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Diversity in Living Organisms

Chapter: Diversity in Living Organisms


Various forms of life exist on earth. There are microscopic bacteria - which are few micrometers in size; blue whale – which is of 30 metres in height and red wood Californian tree - of 100 metres in height. There are some bright coloured flowers and birds whereas some are colourless. Some trees like pine live for thousands of years and some insects like mosquitoes die within a few days. This puzzling diversity of life around us has evolved on the earth over millions of years. However, we do not have much time to spend observing each organism. Therefore, we try to sort them by the similarities present in the organisms and put them into different classes. These classes are sub classified as per their characteristics.

Basis of Classification:

Since long ago, several attempts were made to classify living things into groups. Aristotle is a Greek scientist who classified animals as per their place of existence i.e. whether they live on land, water or in the air. However, this way of classifying is misleading too, as creatures like whales, octopuses; sharks though they live in water, are very different from each other in many ways. Only their environment is what they share in common. Hence, we need to find more ways to classify a group of organisms. Appearance, behaviour, forms and functions can be termed as different ways in which they can be classified. Let us consider an example to understand this well.  A wall is built with stones of different sizes and shapes. The stones at the top surface of the wall will not affect the stones that are below them. Moreover, we can see that the sizes and shapes of the stones present in the lowermost layer are the deciding factors of the size and shape of its next layer and it continues. The characteristics in the next level would be dependent on the previous one and would decide the variety in the next level. In this way, we can build up a whole hierarchy of characters that are mutually related that can be used for classification.

Let us look at some concrete examples of some characteristics used for hierarchical classification.

1. A eukaryotic cell has a membrane bound nucleus and organelles, which allows cellular processes to be carried out efficiently. In addition, there are some organisms, which do not have a clearly demarcated nucleus and other organelles. Hence, they need to have their biochemical pathways organised in a different way. This would have an effect on every aspect of a cell design. Nucleated cells would have the capability to take part in making multi-cellular organisms as they can take up specialised functions. Therefore, this is a basic characteristic of a cell division.

2. Cells that assemble to form a single organism use the principle of division of labour. Not all cells would be alike in such type of a body design. Group of cells will carry out specific functions. This makes a very fundamental division in the body designs of organisms.

3. The ability of producing one’s own food versus having to get food from outside would make very diverse body designs essential.

The type of body design used for categorization of plants will be very different from categorization of animals. This difference is because of the different standard designs.

Classification and Evolution:

All living creatures are recognized and classified based on their body design, form and function. Some qualities are expected to make more wide-ranging changes in body design than others. Time also plays a vital role in this. Characteristics that came into survival earlier are likely to be more essential than characteristics that have come into survival later. Hence, we can conclude that the categorizations of life forms are closely related to their evolution. Now let us define what evolution is. Evolution is the change in the inherited phenotypic traits of biological population over successive generations. Evolution is raised by the gathering of changes in body design allowing the living organism to survive better. Charles Darwin is the first scientist who described the evolution in 1859 in his book called the “Origin of Species”.

What is biodiversity?

There is a great diversity of plants and animals around us. Life on earth is quite vivid and every living being appear to be different with respect to appearance and mode of life and they maintain their own identity. This phenomenon is known as Bio-Diversity.

The Hierarchy of Classification:

Some biologists like Ernst Haeckel (1894), Robert Whittaker (1959) and Carl Woese (1977) have classified living organisms into kingdoms. Whittaker has classified organisms into five kingdoms and they are Monera, Protista, Fungi, Plantae and Animalia. He classified organisms based on their cell structure, mode and source of nutrition and body organisation. The amendment that Woese introduced by separating the Monera into Archaebacteria (or Archaea) and Eubacteria (or Bacteria) is also in use.

Further classification is done by naming sub-groups at various levels as given in the following scheme:


Phylum (for animals) / Division (for plants)






Species is the basic unit of classification. Species include all the organisms that are similar enough to breed and perpetuate.



The Five Kingdom Classification (WHITTAKER'S Classification):


1. Monera:

Moneras do not have a definite nucleus or organelles, they also do not show any multi-cellular body designs. They show diversity based on many other characteristics. Some of them have cell walls while some do not. The mode of nutrition of organisms belonging to this group can be either be synthesizing their own food (autotrophic) or getting it from the environment (heterotrophic). This group includes bacteria, blue-green algae or cyanobacteria, and mycoplasmas.

2. Protista:

Protista group includes many types of unicellular eukaryotic organisms. Several of these organisms use appendages, such as hair-like cilia or whip-like flagella for moving around. Their mode of nutrition can be autotrophic or heterotrophic. Examples are unicellular algae, diatoms and protozoans.

