Animal Kingdom-
R.H. Whittaker (1969), an American taxonomist, classified all the organisms into five kingdoms. As the viruses are on the border line of living and nonliving, they have not been included in the five kingdoms.
Whittaker's five kingdoms include Monera, Protista, Plantae, Fungi and Animalia.
The Kingdom Animalia is very diverse and includes varied forms of vertebrates and
invertebrates as sponges, corals, worms, insects, snails, starfishes, sharks, bonyfishes
frogs, lizards, snakes, turtles, crocodiles, birds and mammals.
.Animals are the consumer organisms of various orders, and form links in the different
food chains and complex food webs.
The earliest animals arose in the sea during the Precambrian era. Most phyla existing today appeared in the beginning of the Cambrian period, about 570 milion years ago.
Over a million species of animals have been described till now. These species differ in structure and form.
Inspite of these differences, there are fundamental features common to all animals
w.r.t. the arrangement of cells, body symmetry, nature of coelom, patterns of various organ-systems etc.
The study of general animal features, however, reveals evolutionary trends in almost each structure and function.
A gradual increase in complexity is noticeable from the lower group towards the higher group of animals.
. Animals are classified on the basis of following features:
ORGANISATION OF BODY
Scheme of organisation Animals are multicellular, heterotrophic eukaryotes.
The different types of cells are organised into functional units of progressively
increasing complexity, forming tissues.
There are four main types of tissues in animals: epithelial, muscular, connective and nervous. In more complex animals
these tissues function together as a coordinated unit for a common purpose.
Such a unit is termed an organ. For example, stomach,which is one of the digestive organs, contains all the four tissues In the most complex animals, groups of organs work together for a common function.
Such a group of organs is known as an organ system.
For instance, the organs, such as stomach, intestine, liver and pancreas, aid in the digestion of food, and constitute the digestive system.
Grades or levels of organisation
. Animals show three different grades of organisation:
Cellular grade : This level of organisation is the characteristic of sponges and mesozoans because in a sponges, only cells exhibit division of labour for
performing specialised functions
Tissue grade : This is of two types:
(i) Cell-tissue grade It is seen in coelenterates as the similar cells are specialised for different
functions and also form poorly defined tissues as
well.
(i) Tissue - organ grade: It appears in flatworms
(Platyhelminthes) with arrangement of tissues to form organs.
Organ system : Here organs function together ina coordinated system to perform a basic function.
Organ system is found in higher invertebrates and all vertebrates.
