A mammal classified in the order Xenarthra along with anteaters, tamanduas, and armadillos. The sloth differs from all other mammals by having additional articulations (xenarthrales) between their lumbar vertebrae (called xenarthrous vertebrae). They also have a double rather than a single inferior vena cava, the vein that returns blood from the posterior portion of the body to the heart. The dental formula is I 0/0, C 0/0, PM 0/0, M 5/4 × 2, for a total of 18 teeth which grow throughout life. Females have a primitive, divided uterus and a common urinary and genital tract; males have internal testes.