The muscles that connect to Bacoon's shell are unique.
The "Bend1" and "Bend2" meshes act like morph targets to create the bend effect.

The position and scale of the muscles depends on how open the shell is.
Bacoon usually only opens the shell by up to 28 degrees.
The Y position is (15 * degrees) (min: 0cm / max: 410cm) (relative to base point)
The Y scale is (degrees / 28) + 0.5 (min: 0.5 / max: 1.3)
The X scale is (1.5 - scaleY) / 2 + 0.75
The Z scale is always 1