NGC 3521 is similar in appearance to M63
and belongs to the class of galaxies known for lacking well defined
spiral arm structure. These galaxies are known as "flocculent"
galaxies because they have short, patchy spiral arms which are
fragmented and loosely organized. About one third of all spiral
galaxies have flocculent structure to some degree. The other two
thirds have the classic "grand design" structure with
two symmetric prominent spiral arms. Classic "grand design"
galaxies include well known objects such as M51, M100, and M81.
Other well known flocculent spirals include M63, NGC 2403, M33,
and NGC 2841. A system for classifying spiral arm structure was
set up by in 1982 (Elmegreen and Elmegreen 1982) which divided
spiral galaxies into 12 classes. The "Arm" classification
sequenced spiral galaxies on a scale from 1 to 12. The ones with
the most irregular and fragmented arms were class I to III, while
the most symmetric and smooth spiral arms belonged to classes
10 to 12. Flocculent galaxies make up classes I through III, with
NGC 3521 falling into class III. Flocculent galaxies often show
a large halo to disk ratio for unknown reasons. The optical counterpart
to the extensive gaseous halo of NGC 3521 can be seen shining
well above the galactic plane.