MAMMALS

General Characters of Mammals

Although mammals have evolved from reptiles they show many important structural differences. These differences are in fact the general characters of mammals which are as follows:

1.Most mammals have a body covering of hair instead of scales.

2.There is a muscular diaphragm in mammals that separates the thoracic and abdominal cavities. This structure is not found in any previous group.

3. The lower jaw in mammals is composed of only one large bone and articulates directly with skull.

4.External ear or pinna in present. There is a chain of three bones in the ear Malleus, Incus & Stapes.

5.Mammals have deciduous and permanent teeth in some mammals e.g. man the
teeth are in two sets, one in early life the milk teeth and later the permanent teeth.

6.Mammals have 4-chambered heart and only left aortic arch (in birds it is right).

7. Mammals are warm blooded (Homeothermic) animals. 

8.The red blood cells are non-nucleated.

9.Mammals have well developed voice apparatus, the larynx and epiglottis.
Most mammals give birth to young (viviparous).

10.Mammals feed their young on milk produced by mammary glands of mother.

11.Mammals are classified into three sub-classes.

1. Prototheria

egg-laying mammals

2 Metatheria

pouched mammals

3.Eutheria

Placental mammals including man

Sub-Class Prototheria

 The Prototheria is that group which has characteristics of both reptiles and mammals and therefore form a connecting link between the two. They also provide evidence of the evolution/origin of mammals from reptilian stock. Certain members of this sub-class are adapted for aquatic life as the duck bill which has a bill similar to that of a duck and has webbed toes. It has thick fur on its body. The female has mammary glands to feed the young. Both these are mammalian characters. At the same time these animals. have cloaca and cloacal opening instead of separate openings for digestive system and urinogenital system. Both these characters are reptilian characters. These animals are found in Australia, e.g. Duck bill Platypus & Echidna (Spiny ant eater).

Sub-Class

 Eutheria This sub-class includes placental mammals. In the body of mother development of young is maximum and the young when born are fully developed. In these mammals during development a structure known as placenta is formed through which the fetus is nourished. Also the placenta has endocrine function i.e. it produces certain hormones, for this reason these mammals are also called placental mammals. Placental mammals have maximum mammalian characters but in some the hair have become modified into scales (pangolin) and spines (porcupine). Examples are man, whale, elephant, horse, rat, mice, bat, dolphin,e.t.c.