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mammalany animal of the class Mammalia, a group of about 4250 species, often regarded as the most highly evolved animals, and so named because they all possess MAMMARY GLANDS. There are three living subclasses:
- Monotremata - MONOTREMES, primitive egg-laying mammals such as the duck-billed platypus and Echidna, the spiny ant eater.
- Marsupialia - MARSUPIALS, which transfer their young to pouches for the latter part of their early development.
- Eutheria - EUTHERIANS, which have a placenta.
Mammals are characterized by the presence of hair, a DIAPHRAGM used in AERIAL RESPIRATION, milk secretion in the female (LACTATION) for suckling the young, presence of only the left systemic arch in the blood circulatory system, three auditory ossicles in the ear, and a lower jaw of a single pair of bones. In all classes except the monotremes, the young are born live (see VIVIPAROUS).