To utilize the air trapped between soil particles for cutaneous respiration.
Explanation
Earthworms are terrestrial invertebrates that lack specialized respiratory organs such as lungs or gills. Instead, they rely on cutaneous respiration, a process where gas exchange occurs directly through their skin.
Detailed Analysis:
- To avoid exposure to direct solar radiation. is incorrect: While earthworms exhibit negative phototaxis (avoidance of light) and solar radiation can be harmful, this is a behavioral response. The requirement for moisture is fundamentally tied to the physiological necessity of respiration rather than just light avoidance.
- To access organic nutrients found only deep underground. is incorrect: Earthworms primarily feed on decaying organic matter and humus, which are most abundant in the topsoil layers, not deep underground.
- To utilize the air trapped between soil particles for cutaneous respiration. is correct: For cutaneous respiration to occur, oxygen must first dissolve in a layer of moisture on the skin before diffusing into the blood capillaries. The sub-surface soil provides a moist environment that prevents the skin from drying out, while the spaces between soil particles contain the trapped air necessary for oxygen intake. If the skin dries, the earthworm cannot breathe and dies of asphyxiation.
- To escape from avian predators. is incorrect: While burrowing helps escape predators, this is an ecological or behavioral advantage, not the primary physiological reason for their habitat preference.
Key Takeaway: Earthworms inhabit moist sub-surface soil primarily to facilitate cutaneous respiration, as their skin must remain moist to absorb oxygen from air trapped between soil particles.