3. Fungi:

Fungi are heterotrophic eukaryotic organisms. They are called saprophytes as they employ decaying organic material as food. Cell walls in fungi are made up of a tough complex sugar that is called chitin. Yeast and mushroom are some examples. Some fungal species exist in permanent mutually dependent relationships with blue- green algae (or cyanobacteria). Such relationships are called symbiotic relationships. These symbiotic life forms are called lichens. These lichens can be shown as the slow-growing large coloured patches on the bark of trees.

4. Plantae:

Plantae are multi-cellular eukaryotes with cell walls. They are autotrophs and they utilise chlorophyll for photosynthesis.  Hence, we can include plants in this group.

5. Animalia:

Animalia include all organisms, which are multi-cellular eukaryotes without cell walls. They are heterotrophs.

Let us look at plantae and animalia in detail:

Kingdom Plantae:

The primary level of categorization among plants depends on whether the plant body has well- differentiated, distinct components.

The later level of categorization is based on whether the differentiated plant body has special tissues for the transport of water and other substances present in it.

Classification further goes on what considers the capacity to tolerate seeds and whether the seeds are rooted within fruits.



Division of plantae is as follows:


i. Thallophyta:

This group contains the plants that do not have healthy differentiated body design. The plants in this group are commonly known as algae. These plants are primarily aquatic. Examples are ulothrix, cladophora and chara.

ii. Bryophyta:

Bryophyta are called the amphibians of the plant kingdom. Stem and leaf-like structures come under this classification. Still, there is no specialised tissue for the conduction of water and other substances from one part of the plant body to another. Examples are moss (Funaria) and Marchantia.

iii. Pteridophyta:

The plant body is classified into roots, stem and leaves. It also has some particular tissue, which is specialised for conducting water and other substances from one part of the plant body to another. Some examples are Marsilea, ferns and horsetails. Spores are the naked embryos, which are present in the Thallophytes, the Bryophytes and the Pteridophytes. The reproductive organs of plants in all these three groups are very ordinary, and they are therefore, called ‘cryptogamae’, or ‘those with hidden reproductive organs’.


Phanerogams are the flowering plants. Plant body of phanerogams is well differentiated into roots, stem and leaves. They have a vascular tissue and reproductive tissue, which ultimately make seeds. Phanerogams are further classified considering the seeds are naked or enclosed in a fruit.

a. Gymnosperms (gymno- naked, sperma- seeds):

These plants are evergreen, perennial having unbranched stem and conifer leaves.  These plants bear separate male and female flowers called as "cones". They do not form fruits and their seeds are open (naked).

Examples are cycas, pinus (pines), cederus (deodar).

b. Angiosperms:

Angiosperm is derived from two Greek words angio and sperma. Angio means covered and sperma means seed. The seeds grow inside an organ and are thus modified as a fruit. These are also called as flowering plants. Cotyledons are the structures present as plant embryos in seeds. Cotyledons are defined as ‘seed leaves’ because in many cases they come out and become green when the seed germinates. Hence, cotyledons correspond to a bit of pre-designed plant in the seed.
The angiosperms are further classified into two groups depending on the number of cotyledons present in the seed. Plants with seeds having a single cotyledon are called monocotyledonous or monocots. Plants with seeds having two cotyledons are called dicots..

Kingdom Animalia:

Kingdom animalia are organisms, which are eukaryotic, multi-cellular and heterotrophic. Cell walls are not present in them. Most of the animals are mobile in nature. They are further classified depending on the extent and type of the body design differentiation found.

i. Porifera:

The word means organisms with holes. These are non-motile animals attached to some solid support. There are holes or ‘pores’, all over the body. These lead to a canal system that helps in circulating water throughout the body to bring in food and oxygen. These animals are covered with a hard outside layer or skeleton. The body design involves minimal differentiation and division into tissues. They are commonly called sponges, and are mainly found in marine habitats.

ii. Coelenterata (cnidaria) :

These are the animals living in water. They show more body differentiation. The body of these animals consists of two layers of cells: one layer of cells makes up the outside of the body, and others make the inner lining of the body. They have a body cavity called coelom. Some of these species live in colonies (corals), while others have a solitary life span (Hydra). Jellyfish and sea anemones are common examples.

iii. Platyhelminthes:

The body of animals of this group is bilaterally symmetrical, which means that the left and the right halves of the body have the identical design. There are three layers of cells from which, different tissues can be made. Hence, these are called triploblastic. There is some degree of tissue formation. Still, there is no true internal body cavity or coelom, in which well- developed organs can be accommodated. The body is flattened dorsiventrally from top to bottom so these are called flatworms. They are generally free- living or parasitic. Planarians, or parasitic animals like liver flukes are some examples of free living organisms.

