Ichthyology is the branch of zoology devoted to the study of fish. This includes skeletal fish , cartilaginous fish , and jawless fish . At least 30,700 fish species have been described, comprising a majority of vertebrates.... , the isthmus is the narrow area between the gill
Gill
A gill is an anatomical structure found in many aquatic ecosystem organisms. It is a respiration organ whose function is the extraction of oxygen from water and the excretion of carbon dioxide.... openings on the underside of a fish's head.
