Organisms are classified into different groups in order to identify any specific organisms i,e for easier and convenient study of different organisms. At a time we can study the characteristics of a large number of organisms and the relationship between them.
Examples of range of variations observed in daily life are:
(i) Variation in size: size of organisms can vary from microscopic bacteria to giant redwood trees (about 100 metres).
(ii) Variation in lifespan: lifespan varies from organisms to organisms i.e for bacteria it is of few hours whereas in case of large trees it can be thousands of years .
(iii) Variation in body colour: most of the organisms (insects, flowers, birds) show a vast range of colours to attract and protect from predators.
The more basic characteristic for classifying organisms is the kind cells they are made of because there are a number of organisms which are living in a common habitat but they belong to different taxonomic groups. Organisms can be classified into eukaryotes (protista, fungi, plants and animals) and prokaryotes (monera) on the basis of cell type.
The primary characteristic on which the first division of organisms is made is the nature / kind of the cell - prokaryotic and eukaryotic cells. Nature of the cell includes the presence or absence of membrane-bound organelles. Therefore, on the basis of this fundamental characteristic, we can classify all living organisms into two broad categories of eukaryotes (protista, fungi, plantae, animalia) and prokaryotes (monera). Then, further classification is made on the basis of cellularity or modes of nutrition, sexual reproduction and phylogenetic relationship.
Plants and animals are put in different categories on the basis of whether the organisms produce their own food through the process of photosynthesis (plants). But, locomotion is considered as the characteristic feature that separates animals from plants. This is because the absence of locomotion in plants gave rise to many structural changes such as the presence of a cell wall (for protection), the presence of chloroplasts (for photosynthesis) etc.
Primitive organisms are those which have a simple body structure (lower organisms) and ancient body design that have not changed much. The organisms that have acquired more complex structure and body design are known as advanced organisms or higher organisms. For example, an Amoeba is more primitive as compared to a starfish. Amoeba has a simple body structure and primitive features as compared to a starfish. Hence, an Amoeba is considered more primitive than a starfish.
The advanced organisms will be the same as complex organisms . It is not always true that an advanced organism will have a complex body structure. This is because there is a possibility that over the evolutionary time, complexity in body design will increase.
The criterion for the classification of organisms belonging to kingdom Monera or Protista is the presence or absence of a well-defined nucleus. Kingdom Monera has no nuclear membrane and these are known as prokaryotes while kingdom. Protista shows well defined nucleus and these organisms are called eukaryotes.
Protista includes single celled, eukaryotic, and photosynthetic organisms.
In the hierarchy of classification, the grouping that includes the smallest number of organisms with a maximum of characteristics in common is species, whereas the kingdom will have the largest number of organisms.
Thallophyta or algae is the division of plants that has the simplest organisms. This group includes plants, which do not contain a well differentiated plant body. Their body is not differentiated into roots, stems, and leaves.
| Pteridophyta | Phanerogams |
| The reproductive organs of pteridophytes are inconspicuous or less differentiated. | The reproductive organs of phanerogams are well developed. |
| Pteridophytes have naked embryos that are called spores. | Phanerogams produce seeds which consist of embryo and stored food. |
| Example: Ferns, Marsilea, Equisetum, etc. | Example: Pinus, Cycas, fir, etc. |
| Gymnosperm | Angiosperm |
|---|---|
| Gymnosperms are non-flowering plants. | Angiosperms are flowering plants. |
| The plants of seeds of this group bear naked seeds and are usually perennial, evergreen and woody. | The seeds develop inside an organ which is modified to become a fruit. |
| Example: Pinus, Cedar, fir, Cycas, etc. | Example: Coconut, palm, mango, etc. |
| Porifera | Coelenterate |
|---|---|
| They are non-motile and found attached to rocks. | They either live in colonies (corals) or have a solitary life-span (Hydra). |
| The body of Porifera is made up of a single layer of cells. | The body of Coelenterate is made up of two layers of cells. |
| Example : Spongilla, Euplectella, etc. | Example : Hydra, sea anemone, corals, etc. |
| Annelids | Arthropods |
|---|---|
| The true body cavity called ‘coelom‘ is present. | The body cavity is present. It contains blood called ‘haemocoel’. |
| The body is divided into several identical segments. | The body is divided into a few specialized segments. |
| Amphibian | Reptiles |
|---|---|
| Respiration is either through gills or lungs. | Respiration is through lungs. |
| The skin of Amphibian is moist and soft. | The skin of Reptiles is hardened. |
| They lay eggs in water. | They lay eggs on land. |
| Example : frogs, toads, and salamanders. | Example: lizards, snakes, turtles, chameleons, etc. |
| Aves | Mammals |
|---|---|
| Their body is covered with feathers and they possess a beak. | They do not have feathers and the beak is also absent and their body is covered with hairs. |
| The bones are hollow for flying. | No hollow bones. |
Therefore, classification serves the following advantages:
(i) It determines the evolutionary relationship by establishing the gradually increasing complexity of form and structure in different groups of organisms .
