While it appears as long, filamentous threads, these threads are formed by individual cells joined end-to-end. These cells are not independent and work together, which is why it's considered multicellular.

Here's why Spirogyra is not colonial:

* Colonial organisms are composed of independent cells that can survive on their own. They may be loosely connected or exhibit some degree of specialization, but each cell is capable of independent existence.

* Multicellular organisms have cells that are highly specialized and dependent on each other for survival. They form tissues and organs, and the organism as a whole functions as a coordinated unit.

Spirogyra fits the definition of a multicellular organism because its individual cells are specialized and interdependent, forming a filamentous structure that functions as a single organism.