Animals in the subphylum Vertebrata are distinguished by several features. Most prominent is the endoskeleton of bone or cartilage, centering around the vertebral column (spine or backbone). Composed of separate vertebrae, a vertebral column combines flexibility with enough strength to support even a large body. Other vertebrate features include: 1) complex dorsal kidneys; 2) a tail (lost via evolution in some groups) extending between the anus; 3) a closed circulatory system with a single, well-developed heart; 4) a brain at the top end of the spinal cord, with ten or more pairs of cranial nerves; 5) a cranium (skull) protecting the brain; 6) paired sex organs in both males and females; and 7) two pairs of movable appendages (this would be fins in the fish that evolved into legs in land vertebrates).