Biologically, mammals are classified into three groups: placentals, marsupials, and which other group that includes the platypus and the echidna?
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Monotremes
The name 'monotreme' comes from the Greek words for 'single hole,' referring to the cloaca, a single opening used for both excretion and reproduction.
Also asked as:
- Placentals and marsupials are two of the three biological groups of mammals; what is the third group, which includes the echidna?