Central cavity: Inner space delimited by the inner wall of two-walled cup.
In one-walled cup = internal cavity.
Empty: Without any skeletal structure in the central cavity or in the inner cavity.
Reduce: Due to the large development of the inner wall.
Inner wall: Internal part of the skeleton, separating central cavity and skeletal structures, composed by a porous carcass, simple or coated by an additional sheath.
Full: With secondary skeletal structures in central cavity or in inner cavity.