The snapshot files contain the following lines of data:

  nbods               # number of particles
  ndim                # dimensionality [=3]
  tnow                # simulation time
  pmass(1)            # particle masses
   ...
  pmass(nbods)
  x(1),y(1),z(1)      # particle positions
   ...
  x(nbods),y(nbods),z(nbods)
  vx(1),vy(1),vz(1)   # particle velocities
   ...
  vx(nbods),vy(nbods),vz(nbods)
  eps(1)              # gravitational smoothing lengths
   ...
  eps(nbods)
  pot(1)              # gravitational potential
   ...
  pot(nbods)

For each block of particle data, the order and number of particles 
in each component are:

disk1  (M101's disk: 800,000 particles)
halo1  (M101's dark halo: 200,000 particles)
disk2  (N5474's disk: 100,000 particles)
bulge2 (N5474's bulge: 40,000 particles)
halo2  (N5474's halo: 25,000 particles)

(note that the M101 model does not have a bulge component)