Within Chordata, Craniota or Vertebrata are the largest and most ecologically successful taxon, with about 60,000 recent species. They have colonized all habitats on earth, from the sea to freshwater, from the land to the air. In addition to primarily aquatic forms, mostly cartilaginous and bony fish, a whole range of secondary aquatic vertebrates have evolved from terrestrial forms and have again adopted a fish-like body. For these reasons, the vertebrates show extraordinary morphological and ecological diversity.