The diagram shows a longitudinal section of a snail (a gastropod mollusk). Four parts are labelled:
I points to the hard, outer protective covering that sits on top of the snail's body. This is the shell.
II is the mantle, the tissue layer just beneath the shell.
III represents the visceral mass containing the internal organs.
IV is the broad, flat, muscular foot used for locomotion.
The shell is a defining feature of most gastropods. It is secreted by the mantle and is composed largely of calcium carbonate. It serves as protection against predators and desiccation. Because label I points directly to this hard outer covering, the correct answer is shell.
