Gills are the respiratory organs in fishes, allowing them to extract oxygen from water. These specialized structures are responsible for the exchange of gases, where oxygen from the water is taken up and carbon dioxide is released.

Gills are projections of the skin, which means they are extensions or outgrowths of the fish's skin. They are not associated with the nostrils or the alimentary canal, as mentioned in options b and c, respectively.

- Gills as projections of the skin:
Gills are located on the sides of the fish's head, behind the mouth, and are covered by a bony structure called the operculum. When a fish opens its mouth, water enters the mouth and passes over the gills. The gills are made up of thin, flat filaments that are rich in blood vessels. These filaments increase the surface area available for gas exchange. As water flows over the gills, dissolved oxygen in the water diffuses across the thin walls of the gill filaments and enters the bloodstream, while carbon dioxide diffuses out of the bloodstream and into the water to be expelled. The oxygen-rich blood is then circulated throughout the fish's body, providing oxygen to the cells.

- Function of gills:
Gills are highly efficient respiratory organs that allow fishes to extract oxygen from water. Unlike mammals, fishes do not have lungs, so gills are essential for their survival. Water passing over the gills is pumped by the fish's mouth and gill covers, creating a continuous flow of fresh water over the gill filaments. This ensures a constant supply of oxygen for the fish's respiratory needs.

- Adaptations for gill respiration:
Fishes have evolved various adaptations to enhance the efficiency of gill respiration. These adaptations include the presence of gill rakers, which help filter out debris and prevent clogging of the gill filaments. Additionally, countercurrent exchange systems in the gill filaments allow for efficient gas exchange by maximizing the concentration gradient between the water and the fish's blood.

In conclusion, gills in fishes are projections of the skin. They are responsible for extracting oxygen from water and releasing carbon dioxide, allowing fishes to respire and survive in aquatic environments.

Fishes and their respiratory organs
Introduction:
Fishes, being aquatic organisms, require specialized respiratory organs to extract oxygen from water. Gills are the primary respiratory organs in fishes, allowing them to respire in their aquatic environment.
Gills:
Gills are specialized organs that are responsible for extracting oxygen from water and removing carbon dioxide. They are located on the sides of fishes' heads, protected within gill chambers.
Origin of gills:
Gills are not projections of the skin, nostrils, or the alimentary canal. They have a different origin and are developed from the pharyngeal arches in fishes.
Function of gills:
Gills perform the following functions in fishes:
1. Oxygen uptake: Gills extract dissolved oxygen from water, which is then transported to the fish's bloodstream for distribution to the body tissues.
2. Carbon dioxide release: Gills eliminate carbon dioxide, a waste product of cellular respiration, by diffusing it out into the surrounding water.
3. Ion regulation: Gills also play a role in regulating the balance of ions such as sodium, chloride, and calcium in the fish's body.
4. Acid-base balance: Gills help maintain the pH balance of the fish's body fluids by regulating the levels of bicarbonate ions.
5. Osmoregulation: Gills assist in maintaining the proper balance of water and salts in the fish's body, allowing them to live in various aquatic environments.
Conclusion:
Gills are the respiratory organs in fishes, responsible for extracting oxygen from water and removing carbon dioxide. They are not projections of the skin, nostrils, or the alimentary canal. Understanding the structure and function of gills is crucial in studying the respiratory adaptations of fishes to their aquatic habitat.