Modern mammals have following characteristics:

1. They have hair on the body.

2. They have mammary glands.

3. They have specialized teeth. They have heterodont dentition

4. Three middle-car ossicics are present.

5. Diaphragm is present in them.

6. Their skin has sweat, sebaceous. scent glands.

7. Theirheart is four- chambered.

8. They have large cerebral cortex.

The zoologists disagree on subclass classification. Most zoologists consider that mid Cretaceous mammals diverged to form two sub-classes. These sub classes were: Prototheria and the Theria.

1. Sub-class Prototheria

The monotremes (the duck-billed platy pus and the echidna) were earlier classified in the subclass Prototheria. But recent fossil evidence shows that the monotremes have dentition like subclass theria. Thus monotremes were placed in subclass theria. Therefore, the Prototheria contains only extinct forms.

2. Subclass Theria

The subclass Theria diverged into three infraclasses in late Cretaceous period. These infraclasses are:

(a)   Infra class Ornithodelphia: (ornis, bird + delphia, birthplace). It contains monotremes (mono, one + trema, opening). The monotremes possess a cloaca. Monotremes are oviparous. The six species of monotremes are found in Australia and New Guinea.

(b)   Infraclass Metatheria: (meta, after) It contains marsupial mammals. They are viviparous. But they have very short gestation periods. They develop a protective pouch called the marsupium. It covers the female mammary glands. The young crawl into the marsupium after birth. They feed in the marsupium and complete their development. About 250 species of marsupials live in the Australian region and the Americas.

(e) Infraclass Eutheria: (Eu, true). It containsJN0-541 placental mammals. They are born at an advanced stage of development. The young is nourished within the uterus. Theydevelop placenta. The exchanges between maternal and fetal circulatory systems occur through placenta. Placenta is composed of both maternal 3101 and fetal tissue.This subclass contains 3,800 species. It has 17 orders.

By saadia

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