Polymesh rendered as a volume (rollover image).
Noise connected (through Range shader) to Displacement of Standard Volume.
Step Size 0.1, Volume Padding 0.05.
When the Step Size is positive, it turns a polymesh into a volume and its value is used as the base step for raymarching the volume.
The mesh should be closed and not have anything self-intersecting, otherwise, the volume may not render correctly.
You must add a Step Size value and assign a Standard Volume shader for the mesh to render as a volume.
A tutorial that covers how to render a polymesh as a volume can be found here.
More information about volume Step Size can be found here.
Head models rendered as volumes with different textures connected to displacement of Standard Volume
Pepe model by Daniel M. Lara (Pepeland)