 The most basal of the living teleosts are the <unk> ( eels and allies ) and the <unk> ( <unk> and allies ) . There are 800 species of <unk> . They have thin leaf @-@ shaped larvae known as <unk> , specialised for a marine environment . Among the <unk> , eels have elongated bodies with lost pelvic girdles and ribs and fused elements in the upper jaw . The 200 species of <unk> are defined by a bony element in the tongue . This element has a basibranchial behind it and both structures have large teeth which are paired with the teeth on the <unk> in the roof of the mouth . The clade <unk> includes the <unk> ( herrings ) and Ostariophysi ( carps , catfishes and allies ) . <unk> consist of 350 living species of herring and herring @-@ like fishes . This group is characterised by an unusual abdominal scute and a different arrangement of the hypurals . In most species , the swim bladder extends to the braincase and plays a role in hearing . Ostariophysi , which includes most freshwater fishes , have developed some unique adaptations . One is the Weberian apparatus , an arrangement of bones ( Weberian ossicles ) connecting the swim bladder to the inner ear . This enhances their hearing , as sound waves make the bladder vibrate , and the bones transport the vibrations to the inner ear . They also have a chemical alarm system ; when a fish is injured , the warning substance gets in the water , alarming nearby fish . 
