What determines whether or not a body system is likely to remain in a phylum over the course of evolution

Bauplan is the assembly of the body structure which is common in the organisms belonging to the same phylum. The Same bauplan determines that a body part of the organisms are same and they will be in the same phylum in the Evolution also. Bauplan includes symmetry, segmentation, and limb disposition. 

The diaphragm is what i think ur talking about