(ii) It helps in understanding millions of life forms in detail.
(iii) It also helps in the development of other life sciences.
For classifying an organism in a hierarchical model, we choose the fundamental characteristic among several other characteristics. For example, plants differ from animals in having chloroplast and lacking locomotion. But, only locomotion is considered as the basic or fundamental feature that is used to distinguish between plants and animals. This is because the absence of locomotion in plants gave rise to many structural changes such as the presence of a cell wall for protection, and the presence of chloroplast for photosynthesis.
The five kingdoms proposed by R.H. Whittaker are Monera, Protista, Fungi, Plantae, and Animalia.
The basis for grouping organisms into five kingdoms is as follows:
(i) On the basis of Cell type, all living organisms are divided into two broad categories of eukaryotes and prokaryotes . This division led to the formation of kingdom Monera, which includes all prokaryotes.
(ii) On the basis of body organisation, organisms can be classified into cellular tissue, organ and organ system level. Unicellular eukaryotes form kingdom Protista, and multicellular eukaryotes form kingdom Fungi, Plantae, and Animalia.
(iii) Animals are then separated on the basis of presence or absence of a cell wall.
(iv) On the basis of mode of nutrition , since fungi and plants both contain a cell wall, they are separated into different kingdoms. Fungi have heterotrophic mode of nutrition, whereas plants have autotrophic mode of nutrition. This results in the formation of the five kingdoms.
The kingdom Plantae is divided into five main divisions: Thallophyta, Bryophyta, Pteridophyta, Gymnosperms, and Angiosperms.
The classification depends on the following criteria:
• Differentiated / Undifferentiated plant body
• Presence / absence of vascular tissues
• With/without seeds
• Naked seeds/ seeds inside fruits
(i) Thallophyta : these are simple , thalloid and undifferentiated body parts. They do not have roots, stems and leaves.
(ii) Bryophyta : their plant body is more differentiated and has root ,stem and leaf-like body parts. They do not have vascular tissue.
(iii) Pteridophyta : they have true root, stem and leaf. They possess well differentiated vascular tissue.
(iv) Gymnosperm : the plants in which ovules are not enclosed by any ovary wall i.e naked - seeded plants.
(v) Angiosperm : seeds are enclosed in fruits.
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Animals in Vertebrates are classified into five classes:
(i) Class Pisces: This class includes fish such as Scoliodon, tuna, rohu, shark, etc. These animals mostly live in water. Hence, they have special adaptive features such as a streamlined body, presence of a tail for movement, gills, etc. to live in water.
(ii) Class Amphibia: The amphibians have adopted to live both on land and water. They respire with the help of gills, lungs and through skins. They are cold - blooded animals. They lay eggs and development through larval stages. e.g : Frog, Salamander etc.
(iii) Class Reptilia: The class name refers to their creeping or crawling mode of locomotion. The body of a reptile is covered with dry and cornified skin to prevent water loss . They are cold - blooded animals. They lay eggs on land. e.g : Snakes, Chameleon etc.
(iv) Class Aves: The characteristics features of aves are the presence of feathers. Most of them have feathers. Their forelimbs are modified into wings for flight, while hind limbs have scales modified for walking and clasping. They are warm blooded and lay eggs. e.g : Crow, Pigeon etc.
(v) Class Mammalia: The most unique mammalian characteristic is presence of milk producing glands by which the young ones are nourished. Their skin has hair as well as sweat glands to regulate their body temperature. e.g : Humans, Lions, Dogs etc.
(a) True
(b) True
(c) False
(d) True
(e) False
(f) True
(a) saprophytic
(b) chitin
(c) lichen
(d) carbohydrate
(e) Species
(f) Thallophyta
(g) Bryophytes
Dicot seeds (having two cotyledons) : Gram, Pumpkin, Pea.
Monocot seeds (having single cotyledons) : Wheat, Rice, Maize
(A) (B)
(a) Naked seed (B) Gymnosperms
(b) Covered seed (A) Angiosperms
(c) Flagella (D) Euglena
(d) Marchantia (C) Bryophytes
(e) Marsilea (F) Pteridophyta
(f) Cladophora (E) Thallophyta
(g) Penicillium (G) Fungi
(A) (B)
(a) Pore bearing animals (C) Porifera
(b) Diploblastic (B) Coelenterata
(c) Metameric segmentation (F) Annelida
(d) Jointed legs (A) Arthropoda
(e) Soft bodied animals (E) Mollusca
(f) Spiny skinned animals (D) Echinodermata
Acoelomates (coelom absent) : Spongilla, Sea anemone, Planaria, Liver fluke.
