  We carry on our studies related to the fully parabolic quasilinear
Keller-Segel system started in [6] and continued in [7]. In the above mentioned
papers we proved finite-time blowup of radially symmetric solutions to the
quasilinear Keller-Segel system if the nonlinear chemosensitivity is strong
enough and an adequate relation between nonlinear diffusion and
chemosensitivity holds. On the other hand we proved that once chemosensitivity
is weak enough solutions exist globally in time. The present paper is devoted
to looking for critical exponents distinguishing between those two behaviors.
Moreover, we apply our results to the so-called volume filling models with a
power-type probability jump function. The most important consequence of our
investigations of the latter is a critical mass phenomenon found in dimension
2. Namely we find a value m0 such that when the solution to the two-dimensional
volume filling Keller-Segel system starts with mass smaller than m0, then it is
bounded, while for initial data with mass exceeding m0 solutions are unbounded,
though being defined for any positive time.
