Mammalia

All the animals relating to class Mammalia are termed as mammals. In the animal kingdom, one of the most developed species are mammals that are categorized under Vertebrata. Around 6,000 species of living mammals are present on the planet. They are well adapted to their surroundings such as from deserts, oceans, and polar regions to rivers and rainforests, etc because of their distribution all over the world They are depicted by the presence of mammary glands which helps them to feed their offsprings.

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Main Characteristics of Mammals

Below is the list of characteristics of class Mammalia:

• Sweat glands and oil glands are found on the skin.

• Presence of mammary glands, which aid in the production of milk to feed their young.

• All these animals are warm-blooded.

• Their hearts are divided into four chambers.

• The tooth comes twice in these animals (Diphyodont).

• No nucleus is found in its red blood cells (except in camels and llama). 

• The skin of mammals has hair.

• External ear (pinna) is present in mammals.

• They are of different types - heterodont and thecodont. 

• The skull's form is dicondylic.

• A four-chambered heart is found in mammals. Both the renal portal and sinus venous systems are absent.

• They possess muscular diaphragms.

• They respire through lungs.

Classification of Mammals

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The largest class in the animal kingdom is Mammalia. Several types of mammals are there which can be differentiated up into smaller mammals, larger mammals, and marine mammals, etc. On the basis of their reproduction, they are classified into  2 subclasses. 

• Prototheria

• Theria

Subclass Prototheria

Prototherians are considered as the most primitive mammals and they are only restricted to Australia and its adjacent islands such as Tasmania and New Guinea. Prototheria is also named as Monotremes. This subclass includes egg-laying mammals. Prototheria includes one order that comprises 6 species. 

Order:  Monotremata

Example: Duck Billed platypus (Ornithorhyncus), Echidna (Tachyglossus).

Subclass Theria

The members of this subclass give birth to young ones. It is divided into 2 infraclasses - 

1. Metatheria

2. Eutheria

1. Infraclass Metatheria

The females of this infraclass possess a marsupium or brood pouch and are thus known as marsupials or pouched mammals. Immature young are born to the mammals in this infraclass and remain in their mother's pouch until they reach adulthood. There are approximately more than 250 species in this infraclass and they are inhabited throughout Australia, Papua New Guinea and some parts of America. 

For example, kangaroos, koalas and possums.

Metatheria Sub-class is Divided into 7 Orders Including 250 Species.

2. Infraclass Eutheria

Mammals that give birth to well developed young ones or the child directly belong to this infraclass. An embryo is formed by the young ones in their mother's stomach where they grow for a particular period of time. They derive nutrition for their growth and development from the placenta of the mother. This infraclass comprises 19 orders.

The orders of this infraclass are listed below-

Characteristics of Mammals

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Appendages:

• 2 pairs of pentadactyl limbs are present. In this, each foot has either 5 or fewer toes given with horny nails, claws or hooves.

• They are adapted variously for running, jumping, swimming, climbing, etc.

Digestive System:

• Relatively smallmouth with movable lips.

• True salivary glands present in the buccal cavity.

• Different types of teeth are attained by them (Heterodont).

Respiratory System:

• They breathe only through the lungs which are elastic and spongy.

• In breathing, both the rib muscles and diaphragm play a vital role for the purpose of respiration.

Circulatory System

• The circulatory system of mammals is closed.

• The heart pumps oxygenated blood which is transported to the several parts of the body.

• The hepatic portal system is present while the renal portal system is not.

• In most mammalian species, RBCs are concave, circular, and non-nucleated.

Excretory System:

• A pair of bean-shaped kidneys are found which are metanephric.

• They are ureotelic animals (excrete urea).

Sense Organ: 

• With the help of internal nasal passages, the olfactory sacs open far back into the pharynx.

• Movable lids are present in their eyes.

• An organ of Corti is present in the internal ear and 3 bony ear ossicles (malleus, incus, and stapes) are present in the middle ear.

Fun Facts of Mammalia

• In a mammal, the highest body temperature is of a goat (Average 39°Celsius).

• Echidna and Duck-billed Platypus are the egg-laying mammals.

• Atlantis of mammalian embryos help in the purpose of respiration.

• Excretion in hemichordates takes place by the glomerulus.

• The smallest man like Ape is an Orangutan.