In most, if not all, galaxies in Fig. 1 we find small scale deviations from axial symmetry, both in the HI distribution and in the velocity field (i.e. corresponding points have somewhat different properties). If we are interested in the large scale properties of these galaxies these deviations do not play a significant role because they are averaged out. They do, however, lay a burden on the determination of precise fits of complicated models like oval distortions, kinematical warps, and density wave models. Moreover, it is not easy to give a proper explanation for all these asymmetries.
Part of the asymmetries in the velocity fields might be explained by local concentrations in the HI distribution combined with the effects of beamsmoothing. Others might be associated with the presence of peculiar arms, like the asymmetries in NGC 7331, NGC 5033 and NGC 6946. Mild effects from tidal interaction with (small) companions cannot be, excluded either. In that connection we can mention NGC 5033 (with NGC 5005), NGC 5055 (with UGC 8313), NGC 4151 (with NGC 4145; Arp's argument that NGC 4156 is responsible for the asymmetry in the outer parts of NGC 4151 (Arp, 1977) is interesting, but not compelling), NCC 4258 (with NGC 4248), and so on.
A rather frequent phenomenon is the occurrence of HI "tails" or "outlying" clouds in the outer parts of spiral galaxies. Such features have been found in M83 (Rogstad et al., 1974), NGC 3198, NGC 5033, NGC 2841 and NGC 7331 (see chapter 4). Isolated HI clouds have been found around M33 (Wright, 1973), NGC 55 and NGC 300 (Mathewson et al., 1975), M31 (Newton and Emerson, 1977) and a few other galaxies as discussed by Davies (1974). In all cases no optical emission has been detected at the same spot on the sky. It is not clear whether the presence of these clouds is related to the presence of warps in the outer parts (see section 5). Most of these clouds have more or less the radial velocity expected for their position with respect to the centre of the galaxy. Still their existence as identifiable features casts some doubt whether they can be used as tracers of the mass distribution. Since these clouds have not been detected in the general field (see Shostak, 1977) they might be remnants from the formation stage of a galaxy.