When 18th-century scientists first came across Australia's platypus , they thought it was a trick. It is not surprising that the platypus made people confused. This funny-looking animal has feet and is a kind of warm-blooded mammal. While other mammals usually keep their blood at around 37 degrees, the platypus has a lower body temperature of 32 degrees. As to its appearance, the platypus'mouth is not really like ducks'at all; its mouth actually looks a bit soft. The platypus closes its eyes when swimming. It uses its mouth to pick up outside information made by the creatures underwater. Strangest of all, the platypus is a mammal that can lay eggs. And there is only one other kind of mammal that can lay eggs, the echidna   of Australia. Both the platy-pus and the echidna lay soft-shelled eggs, and both feed their young with their own milk that comes out of their skin. These animals also walk in a way that is similar to crocodiles , with legs on the sides of their bodies rather than under them. Though pretty, in a special way, the platypus is actually one of the few mammals that are poisonous. A male platypus has knife-like bones on its back legs which have enough poison to kill a dog. The platypus has the honor of being one of the oldest mammals in the world. Until the early 20th century, it was hunted for its fur, but the situation has changed. Although the platypus is easily affected by pollution, it is not under any immediate threat. Which of the following is NOT used as a comparison with the platypus in the text?
The crocodile. The echidna. The duck. The dog.