Fish are aquatic species which display a large range of diversity due to the vast array of habitats which they can survive in from streams found in high mountains to habitats in the deep ocean. The earliest fish which came about were the Chordates and from there fish have evolved tremendously.
In terms of their anatomy, fish possess many of the systems which are found in humans. Some of these systems include:
- Respiratory System: Use gills to exchange gases (and thus breathe), although there are some air-breathing fish. Air-breathing fish usually live in shallower habitats and need to come up for air occasionally. They take in air using their mouth and this causes their buccal cavity to expand allowing air to enter and move into their air breathing organ (Lungfish have a paired lung which bears similarities to the lungs of tetrapods).
- Circulatory System: Have a closed circulatory system similar to how all vertebrates do. Water breathing fish have a circulatory system which is comprised of a single circuit.
- Digestive System: Have similar organs to humans such as an esophagus, stomach, pancreas and liver for example. With the exception of Agnathans, which are jawless fish, fish are capable of feeding on a variety of food products such as plant materials.
- Excretory System: Most nitrogenous wastes excreted by fish leave in the form of ammonia. The gills act as an organ which allow for waste diffusion and the kidneys are used to filter blood wastes.
- Sensory and Nervous System: Although the brain of a fish is much smaller than that of a human, fish have a developed nervous system. Furthermore, fish possess sensory organs which are highly developed and are needed for functions such as hearing and olfaction.
In addition to these systems, fish also have a muscular system, reproductive system and an immune system. Evidently fish are rather developed vertebrates. All of the systems which exist in fish have evolved to function in a way which allows fish to survive in all parts of the globe which they inhabit.