All living beings are divided into plants and animals.
The animals are divided into two groups namely invertebrates and vertebrates.
The invertebrates are without back-bone and include single cell protozoa, corals, flukes, segmented worms, snails, insects and spiders.
The vertebrates have back-bone and include fish, amphibians, reptiles, birds and mammals.
There are two types of fish namely cartilaginous fish and bony fish.
The cartilaginous fish include fish like sharks and rays.
The bony fish include a number of marine and fresh water fish.
Fish have different food habits. There are plant eaters, plankton (very small plant and animals) eaters and flesh eaters.
Most of the fish are used as food for us and there are a number of very colourful fish which are called ornamental fish and we keep them in aquaria as pets.