Which of the following respiratory systems is not closely associated with a blood supply?
a. The lungs of a vertebrate
b. The gills of a fish
c. The tracheal system of an insect
d. The skin of an earthworm

Respiratory systems are biological structures that facilitate gas exchange, primarily oxygen and carbon dioxide, between an organism and its environment. Different organisms have evolved various respiratory systems, such as lungs, gills, tracheal systems, and skin, to efficiently meet their metabolic needs. Each system is adapted to the organism's habitat and lifestyle.

The tracheal system in insects is a network of air-filled tubes that directly deliver oxygen to tissues and cells, bypassing the need for a circulatory system to transport gases. Air enters through spiracles and diffuses through the tracheae and tracheoles, allowing for efficient gas exchange without the involvement of blood. This system is highly effective for small, terrestrial organisms like insects.

In many organisms, respiratory systems are closely linked with the circulatory system to transport gases throughout the body. For example, vertebrate lungs and fish gills are richly supplied with blood vessels to facilitate the exchange of gases between the respiratory surface and the bloodstream. However, some systems, like the insect tracheal system, operate independently of the blood supply.