It's likely that the spiral structure itself reminded him of a whirlpool or a pinwheel, which were common analogies used to describe these galaxies.
While Hubble didn't specifically mention a comparison, he did contribute greatly to our understanding of the universe by showing that galaxies are not just random collections of stars, but have distinct shapes and structures. This discovery, alongside his work on the expanding universe, solidified his place as a key figure in modern astronomy.
