- Each organism is different from the other to a lesser or greater extent.
- On one hand we have microscopic bacteria of a few micrometers in size, while on other hand blue whale and red wood trees of 30 metres and 100 metres respectively.
- Some of pine trees live for thousands of years while insects like mosquito die within a few days.
- Life also ranges from colourless or even transparent worms to brightly coloured birds and flowers.
Biologist, such as Ernest Haeckel (1894), Robert H Whittaker (1969) and Carl Woese (1977) have tried to classify all living organisms into broad categories called kingdoms.
The classification proposed by R H Whittaker is widely used and it is known as Five Kingdom Classification. These groups are formed on the basis of their cell structure, mode and source of nutrition and body organisation.
The major characteristics considered for classifying all organisms into five kingdoms are:
(i) Whether they are made of prokaryotic or eukaryotic cells.
(ii) Whether the cells are living singly or organized into multicellular organism.
(iii) Whether the cells have a cell wall and do they prepare their own food.
These organisms do not have a defined nucleus or organelles, nor do any of them show multi-cellular body. On the other hand, they show diversity based on many other characteristics.
Some of them have cell walls while some do not.
The mode of nutrition of organisms can be either by synthesizing their own food (autotrophic) or getting it from the environment (heterotrophic).
This group includes bacteria, blue-green algae or cyanobacteria, and mycoplasma.
This group includes many kinds of unicellular eukaryotic organisms.
Some 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 (Amoeba, Euglena, Paramoecium).
These are heterotrophic eukaryotic organisms.
Some of them use decaying organic material as food and are therefore called saprotrophs.
Others require a living protoplasm of a host organism for food. They are called parasites.
Many of them have the capacity to become multicellular organisms at certain stages in their lives.
They have cell walls made of a tough complex sugar called chitin.
Examples are yeasts, molds and mushrooms.
Some fungal species live in permanent mutually dependent relationships with blue green algae (or cyanobacteria). Such relationships are called symbiotic. These symbiotic life forms are called lichens. Lichens are found as the slow growing large, coloured patches on the bark of trees.
These are multicellular eukaryotes with cell walls. They are autotrophs and use chlorophyll for photosynthesis. Thus, all plants are included in this group.
These include all organisms which are multicellular eukaryotes without cell walls. They are heterotrophs (depend for food on others).
- The modification Woese introduced by dividing the Monera into Archaebacteria (or Archaea) and Eubacteria (or bacteria).
- Further classification is done by naming sub-groups at various levels as shown in figure:
- The first level of classification among plants depends on whether the plant body has well differentiated, distinct parts.
- The next level of classification is based on whether the differentiated plant body has special tissues for the transport of water and other substances.
- Further classification looks at the ability to bear seeds and whether the seeds are enclosed within fruits.
Plants that do not have well-differentiated body design fall in this group.
The plants in this group are commonly called algae.
These plants are predominantly aquatic.
Examples are Spirogyra, Ulothrix, Cladophora, Ulva and Chara
These are called the amphibians of the plant kingdom.
The plant body is commonly differentiated to form stem and leaf-like structures.
However, there is no specialized tissue for the conduction of water and other substances from one part of the plant body to another.
Examples are Moss (Funaria) and Marchantia
In this group, the plant body is differentiated into roots, stem and leaves and has specialized tissue for the conduction of water and other substances from one part of the plant body to another.
Some examples are Marsilea, ferns and horsetails
- The reproductive organs of plants in all these three groups are very inconspicuous, and they are therefore called ‘cryptogams’, or those with hidden reproductive organs.
- On the other hand, plants with well differentiated reproductive parts that ultimately make seeds are called phanerogams.
Seeds are the result of sexual reproduction process.
- They consist of the embryo along with stored food, which assists for the initial growth of the embryo during germination. This group is further classified, based on whether the seeds are naked or enclosed in fruits, giving us two groups: gymnosperms and angiosperms.
This term is derived from two Greek words: gymno– means naked and sperma– means seed.
The plants of this group bear naked seeds and are usually perennial, evergreen and woody.
Examples are pines (deodar), Cycas.
This word is made from two Greek words: Angio means covered and sperma– means seed. These are also called flowering plants.
The seeds develop inside an ovary which is modified to become a fruit.
Plant embryos in seeds have structures called cotyledons.
Cotyledons are called ‘seed leaves’ because in many instances they emerge and become green when the seed germinates.
The angiosperms are divided into two groups on the basis of the number of cotyledons present in the seed.
-Plants with seeds having a single cotyledon are called monocotyledonous or monocots. For example, Maize, wheat, rice etc.
-Plants with seeds having two cotyledons are called dicotyledonous or dicots. For example, sunflower, Peas, etc.
-These are organisms which are eukaryotic, multicellular, and heterotrophic.
-Their cells do not have cell-walls.
-Most animals are mobile.
The word Porifera means organisms with holes. There are holes or ‘pores’, all over the body.
These leads to a canal system that helps in circulating water throughout the body to bring in food and oxygen.
These are non-motile animals attached to some solid support.
