Explanation

The $\ 9^{th}$ or sacral vertebra of frog is acoelous and highly specialised. The anterior face is convex for this give greater strength to them, it would be given if its anterior end are hollowed out, the transverse process are stout and downwardly directed. The ileum of the pelvic girdle articulates with the transverse process of $9^{th}$ vertebrae.