MONOTREMATA, the lowest existing order of mammals, including the duck-billed platypus of Australia and the echidnas or spiny anteaters of Australia and New Guinea. Unlike all other existing mammals, these animals lay eggs ; their right and left ovi ducts remain separate, not uniting to form a median uterus, and the ducts, leading respectively from the ovaries or testes, from the bladder and from the digestive tract, all open into a single cloaca or common outlet (whence the name Monotremata).
The platypus (q.v.) is adapted for semi-aquatic life, having an otter-like form, with webbed feet ; it is covered with smooth sleek fur and has a long beaver-like tail. It eats small crustaceans, worms and insect larvae, obtaining them by nuzzling in the mud with its duck-like bill. It has true molar teeth when young, but later these are replaced by horny plates.
The echidna (q.v.) lives in dry country, is covered with quills and coarse hair and has a short tail and a long pointed toothless bill with an extensile tongue for feeding upon ants. It is more compactly built than the platypus and has immense strength for digging. The New Guinea echidna (Zaglossus) is larger, and has a very long bill and short spines almost covered by long under-fur.
In 1792 the "spiny anteater" of New South Wales, Australia, was described and figured by Shaw, as Myrmecophaga aculeata, the impression of the author being that it was related to the South American anteater. In 1799 Shaw published the first description and figures of the duckbill, naming it the "Platypus anatinus," and noting the beaver-like tail and body, the serrated duck like bill, the small eyes, the ear-holes behind the eyes, the webbed and clawed feet and other details. He came to the conclusion that it must be a resident in watery situations ; that it has the habits of digging or burrowing in the banks of rivers, or under ground; and that its food consists of aquatic plants and animals. In 1800 Blumenbach received a stuffed specimen collected by Sir Joseph Banks, which he described under the name "Ornitho rhynchus paradoxus," referring it to the Edentata (q.v.). The gen eric name Platypus of Shaw had already been invented by another author for a genus of beetles, so that the correct scientific name of the typical species is Ornithorhynchus anatinus; but the name platypus is now the common one for the animal.
In 1802 Home published a description of the anatomy of two spirit specimens of the platypus, in which he noted that "the structure of the female organs is unlike anything hitherto met with in quadrupeds. . . ." He was led to examine the corresponding
parts in birds and reptiles and found most resemblance to the organs of ovoviviparous lizards. Later in the same year Home examined the anatomy of the spiny anteater. He at once recog nized its affinity to the platypus and included it in the same genus as "Ornithorhynchus hystrix." Home also concluded that Ornithorhynchus represented a new tribe of quadrupeds, "so that it may be considered as an intermediate link between the classes Mammalia, Ayes and Amphibia. . . . Between it and the bird, no link of importance seems to be wanting." Thus arose the mis leading and still widely prevalent notion that the platypus is a link between mammals and birds, whereas its true position is more that of a link between mammals and reptiles.
In 1803 Etienne Geoffroy, the colleague of Cuvier, established a new order, Monotremata, characterized by : "Digits clawed, no true teeth; a common cloaca opening to the exterior by a single orifice." In 1809 Lamarck added a class, Prototheria, for the platypus and echidna, deciding that they were not mammals, for they had no mammary glands and were probably oviparous; they were not birds, for their lungs differed, and they had no wings; and they were not reptiles, for they possessed a four-chambered heart. Then followed a long argument whether the monotremes were to be classed as mammals. Finally in 1834 De Blainville arranged the mammals under three subclasses : (I) Ornithodelphia (bird-like uteri) : including the monotremes; (2) Didelphia (double uteri) : including the marsupials; (3) Monodelphia (single uteri) : including the placental mammals. In 1884 Caldwell obtained evidence enabling him to cable to the British Association meeting at Montreal, "Monotremes oviparous, ovum meroblastic." In 1891 Richard Semon secured monotreme material in Queens land, including new-laid eggs and a series of embryos of the echidna. He deduced that the egg of echidna, unlike the eggs of reptiles or birds, grows within its shell by receiving nutriment through the shell from the uterine tissues of the mother. During the past 20 years H. Burrell, working under Professor Launcelot Harrison, has given accurate descriptions of the underground tunnels dug by the platypus, and of the carefully constructed nests of leaves and grass in which their eggs are laid.