A classic example of an organism that can only exist as a group of cells is a volvox.

Here's why:

* Colonial Green Algae: Volvox are a type of green algae that form spherical colonies.

* Specialized Cells: These colonies contain thousands of individual cells, each with a specific role. Some are specialized for reproduction, others for movement, and others for nutrient absorption.

* Interdependence: The cells within the colony are dependent on each other for survival. They can't exist independently.

* Cooperative Function: The cells act together as a coordinated unit, allowing the colony to move, reproduce, and survive as a whole.

While individual Volvox cells can reproduce asexually, they are not capable of surviving on their own for long periods. They require the cooperation and interaction of other cells to form the colony that defines their existence.

Let me know if you'd like to explore other examples of organisms that exist only as groups of cells!