Basa Fish Health Benefits Nutrition and Risk Organic FactsThe inner skeleton consists of the skull bones apart from the roofing bones of the top, which are really a part of the exterior skeleton, the vertebral column, and the fin helps fin rays. The fin supports are derived from the exterior skeleton but may be treated here because of their close purposeful dating to the inner skeleton. The internal skeleton of cyclostomes, sharks, and rays is of cartilage; that of many fossil groups and a few primitive living fishes is usually of cartilage but may include some bone. In place of the vertebral column, the earliest vertebrates had a fully built notochord, a versatile stiff rod of viscous cells surrounded by a strong fibrous sheath. During the evolution of recent fishes the rod was changed partly by cartilage after which by ossified cartilage. Sharks and rays retain a cartilaginous vertebral column; bony fishes have spool shaped vertebrae that during the more primitive living forms only partially replace the notochord.