Class Mammalia is divided into three subclasses based on the nature of reproduction. These are Prototheria, Metatheria and Eutheria.
1. Prototheria: The word Prototheria is formed from the Greek word protos meaning first and ther meaning mammal. Mammals that lay eggs are called ovipositors or prototheria. They live in Australia and New Guinea. Dr. Siddique Publications
(i) They are primitive egg-laying mammals. They are not immortal.
(ii) They lay eggs with a leathery covering. A newborn is born from an egg.
(iii) Newborns suck the mother’s milk. Because their mammary glands do not have nipples.
(iv) They have sharp lips, no teeth and no external ears.
(v) There is only one mucous membrane for excretory, reproductive and digestive system. Hence they are called monotrims.
Examples: Goose-billed Platypus (Ornithorhynchus anatinus), Short-billed Echidna (Tachyglossus aculeatus)
2. Metatheria: The word Metatheria is formed from the Greek word meta meaning medium and ther meaning mammal. Mammals that have a pouch called a marsupium are called marsupials or metatheria. They live in Australia, Tasmania, New Guinea, Central and South America.
(i) They are simple immortal mammals.
(ii) They give birth to very immature neonates.
(iii) Female animals have pouch called marsupium. In this pouch the new-born attain fullness. Hence they are called marsupials.
(iv) Mammary glands including nipples are exposed in marsupium.
(v) They have two uteri and two vaginas.
Examples: Red kangaroo (Osphranter rufus/Macropus rufus), Virginia opossum (Didelphis virginiana), Koala (Phascolarctos cinereus).
3. Eutheria: The word Eutheria is formed from the Greek word eu meaning nature and ther meaning mammal. Mammals that possess true immortality are called immortalized mammals or eutheria. They are spread throughout the world. Dr. Siddique Publications
(i) They are true immortal mammals. Their embryo consists of chorion and allantois.
(ii) Immortals provide nutrients during their flowering. The gestation period is long lasting.
(iii) Their embryos remain in the uterus for some time and give birth to mature neonates.
(iv) Mammary glands of female animals are highly developed. It contains nipples.
(v) Their vagina is one. The testes are located in the scrotal sac.
Example: Human (Homo sapiens), Shushuk (Platanista gangetica), mouse (Mus musculus), Rusa deer (Rusa unicolor), tiger (Panthera tigris), fruit bat (Pteropus giganteus).