Echidna and platypus lay eggs but are considered as mammals. Why?
Hint: Echidna and platypus both belong to the class Mammalia. The most unique mammalian characteristics is the presence of milk-producing glands (mammary glands) by which the young ones are nourished.
Complete Answer: - Echidna and platypus belong to the order of Monotremata of the class Mammalia. They are the only egg-laying mammals. - Monotremes possess mammalian features. In addition to these features, they also possess some of the reptilian features as well. - They are considered as primitive mammals with both mammalian and reptilian features. They are considered as primitive mammals with both mammalian and reptilian features. - Like reptiles, they also lay eggs, have a cloaca, a single opening through which the urine, feces, sperms, or eggs passes out. Thus this is termed as a reptilian feature. - Like mammals, monotremes are warm-blooded animals, their whole body is covered with fur to provide protection to their body. They also nourish and take care of their young ones. - Platypus is a semi-aquatic prototherian found in Tasmania, Australia, lives in burrows on the banks of rivers and streams. - The animal has a broad bill or beak, teeth only in the young stage, flat tail, webbed and clawed digits. Females possess mammary glands, but teats or nipples are absent. - Echidna is an insectivorous prototherian of Tasmania, Australia, and New Guinea. Jaws are prolonged into the beak-like muzzle. - Absence of teeth. The tongue is sticky. Hair present. The second claw of each hind limb is longer and curved.
Note: In Prototheria, the ear is devoid of the pinna, the scrotum is absent in male, vagina and uterus are absent.