Here's why:

* Cells are the fundamental building blocks of life: All living organisms, including animals, are made up of cells. Cells are the smallest units that can carry out all the processes necessary for life.

* Cellular structure: Animals have complex cellular structures with specialized organelles that perform specific functions. These organelles work together to keep the animal alive.

* Viruses: While viruses are not considered living organisms, they are sometimes confused with "non-cellular" life. Viruses are essentially genetic material (DNA or RNA) enclosed in a protein coat. They cannot reproduce on their own and require a host cell to replicate.

Therefore, there are no known animals that are not composed of cells.