 The defining features of the teleosts are mobile premaxilla , elongated neural arches at the end of the caudal fin and unpaired basibranchial <unk> . The premaxilla is unattached to the neurocranium ( braincase ) ; it plays a role in protruding the mouth and creating a circular opening . This lowers the pressure inside the mouth , sucking the prey inside . The lower jaw and maxilla are then pulled back to close the mouth , and the fish is able to grasp the prey . By contrast , mere closure of the jaws would risk pushing food out of the mouth . In more derived teleosts , the premaxilla is enlarged and has teeth , while the maxilla is toothless . The maxilla functions to push both the premaxilla and the lower jaw forward . To open the mouth , an adductor muscle pulls back the top of the maxilla , pushing the lower jaw forward . In addition , the maxilla rotates slightly , which pushes forward a bony process that interlocks with the premaxilla . 
