monotreme [key], name for members of the primitive mammalian order Monotremata, found in Australia, Tasmania, and New Guinea. The only members of this order are the platypus, or duckbilled platypus, and the several species of echidna, or spiny anteater. Although monotremes possess the distinguishing mammalian features of hair and mammary glands, they are unique among mammals in laying eggs rather than giving birth to live young. The eggs are like those of reptiles, with large yolks and leathery shells. Like birds and reptiles, monotremes have a single opening, the cloaca, for the passage of liquid and solid wastes, the transfer of sperm, and, in the female, the laying of eggs. In addition, certain features of the skeletal structure are like those of reptiles, and the regulation of body temperature is less effective than in other mammals. Adult monotremes are toothless. The males possess spurs on their hind feet; these are connected to poison glands and are presumably used as weapons. Mammals are known to have evolved from reptiles; the monotremes probably branched off at an early stage of mammalian evolution and have retained many reptilian features. They are classified in the phylum Chordata, subphylum Vertebrata, class Mammalia, order Monotremata.
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