Gills are the respiratory organs that are capable of extracting dissolved oxygen in water, and those are found in evolutionarily higher and complex aquatic animals. However, the microscopic and simple aquatic organisms do not require a gill apparatus to extract oxygen from water, as their body surface can absorb adequate amounts. The structure of a gill is interesting, and it has a filtering system to trap particles other than water while gas exchange takes place. In fishes, the water is taken from the mouth, projected through gills to absorb oxygen, and sent out through the gill slits (cartilaginous fish) or operculum (bony fish). The primary function of a gill involves a counter current system of flowing blood in the gill and water around the gill in opposite directions. In addition, the comb-like filaments of the gill called gill lamellae aids in increasing the surface area of gas exchange. There are slight differences between the gills of bony fish and cartilaginous fish in structure, but the function of extracting oxygen with