It's just hit me.
A corporeal schema is not a structure, it's a background - as MMP says, it is the flip-side of the projective activity that leaves objects round about it as traces of its acts and at the same time uses them as springboards to future acts.
Existence is not any particular structure, it is a movement from contingency to necessity (sedimentation) and necessity to contingency (innovation).
Our discussion of boundedness is meant to reinstall the subpersonal in a situation, but this situation is never entirely biological.
Merleau-Ponty was concerned to oppose a Kantian transcendental ego that constitutes its world in its entirety. But he would also be opposed to a pan-biologism that sees on biological situation, and which recognize no dialectical relation between this biological situation and a personal one.
But how are we to think of the influence of the personal on the biological?
The idea of the discussion of boundedness was not to trace prepersonal commitment to biological structure, but to interpret those structures in terms of that commitment, i.e. in terms of existence.
We do not exist in a purely biological situation, in fact, it takes incredible suffering to remain in such a situation for any extended period of time. The objection to Kantianism is that the organism is never entirely incorporated into our personal life. But the reverse is also true. Our personal life is rarely lived in that prepersonaltime of pure sensibility.