-fishes.
• The skeleton of the fishes is in the form of the notochord, connective tissues, bones, cartilages, and non-bony scales.
• The digestive tract is complete.
• The heart is 2 chambered with a single auricle and ventricle.
• Kidneys are paired, longitudinal structures that lie above the body cavity.
• The brain is well developed with ten pairs of cranial nerves.
• Cranium surrounds the brain.
• The middle ear is completely absent but the internal ear has well-developed semi-circular canals.
• Sexes are separate.
• Mostly oviparous (egg-laying) but some are viviparous ( direct birth giving).
• Fertilization is generally external.
• Embryonic membranes are absent.
• Eggs are large with much yolk.
• Development is direct ( without any metamorphosis).