These animals are covered with a hard outside layer or skeleton. The body design involves very minimal differentiation and division into tissues.
They are commonly called sponges and are found in marine habitats.
Some examples are Euplectella, Sycon, Spongilla, etc.
b. Coelenterata (Cnidaria)
These are animals living in water. They show more body design differentiation.
There is a cavity in the body. The body us made of two layers of cells: (1) makes up cells on the outside of the body and (2) the other makes the inner lining of the body.
Some of these species live in colonies (corals) while others have a solitary like-span (hydra).
Jellyfish and sea anemones are common examples.
The body of animals in this group are far more complexly designed.
The body is bilaterally symmetrical, meaning that the left and the right halves of the body have the same design.
There are three layers of cells from which differentiated tissues can be made, which is why such animals are triploblastic.
This allows outside and inside body linings as well as some organs to be developed. There is thus some degree of tissue formation.
However, there is no true internal body cavity or coelom, in which well-developed organs can be accommodated.
The body is flattened dorsoventrally (meaning from top to bottom), which is why these animals are called flatworms.
They are either free-living animals like planarians, or parasitic animals like liveflukes.
d. Nematoda (Aschelminths)
The nematoda body is also bilaterally symmetrical and triploblastic.
However, the body is cylindrical rather than flattened
There are tissues, but no real organs, although a sort of body cavity or a pseudocoelom, is present.
These are familiar as parasitic worms causing diseases, such as the worms causing elephantiasis (filarial worms) or the worms in the intestines (Roundworm or pinworms)
Annelid animals are also bilaterally symmetrical and triploblastic, but in addition they have a true body cavity. This allows true organs to be packaged in the body structure.
There is, thus, extensive organ differentiation. This differentiation 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– fresh water, marine water as well as land.
Earthworms and leeches are familiar examples.
This is probably the largest group of animals. These animals are bilaterally symmetrical and segmented.
There is an open circulatory system, and so the blood does not flow in well-defined blood vessels.
The coelomic cavity is blood-filled. They have jointed legs (the word ‘arthropod’ means ‘jointed legs’).
Some familiar examples are prawns, butterflies, houseflies, spiders, scorpions, and crabs.
In the animals of this group, there is bilateral symmetry.
The coelomic cavity is reduced.
There is little segmentation.
They have an open circulatory system and kidney-like organs for excretion.
There is a foot that is used for moving around.
Examples are snails and mussels.
In Greek, echinos means hedgehog (spiny mammal), 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 that they use for moving around.
They have hard calcium carbonate structures that they use as a skeleton.
Examples are sea-stars and sea urchins.
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 a proper 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 Amphioxus
These animals have a true vertebral column and internal skeleton allowing a completely different distribution of muscle attachment points to be 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:
(i) have a notochord
(ii) have a dorsal nerve cord
(iii) are triploblastic
(iv) have paired gill pouches
(v) are coelomate.
Vertebrates are grouped into six classes.
Cyclostomes are jawless vertebrates.
They are characterized by having an elongated eel-like body, circular mouth, slimy skin and are scaleless.
They are ectoparasites or borers of other vertebrates.
Petromyzon (Lamprey) and Myxine (Hagfish) are examples.
These are fish. They are exclusively aquatic animals.
Their skin is covered with scales/plates.
They obtain oxygen dissolved in water by using gills.
The body is streamlined, and a muscular tail is used for movement.
They are cold-blooded and their hearts have only two chambers.
They lay eggs.
Some fishes with skeleton are made entirely of cartilage, such as sharks, and some with a skeleton made of both bone and cartilage, such as tuna or rohu
These animals differ from the fish in the lack of scales, in having mucus glands in the skin, and a three-chambered heart.
Respiration is through either gills or lungs.
They lay eggs. These animals are found both in water and on land.
Frogs, toads and salamanders are some examples.
These animals are cold-blooded, have scales and breathe through lungs. While most of them have a three-chambered heart, crocodiles have four heart chambers.
They lay eggs with tough coverings and do not need to lay their eggs in water, unlike amphibians.
Snakes, turtles, lizards and crocodiles are some examples.
These are warm-blooded animals and have a four-chambered heart.
They lay eggs. There is an outside covering of feathers, and two forelimbs are modified for flight.
They breathe through lungs. All birds fall in this category.
Mammals are warm-blooded animals with four-chambered hearts.
They have mammary glands for the production of milk to nourish their young.
Their skin has hairs as well as sweat and oil glands.
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 very poorly developed young ones.
The system of scientific naming or nomenclature we use today was introduced by Carolus Linnaeus in 18th century.
In general name of the genus and species is written in Latin form.
Certain conventions are followed while writing the scientific names:
i. The name of genus begins with a capital letter.
ii. The name of the species begins with a small letter.
iii. When printed, the scientific name is given in italics.
iv. When written by hand, the genus name and the species name have to be underlined separately.
The binomial nomenclature makes for a uniform way of identification of vast diversity of life around us.
The binomial nomenclature is made up of two words – a generic name and a specific name.