The mammals that lay eggs, or monotremes, are divided into two major groups, the duck-billed platypus and the echidnas. While there is only one species of platypus, there are four species of echidnas, the short-beaked echidna and three species of long-beaked echidnas. All egg-laying mammals have limbs extending horizontally from their bodies, like reptiles, and elongated, leathery beak-like mouthparts which are toothless in adulthood.
Monotremes have many primitive features that resemble those of their reptilian ancestors in addition to laying similar leathery eggs. They possess multiple skeletal features that are unique within mammals but are derivative of those found in reptiles, such as epipubic bones and ribs attached to their neck vertebrae. They are warm-blooded but maintain a lower body temperature than most other mammals. They lack nipples but still exude milk at the surface of their skin to feed their young.
The duck-billed platypus is a partially aquatic species whose bill is a large sensory organ, sensitive to both touch and weak electric currents. Its feet are webbed, and its fur adapted to insulate it in the water. Echidnas, on the other hand, are not especially well adapted to the water, and their limbs bear large digging claws for burrowing. They possess spines all over their bodies, which they use as an effective defense.