Monotremes differ from other mammals in that they have a singoe opening for their urinary, digestive and reproductive tracts (this single opening is known as a cloaca and is similar to the anatomy of reptiles). Monotremes lay eggs and like other mammals lactate (produce milk) but instead of having nipples like other mammals, monotremes secrete milk through mammary gland openings in the skin. Adult monotremes do not have any teeth.
Monotremes are long-lived mammals. They exhibit a low rate of reproduction. Parents take close care for young and tend to them for long periods of time before they become independent.
The fact that monotremes lay eggs is not the only factor that distinguishes them from other mammal groups. Monotremes also have unique teeth that are thought to have developed independently of the teeth that placental mammals and marsupials have (even though the teeth may be convergent evolutionary adaptations due to similarities). Monotremes also have an extra set of bones in their shoulder (the intercalvical and coracoid) which are missing from other mammals.
Monotremes also differ from other mammals in that they lack a structure in their brain called the corpus callosum (the corpus callosum forms a connection between the left and right hemispheres of the brain).
Monotremes are the only mammals known to posess electroreception, a sense that enables them to locate prey by the electric fields generated by its muscle contraction. Of all monotremes, the platypus has the most sensitive level of electroreception. Sensitive electroreceptors are located in the skin of the platypuses bill. Using these electroreceptors, the platypus can detect the direction of the source and the strength of the signal. Platypuses swing their head from side to side when hunting in water as a way of scanning for prey. Thus when feeding, platypuses do not use their sense of sight, smell or hearing and rely instead only on their electroreception.
The platypus (Ornithorhynchus anatinus) is an odd looking mammal with a broad bill (that resembles the bill of a duck), a tail (that resembles the tail of a beaver) and webbed feet. Another oddity of the platypus is that male platypuses are venomous. A spur on their hind limb delivers a mixture of venoms that are unique to the platypus.
Echidnas include four living species. They feed on ants and termites and are solitary animals. They are coverd with spines and course hair. Although echidnas resemble hedgehogs, porcupines and anteaters, they are not closely related to any of these other mammal groups. Echidnas have short limbs that are strong and well-clawed, making them good diggers. They have a small mouth and do not have any teeth. They feed by ripping apart rotten logs, ant nests and mounds then licking up ants and insects with their sticky tongue. Echidnas are named after a monster of the same name, from Greek mythology.