Three species of mammals differ completely from the more common placental animals: the short-beaked echidna (Tachyglossus aculeatus), the long-beaked echidna (Zaglossus bruijni), and the duck-billed platypus (Ornithorhynchus anatinus). Monotremes differ from placental mammals mostly because development of the offspring occurs outside of the female reproductive tract. Monotreme females conceive young via copulation, but fertilized ova then move to a cloaca (a common opening of urinary and reproductive tract) where they are coated with albumen and a shell, and eggs are laid after 12–20 days. Female echidnas carry the egg into a pouch whereas platypus lay the eggs into a nest of grass. At hatching, young echidnas remain in the pouch and subsist on the milk that drips from the fur (monotremes lack nipples). Once large enough, young are deposited in a nest where they are nursed until weaning. In contrast, female platypus lay their eggs in a grass nest inside a burrow, incubate them until hatching 10 days later, and then nurse the young in the den. Young emerge from the burrow when fully furred and approximately 12 in (30 cm) in length.