Historical observations:

"The Hubble Atlas of Galaxies", 1961, Washington, D.C. Carnegie Istitution of Washington" - "This galaxy, seen almost on edge, is of the NGC 2841, NGC 5055 type, though the spiral arms do not seem to be as thin and are more easily visible. The increased visibility of the arms may be an effect of projection, because the dust lanes and interspersed luminous arms are silhouetted against the bright background luminosity of the central regions of the galaxy. Spiral arms can be seen to within about 12 sec of arc (radius) of the center (about 6 mm on the scale of the illustration). This illustration does not show the detail in the central regions. The burned-out central region is not amorphous like the center of M31 or M81 but has a spiral structure like that in NGC 5055.
NGC 7331 is one of the galaxies often used in discussions of the direction of rotation of spiral arms. There is no question about which way the arms open in NGC 7331; the debatable point is which is the near and which the far side of the galaxy. The Lindblad School believes that the east side is the near one, in which event the arms would be leading because spectroscopic data show that the north end of the spindle is approaching. The original plate, PH-64-H, from which this illustration was made, appears to settle the argument. A part of the light from the nuclear bulge can be seen beneath the dust line on the west side. The nuclear light can just be seen on this positive reproduction. But a negative print from PH-64-H is given by de Vaucouleurs in Ap. J., 127, 487, 1958 (Fig. 5) which shows this particularly well. If the identification of the light with the nuclear bulge is correct, the west is the near side and the spiral arms are trailing."

Allan Sandage & John Bedke - 1994 - "Has a spiral pattern of the MAS type, similar to that of M31. However, the nuclear bulge is smaller than in M31, and the arms in NGC 7331 can be traced closer to the center than M31. NGC 7331 is superposed on a more-distant group of early-type galaxies with a mean redshift of about 6000 km s -1, of which Stephan's Quintet is a subgroup. Three galaxies of this more-distant group can be in the map here. As described in thr Hubble Atlas, NGC 7331 played a crucial role in settling an early problem as to the direction of the opening of the spiral pattern relative to the direction of rotation of the galaxy" [Allan Sandage & John Bedke - 1994].

Secondo il "The de Vaucouleurs Atlas of Galaxies, R.J. Buta, H.G. Cprwin, Jr, and S. C. Odewahn NGC 7331 "has complex, partly flocculent spiral structure and considerable nearside extinction. That the bulge is significant is evidence by the " fattening" of the galaxy's isophotes around the minor axis of the outer disk. In the center, a mini disk is seen that has an enhanced ridge on one side. In the B - I color index, this ridge is blue, suggesting the presence of a ring. Only half the ring may be seen because of the severe nearside extinction.
NGC 7331 was recently studied by Regan et al. (2004), who noted dramatic changes in the morphology of the galaxy with increasing wavelength. This includes a complete inner ring that is most noticeable at infrared wavelengths. The ring is also detectable in CO. Regan et al. estimate a gas mass of 5 x 10 9 M is contained within the ring."