  The structures $\langle M,\subseteq^M\rangle$ arising as the inclusion
relation of a countable model of sufficient set theory $\langle
M,\in^M\rangle$, whether well-founded or not, are all isomorphic. These
structures $\langle M,\subseteq^M\rangle$ are exactly the countable saturated
models of the theory of set-theoretic mereology: an unbounded atomic relatively
complemented distributive lattice. A very weak set theory suffices, even finite
set theory, provided that one excludes the $\omega$-standard models with no
infinite sets and the $\omega$-standard models of set theory with an amorphous
set. Analogous results hold also for class theories such as G\"odel-Bernays set
theory and Kelley-Morse set theory.
