  Aims.The main purpose of this study is to investigate aspects regarding the
validity of the active galactic nucleus (AGN) unification paradigm (UP). In
particular, we focus on the AGN host galaxies, which according to the UP should
show no systematic differences depending on the AGN classification. Methods.For
the purpose of this study, we used (a) the spectroscopic Sloan Digital Sky
Survey (SDSS) Data Release (DR) 14 catalogue, in order to select and classify
AGNs using emission line diagnostics, up to a redshift of z=0.2, and (b) the
Galaxy Zoo Project catalogue, which classifies SDSS galaxies in two broad
Hubble types: spirals and ellipticals. Results.We find that the fraction of
type 1 Seyfert nuclei (Sy1) hosted in elliptical galaxies is significantly
larger than the corresponding fraction of any other AGN type, while there is a
gradient of increasing spiral-hosts from Sy1 to LINER, type 2 Seyferts (Sy2)
and composite nuclei. These findings cannot be interpreted within the simple
unified model, but possibly by a co-evolution scheme for supermassive black
holes (SMBH) and galactic bulges. Furthermore, for the case of spiral host
galaxies we find the Sy1 population to be strongly skewed towards face-on
configurations, while the corresponding Sy2 population range in all host galaxy
orientation configurations has a similar, but not identical, orientation
distribution to star-forming (SF) galaxies. These results also cannot be
interpreted by the standard unification paradigm, but point towards a
significant contribution of the galactic disc to the obscuration of the nuclear
region. This is also consistent with the observed preference of Sy1 nuclei to
be hosted by ellipticals, that is, the dusty disc of spiral hosts contributes
to the obscuration of the broad-line region (BLR), and thus relatively more
ellipticals are expected to appear hosting Sy1 nuclei.
