Acoelous means “no hollow”. Centrum in vertebrae is expressed as concavity. Opisthocoelous and procoelous vertebrae have concave ends at back and infront respectively. While platycoelous have flat front and concave back and biconvex is convex from both the ends. So, acoelous vertebrae (without centrum) can apply to either platycoelous or biconex vertebrae.