Pseudocoelomates (false coeloms) : Wuchereria, Ascaris.
Coelomates (true coelom) : Nereis, Earthworm, Scorpion, Birds, Fishes and Horse.
Cartilaginous fishes areTorpedo, Sting ray and Dog fish.
Bony fishes are Rohu, Angler- fish and Exocoetus.
Two-chambered heart : Rohu, Scoliodon.
Three-chambered heart : Frog, Salamander, Flying lizard, King cobra.
Four-chambered heart : Crocodile, Ostrich, Pigeon, Bat, Whale.
Warm-blooded animals : Ostrich, Pigeon, Bat, Whale.
Cold-blooded animals : Rohu, Scoliodon, Flying lizard, King cobra, Frog, Salamander, Crocodile.
Two egg laying mammals are duck billed platypus (Ornithorhynchus) and Echidna (spiny ant eater).
(a) R. Whittaker
(b) Species
(c) Monera
(d) Unicellular eukaryotic organisation
(e) chlorophyll;
(f) like mushroom
(g) yeast;
(h) lichens.
(a) False
(b) True
(c) False
(d) True
(e) True
(f) True
(a) flatworms (,e.g. liver fluke)
(b) filarial worm (e.g. Wuchereria)
(c) arthropods (e.g. cockroach) and molluscs (e.g. Pila)
(d) nematodes (e.g Ascaris)
Caudal or tail fin helps in swimming of fish which acts as a steering organ.
(a) Thallophyta;
(b) Without vascular tissues;
(c) Pteridophyta;
(d) Phanerogams;
(e) Bear naked seeds;
(f) Angiosperms;
(g) Have seeds with two cotyledons;
(h) Monocots.
Names of some thallophytes are: Dictyota, Graceleria, Ectocarpus, Porphyra, Ulothrix, Spirogyra, Chara, Laminaria etc.
Labelled diagram of Spirogyra is as follows:
Cryptogams (Crypt-hidden, gamos-to marry) refers to plants having hidden sex organs. These are seedless plants including thallophyta, bryophyta and pteridophyta.
Gymnosperms and Angiosperms are collectively called “phanerogams” (Phaneros-visible, gamos-to marry) as both the groups have seeded plants and conspicuous reproductive organs.
Fig. A: Female Cycas plant
B: Male Cycas plant
(a) Bilateral symmetry: Animals like annelida, arthropoda, chordata etc. where the body can be divided into identical left and right halves in only one plane, exhibit bilateral symmetry. Examples-Planaria, earthworm, cockroach, all chordates such as birds, reptiles, humans etc.
(b) Coelom: The body cavity which is lined by mesoderm (cavity between the body wall and gut wall) is called coelom. Examples of animals in which true coelom is present are Earthworm, cockroach, snail, all chordates such as birds, humans etc.
(c) Triploblastic: Those organisms in which the developing embryo has a third germinal layer, mesoderm, in between the ectoderm and endoderm are called triploblastic organisms. Examples- Tapeworm, Ascaris, cockroach, Pila, fish, frog, crocodile, mammals etc.
As there are some characteristic features which are different among these organisms so we can not classify them in one group that's why they are separated from each other and placed in two different groups.
Leech and Nereis are placed in Phylum Annelida because:
Both are metamerically segmented.
Both have a closed circulatory system.
Scolopendra, Prawn and scorpion are placed in Phylum Arthropoda because :
Their body is externally segmented into three parts and consists of head thorax and abdomen.
These animals have an open circulatory system .
They have jointed appendages.
Mango tree is more complex and evolved among bacteria, mushroom and mango tree because of the :
Differences between flying lizard and bird are as follows:
| S.No. | Flying Lizard | Bird |
|---|---|---|
| 1 | It belongs to Class- Reptilia. | It belongs to Class-Aves. |
| 2 | It is a cold-blooded animal. | It is warmblooded animal. |
| 3 | Body covered by dry and rough scales. | Body covered by feathers. |
| 4 | Heart is three- chambered and is divided into two auricles and incompletely divided ventricles. | Four-chambered heart with two auricles and two ventricles. |
| 5 | Two pairs of pentadactyl limbs. | Forelimbs modified into wings. |
| 6 | Jaws and teeth present. | Jaws and teeth replaced by horny beak. |
Cat, rat and bat belong to Class-Mammalia and have following common features:
Both snake and turtle belong to the same Class Reptilia because they share reptilian characters like:
