Dictionary of Biological Psychology
The members of the class Mammalia (see TAXONOMY) are distinguished, generally, by the presence of hair (those without hair, such as the naked mole rat, have lost their hair for specific evolutionary reasons) and, more importantly, by the presence (in females) of mammary glands for the production of milk. There are three subdivisions of the class: MONOTREMES, MARSUPIALS and the EUTHERIAN MAMMALS. The monotremes are the most primitive, distinguished by laying eggs. Platypuses and echidnas are monotremes.
The marsupials—kangaroos, opossums, bandicoots and koalas—are distinguished by early birth and subsequent nursing within a maternal pouch (the marsupium). All other mammals are eutherian. The young of eutherian mammals develop for relatively long periods within the UTERUS. The eutherian mammals are divided into many orders, including PRIMATES, RODENTS and many others.
Reference
Campbell N.A., Reece J.B. & Mitchell L.G. (1999) Biology, 5th edn, Addison-Wesley: Menlo Park CA.
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