Evolutionary trend of respiratory system from simple to complex

Diffusion through skin/cell → gills → tracheal tubes → book lungs → lungs

Some groups of snails that live in freshwater respire using gills, whereas other groups need to reach the surface to breathe air. In addition, some are amphibious and have both gills and a lung.

Snails have a very primitive respiratory system, which includes:

- A simple body surface that allows for gas exchange

- A mantle cavity that contains a small amount of water, which helps with gas exchange

- No lungs or tracheae

This type of respiratory system is often referred to as "cutaneous respiration" or "diffusion-based respiration," where oxygen diffuses directly into the body tissues.

In contrast:

- Fish (B) have gills, which are more complex and efficient for gas exchange in aquatic environments.

- Insects (C) have a tracheal system, which includes tubes that bring oxygen directly to cells.

- Mouse (D) has lungs and a complex respiratory system adapted for terrestrial life.

So, snails have the most primitive respiratory system among the options.