The earliest vertebrates that diverged from the invertebrate chordates were the jawless fishes. Fishes with jaws (gnathostomes) evolved later. Jaws allowed early gnathostomes to exploit new food sources. Agnathans include the hagfishes and lampreys. Hagfishes are eel-like scavengers that feed on dead invertebrates and other fishes. Lampreys are characterized by a toothed, funnel-like sucking mouth, and most species are parasitic on other fishes. Gnathostomes include the cartilaginous fishes and the bony fishes, as well as all other tetrapods.
Cartilaginous fishes include sharks, rays, skates, and ghost sharks. Most cartilaginous fishes live in marine habitats, with a few species living in fresh water for part or all of their lives. The vast majority of present-day fishes belong to the clade Osteichthyes, which consists of approximately 30,000 species. Bony fishes can be divided into two clades: Actinopterygii (ray-finned fishes, virtually all extant species) and Sarcopterygii (lobe-finned fishes, comprising fewer than 10 extant species but which are the ancestors of tetrapods).
ampulla of Lorenzini
sensory organ that allows sharks to detect electromagnetic fields produced by living things
jawed fish with paired fins and a skeleton made of cartilage
eel-like jawless fish that live on the ocean floor and are scavengers
jawless fish characterized by a toothed, funnel-like, sucking mouth
sense organ that runs the length of a fish’s body; used to detect vibration in the water
one of the earliest jawless fish covered in bone
clade of lampreys
in fishes, a gas filled organ that helps to control the buoyancy of the fish