Welcome to this brief introduction to the anatomy of fish. As with any group containing 28,000 different species there is a great deal of diversity amongst the animals we call fish.
Accepting that there is a great deal of variety within the world of fish we may wish to ask ourselves what factors control the shape that a fish takes. Many factors are involved but perhaps the most important is where the fish lives, i.e. in water. Water is a very different substance than the air that we live in, it is much more supportive for one thing. This means that it is easier for fish to fly than it is for us, and they can even float, of course we call flying in water swimming, but the fact remains that because they live in water many fish can live their whole lives without ever touching the bottom, or resting on anything. The other side of the coin of this supportiveness is that water is much harder to move through, we can run through air much faster than we can run through water.
In its simplest form this means that fish don't need legs to stand on, or large wings to fly, small wings (fins) will do fine and secondly it means that if you wish to move quickly it is best to have a streamlined shape. And this of course is what we see in most fish, a streamlined shape with no legs and small wings (fins).
Like all creatures fish need to perceive the world they live in, and thus we find they have senses, sight, hearing and smell, but also, because they live in water, they can sense transverse waves generated by motion and so they have evolved a lateral line system to do this, and some have even learned to use electricity, another sense that is only really made possible because water is such a good conductor of electricity.
Within the constraints of the basic form fish have evolved further variation to accommodate different ways of living, some live on the bottom and have become dorsoventrally flatter, others hide in crevices and have become laterally flattened, some have lost their streamlined shape to facilitate a life of slow movement, and so it goes on; where you live, what you eat, who you want to hide from and how you do this all contribute to your eventual shape, both internal and external.
The many forms and shapes of the fish that fill up the waters of our world are not only beautiful but practically evolved to suit a wide range of different life styles. If you are looking for information specifically on the Swim Bladder, Skeleton, the Scales, the Guts, Nerves, Blood and Respiration system please follow the links to the appropriate pages.