Why do some galaxies appear to have no spiral arms at all?

Spiral arms need a rotating, gas-rich disk with structure that can organize star formation into patterns. Elliptical galaxies are usually dominated by older stars, have little cold gas, and lack the thin disk needed for arms. Irregular galaxies can be distorted by interactions, making their shapes chaotic rather than organized. Dwarf galaxies are especially likely to be irregular because their gravity is weaker and external influences can reshape them more easily. The Hubble sequence reflects these structural differences.