Fish are divided into two main groups, jawed and jawless. The jawed group can be broken down further into two groups according to the composition of their skeletons. Fish that have a skeleton composed mostly of bone, are referred to as bony fish. The other group of fish have a skeleton made up of mostly cartilage.

The following is a general overview of the internal anatomy of bony fish.

Respiratory
Like all living things, fish need oxygen to survive. As water is drawn through the mouth of the fish, the gill covers close. Once the water is drawn in, the mouth is closed and the water is passed into the gill chambers. As the water is passing over the gills, any oxygen in the water is absorbed by the gill filaments. The gills are delicate structures and special care should be taken when handling a fish to ensure the safe release of the fish back into the water.

Swim Bladder
Located between the stomach and the backbone, the swim (air) bladder helps the fish maintain buoyancy within the water. The swim bladder does not have anything to do with swimming or movement of the fish. The fish's nervous system automatically regulates the amount of gas in the swim bladder. It some fish, it is not necessary to have a swim bladder. The flounder, for example, primarily remains on the bottom to look for food or avoid harm. Fish brought from deep water may need to have air removed from the swim bladder before they are released back into the water. The process of removing air from the swim bladder is referred to as, venting.

Nervous system
A fish's nervous system, which consists of a spinal cord, brain, and nerves, is not as complex compared to other vertebraes. The spinal cord runs from the brain through the backbone. Sensory nerves carry messages from the sense organs to the brain. Motor nerves carry messages from the brain to the mussels.

Reproduction
The male reproductive organs are called testes and ovaries in females. The male organs produce sperm contained in a fluid substance referred to as milt. The female organs produce the sex cells. The reproductive process in most fish occurs when the male and female simulataneously release their sex cells into the water through an opening near the anus. As the milt and the eggs are released, they are mixed together resulting in a fertilized egg.

Senses

All fish have the ability to smell. The olfactory organs, used to smell, are found on each side of the snout. Lined with sensitive nerve tissues in the snout, the fish is able to detect odors in the water.

The lateral line is a highly developed line of sensory organs. These sensory organs can detect vibrations in the water as well as the direction of water flow.

Believe it or not, fish can possess a tongue. A fish's tongue is flat and cartilginous and only moves when the base of the tongue moves. Also, the tongue has taste buds, giving the fish the ability to accept or reject food taken into its mouth. Other fish, like the catfish, have barbels near the mouth which are used for both taste and touch.

Digestive

The digestive system changes food into materials that will nourish the body. The type of food a fish eats determines the size of the intestines. A piscivorous fish (a fish that eats other fish) will typically have smaller intestines becuase, chemically, the food breaks down easier. Herbivorous (a fish that eats plants), on the other hand, will have longer intestines because it is more difficult to break down plant matter.