iv. Nematoda:

The nematode body is also symmetrical bilaterally and triploblastic. The body of nematode is not flattened but is cylindrical. It has no real organs. Tissues are present along with a kind of body cavity or a pseudo-coelom. These are well known as parasitic worms causing diseases, such as the worms causing elephantiasis (filarial worms) or the worms in the intestines (roundworm or pinworms).

v. Annelida (segmented worms)

Annelida are symmetrical bilaterally and triploblastic. They also have real body cavity because of which real organs are packaged in the body structure. This shows extensive organ differentiation. This separation occurs in a segmental fashion, with the segments lined up one after the other from head to tail. These animals are found in a variety of habitats– i.e. fresh water, marine water and land. Earthworms and leeches are some familiar examples.

vi. Arthropoda (arthros-jointed, podas-feet) (Animals with jointed legs):

Arthropoda is termed to be the largest group of animals. These are symmetrical bilaterally and segmented. There is an open circulatory system and blood does not flow in well- defined blood vessels. The coelomic cavity is blood-filled and they have joint legs. Prawns, butterflies, houseflies, spiders, scorpions and crabs are some known examples.

vii. Mollusca:

These animals are mostly aquatic with unsegmented soft body covered in a shell, triploblastic, bilaterally symmetrical or asymmetric as in pila. The body is distinguished into three parts:

  1. Anterior head with sense organs
  2. Dorsal visceral mass with organ system
  3. Ventral foot for locomotion

They have an open circulatory system and kidney like organ for excretion. Examples are snails and molluscs.

viii. Echinodermata:

In Greek, echinos means hedgehog, and derma means skin. Thus, these are spiny-skinned organisms. These are exclusively free-living marine animals. They are triploblastic and have a coelomic cavity. They also have a peculiar water-driven tube system, which they use for moving around. They have hard calcium carbonate structures that they use as a skeleton. Examples are starfish and sea urchins.

ix. Protochordata:

These animals are bilaterally symmetrical, triploblastic and have a coelom. In addition, they show a new feature of body design, namely a notochord, at least at some stages during their lives. The notochord is a long rod-like support structure (chord=string) that runs along the back of the animal separating the nervous tissue from the gut. It provides a place for muscles to attach for ease of movement. Protochordates may not have an appropriate notochord present at all stages in their lives or for the entire length of the animal. Protochordates are marine animals.

Examples are Balanoglossus, Herdmania and Amphixious.

x. Vertebrata:

Animals coming under vertebra category have a true vertebral column and internal skeleton, which allows a very diverse distribution of muscle attachment points that are used for movement.

Vertebrates are bilaterally symmetrical, triploblastic, coelomic and segmented, with complex differentiation of body tissues and organs. All chordates possess the following features:

  1. have a notochord
  2. have a dorsal nerve cord
  3. are triploblastic
  4. have paired gill pouches
  5. are coelomate


Vertebrates are grouped into five classes:

a. Pisces:

  1. These animals are exclusively aquatic. They have a streamlined rough body having skin covered with scales.
  2. They possess covered or uncovered gill slits for respiration and paired fins to swim in water. Tail fins help to change direction while swimming. Heart is two chambered.
  3. They are cold-blooded animals. (Body temperature varies as the temperature of environment). They lay a large number of eggs. Fertilization is external.
  4. Some have endoskeletons made up entirely of cartilage, such as sharks and some with endoskeleton made up of both bone and cartilage, as in Tuna or Rohu. Examples are Stingray, Dogfish.

b. Amphibia:

  1. These animals vary from the fish having scales, mucus glands in the skin, and a heart with three chambers. 
  2. Respiration is through either gills or lungs. They lay eggs.
  3. These animals are found both in water and on land. Frogs, toads and salamanders are some examples.

c. Reptilia (crawling vertebrates):

  1. They are cold-blooded, having scales and breathe through lungs.
  2. Most of them have a three-chambered heart, crocodiles have four heart chambers.
  3. They lay eggs with tough coverings and do not need to lay their eggs in water, unlike amphibians. Snakes, turtles, lizards and crocodiles fall in this category.

d. Aves:

  1. These are warm-blooded animals and have a four-chambered heart. They lay eggs.
  2. There is an outside covering of feathers, and two forelimbs are modified for flight..
  3. They breathe through lungs. All birds fall in this category.

e. Mammals:

  1. Mammals are warm-blooded animals with four-chambered hearts..
  2. They have mammary glands for the production of milk to nourish their young ones..
  3. Their skin has hairs as well as sweat and oil glands.
  4. Most mammals familiar to us produce live young ones. However, a few of them, like the platypus and the echidna lay eggs, and some, like kangaroos give birth to a very poorly developed young one.