- mouth: most fish have a conical mouth cavity with a relatively small opening in front and an expandable rear compartment to create suction for capturing food.
- nostrils: in most fish the nostrils are only used for smelling but not for respiration.
- eye: fish eyes are optical but are not capable of binocular vision because they are located on either side of the head.
- lateral line: a canal lined with sensory organs which detect vibrations and changes in temperature and pressure. The lateral line helps fish to maintain balance and maintain distance from other fish when schooling.
- first dorsal fin: the anterior fin on the dorsal surface of a fish which is used for balance.
- second dorsal fin: the posterior fin on the dorsal surface of a fish which is used for balance.
- caudal fin: the tail fin of a fish which is used for propulsion, steering, and balancing.
- caudal peduncle: a fish's "tail stem", the caudal peduncle is usually heavily muscled and is used for propulsion.
- anal fin: the unpaired fin located on the posterior ventral side of a fish and is used for swimming.
- pelvic (ventral) fins: a pair of fins located on the ventral surface of a fish.
- pectoral fin: one of a pair of fins located on either side of a fish behind the gills used for balancing and braking.
- gill cover / operculum: a protective cover over the gills.
- gills: gills absorb oxygen from the water as it passes into the mouth and out through the gill cavity. Gills also maintain salt balance and are excretory organs.
- scales: scales provide protection and reduce friction for fish: two common types are ctenoid (small teeth on rear edge) and cycloid (smooth edged) scales.
- brain: the central point of a fish's nervous system.
- kidney: the kidney is part of a fish's excretory system and has immune function.
- vertebra: a fish's backbone used for structural support and for protecting the nerve chord.
- muscle segments: because fish require great strength to move through water, 40% to 60% of a fish's body weight is made up of large muscles called myotomes.
- gonads: the reproductive organs of a fish
- swim bladder: by altering the amount of gas in the swim bladder, a fish can adjust its buoyancy, and therefore its depth, in the water. (sharks and other cartilaginous fish do not have a swim bladder)
- intestine: an organ for digesting food.
- stomach: an organ for breaking down food.
- liver: an organ for detoxifying blood.
- heart: a two-chamber pump in a fish's circulatory system.
- gill rakers: a set of tooth-like structures which strain the water passing through the gills to prevent debris from entering the gills.