Key Characteristics

Key Characteristics: 

• Gill Breathing
  Most efficient respiratory organ in the animal kingdom
  Gills extract oxygen from a medium (water) that has 5% of the oxygen present in air
• Ectothermic
  
• Aquatic
  
• Fins
  
• Skin usually covered with scales
  
• Excellent olfactory and visual systems
  
• Many species have a sensory system called a lateral line
  Lateral line cells are extremely sensitive to water currents and vibrations

Major groups of fishes

• Hagfish (Class Myxini) 
• Distinct head
• Tripartie brain
• Specialized sense organs and one or more pairs semicircular canals
• Lacks a well developed visceral skeleton
• Lamprey (Class Cephalaspidomorphi)
• Well-developed visceral skeleton
• Two or more pairs of semicircular canals
• Elasmobranchii (Sharks, Skates, and Rays) & Holocephali (Chimeras)
• Jaws
• Three pairs semicircular canals
• Teeth with dentine
• Supporting elements for jaw
• Gills not covered by an operculum 
• Cartilaginous skeleton
• Actinopterygii (Bony Fish)
• Bony skeleton
• Gills covered by bony operculum
• Lung or swim bladder
• Sarcopterygii (Fleshy-finned fishes)
• Supportive elements in the skeleton, pectoral or pelvic girdle, fins, and legs 

Resources

• Check out this UC Berkley resource on the evidence showing how the Coelacanth is a likely common ancestor to tetrapods (land vertebrates)