Here's why:

* Complex Structure: Planarians have a variety of specialized tissues and organs, including a nervous system, digestive system, and reproductive system. This level of complexity requires multiple cells working together.

* Organisms: Planarians are considered complete organisms, capable of independent survival and reproduction. This wouldn't be possible if they were just a single cell.

In contrast, unicellular organisms like bacteria and amoebas consist of just one cell that performs all necessary functions.