Classification of Kingdom Animalia 4

 

4- Class Amphibia

It is the first class of tetrapods. They have bony endoskeleton. Unlike fishes, amphibians have a neck. The first vertebra (cervical vertebra) moves against the back of skull and allows the skull to nod vertically. Their skin is smooth (without scales) and moist. It helps in gas exchange, temperature regulation, and absorption and storage of water. Their heart is double-circuit. It is three-chambered, with two atria and one ventricle. 

They respire by gills in the larval stage and by lungs and skin in the adult stage. They depend on external heat source and so are ectotherms. They cannot regulate their body temperature and cannot maintain it constant. So, they are poikilothermic animals and hibernate in winter. Salamander, newts, and mud puppies are tailed amphibians. Frogs and toads are tail-less amphibians, and caecilians are leg-less amphibians. Amphibians are unisexual. Fertilization is usually external.

5- Class Reptilia

Reptiles are the first animal group that possess amniotic eggs. Amniotic eggs make protective extra-embryonic membranes i.e., amnion, allantois, and chorion. These membranes protect the embryo from drying out, nourish it and enable it to develop on land. The amniotic eggs also contain a large amount of yolk, the primary food supply for the embryo. Such eggs have abundant albumin, which provides additional nutrients and water. The amniotic eggs are also covered with leathery calcareous shell which is partly permeable to gases but not to water. 

Reptiles have dry scaly skin. The bony endoskeleton of reptiles is harder than amphibians. The skull is longer than amphibians. In reptiles, first two cervical vertebrae (atlas and axis) allow more movements of head. In their heart, ventricle is incompletely partitioned, into left and right ventricles.

Reptiles, like amphibians, are ectothermic and use external heat source for thermoregulation. They cannot keep their body temperature at constant, and are poikilotherms. Fertilization in internal. They are oviparous (egg-laying). The present-day reptiles are lizards, snakes, tuatara and crocodiles.

6- Class Aves

Birds have a covering of feathers on the body. Feathers form the flight surfaces that provide lift and aid in steering. Feathers also prevent heat and water loss. Birds are endotherms. It means that they can obtain heat from cellular processes. A source of internal heat allows them to maintain a nearly constant core temperature. The animals who can maintain their core temperature are known as homeotherms.

The body of birds is streamlined and spindle shaped. The forelimbs are modified into wings. Their bones are light due to large air spaces. A lighter sheath called bill replaces the teeth. The sternum (chest bone) bears a large, bone called keel for the attachment of flight muscles.

In many birds a diverticulum of the oesophagus, called crop, is a storage structure that allows birds to quickly ingest large quantities of food. A region of stomach, called gizzard, has muscular walls to crush food. Their heart is four-chambered, with complete separation of atria and ventricles. Birds have much developed nervous system. Vision and hearing are important senses for most birds.

Their external nares open in pharynx through nasal passage ways. The pharynx leads to trachea and then bronchi. The organ of voice, called syrinx is situated at the lower end of trachea. The bronchi lead to a complex system of air sacs that occupy much of the body and even extend to some of the bones. The air sacs connect to lungs, which are made of small air tubes called parabronchi.

Like reptiles and mammals, birds have amniotic eggs with large amounts of yolk and albumin. Such eggs are also covered with leathery shell. In birds, fertilization is internal and development is external i.e., they are oviparous. Some birds have secondarily lost the power of flight and are called running birds e.g., ostrich, kiwi, rhea, cassowary, and emu. The flying birds include pigeon, parrot, crow, eagle, robin etc.

7- Class Mammalia

Mammalia includes the group of vertebrates which are nourished by milk from the mammary glands of mother, and have hair on their body. Mammals have skin glands, developed from epidermis. Sebaceous (oil) glands secrete oily secretion. Sudoriferous (sweat) glands release watery secretions used in evaporative cooling. Mammary glands are functional in female mammals. Most mammals have two sets of teeth during their lives i.e., milk teeth and permanent teeth. External ear or pinna is present. The middle ear has a chain of three bones i.e., incus, malleus and stapes. Mammals are endothermic and homoeothermic animals. They possess four-chambered heart. They have a muscular diaphragm that separates the coelom into thoracic and abdominal cavities. They have well developed voice apparatus in the form of larynx (with vocal cords) and epiglottis. In mammals, fertilization is internal. There are three groups of mammals:

Most mammals (placental mammals) give birth to young ones i.e., they are viviparous. Some mammals lay eggs and so are oviparous. While some (marsupials).

In egg-laying mammals lay eggs in which whole development of their embryo proceeds. These mammals are found in Australia e.g., Duckbill platypus and echidna (spiny anteater).

Some mammals (marsupials) have a pouch (marsupium) on the abdomen of female. These mammals give birth to immature young ones which complete their development in mothers' pouch. They are called ovoviviparous.

Opossum, kangaroo and Tasmanian wolf are the examples of such mammals.

-Duckbill platypus

-Spiny anteater

Placental mammals are the most advanced mammals. During development, a structure called placenta, is formed between mother's uterus wall and foetus body. The foetus is nourished and wastes from foetus are removed through this placenta. Dolphin, rat, monkey, bat, elephant and human are some examples of placental mammals.