Monotremes, the most primitive living mammals, are the only ones that lay eggs. They do not have the bridge of nervous tissue that connects the hemisphers of the brain in more advanced mammals. Like reptiles, they have only one posterior opening to the body - the anal and urion-genital apertures open into a common chamber. The way in which they regulate body temperature is more like mammals than reptiles. The control is fairly constant at ambient temperaures between 28 and 32 degrees C. but outside tehse limits it tends to vary. After hatching, the young are nourished on milk from teatless mammary glands. There are no functional teeth in the adults. The males have a poison spine on each ankle which is used in intra-sexual competition.
The single species, Ornithorhynchus anatinus is found in rivers and lakes in eastern Australia and Tasmania. Fossils suggest that these animals have been in Australia for at least 110 million years. Well adapted to water with dense underfur, a flattened tail, webbed feet, and no external ears. The sensitive snout is covered with soft rubbery skin and has sense organs capable of detecting the weak electrical fields given out by the aquatic invertebrates on which the platypus feeds.
Echidnas or spiny anteaters
Echidnas are to be found in New Guinea, Australia and Tasmania. They forage for invertebrates in soil or in the leaf litter on the forest floor. They are covered with short spines except on the belly. The snout is long and tubular (especially in the case of Zaglossus from New Guinea) and the long sticky tongue is used to sweep up ants and termites. There are no teeth but the palate has horny serrations which are used to grind up the prey. In the breeding season teh female develops a pouch into which she places her egg (usually one but sometimes two or three).
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