Animals in the subphylum Vertebrata are distinguished from other chordates by several features. Most prominent is the endoskeleton of bone or cartilage, centering around the vertebral column (spine or backbone). Composed of separate vertebrae (showing internal metamerism), 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 by the anus; 3) a closed circulatory systems with a single, well-developed heart; 4) a brain at the anterior 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—fins in the fishes, which evolved into legs in land vertebrates.