Which organism has all its cells directly in contact with the external environment?

The organism where all its cells are directly in contact with the external environment is the amoeba. This single-celled organism is a prime example of a eukaryotic protist that has no specialized tissues or systems for internal transport or exchange. Instead, its cell membrane interacts directly with the surrounding environment, allowing for the exchange of gases, nutrients, and waste products through simple diffusion.

Amoebas employ processes like phagocytosis to engulf food directly from their surroundings, emphasizing their reliance on direct contact with their environment for sustenance and to perform basic life functions. This characteristic of being unicellular means that each individual cell of the amoeba can immediately interact with its environment, showcasing a highly efficient adaptation for small organisms.

In contrast, multicellular organisms such as the hydra, earthworm, and oak tree feature specialized tissues and structures that compartmentalize functions and do not allow all cells to have direct contact with the environment. Instead, they rely on systems like circulatory and respiratory structures to facilitate these exchanges, distinguishing them from the amoeba's direct and immediate interaction with its surroundings.