Classifying living things into groups have been made since time immemorial. Greek thinker Aristotle classified animals according to whether they lived on land, in water or in the air.
The process of classification within each group can then continue using new characteristics each time. Now-a-days, we look at many inter-related characteristics starting from the nature of the cell in order to classify all living organisms.
The classification of life forms will be closely related to their evolution.
Charles Darwin first described this idea of evolution in 1859 in his book, The Origin of Species.
There is a possibility that complexity in design will increase over evolutionary time, it may not be wrong to say that older organisms are simpler, while younger organisms are more complex.
Biologists, such as Ernst Haeckel (1894), Robert Whittaker (1969) and Carl Woese (1977) have tried to classify all living organisms into broad categories, called kingdoms. The classification Whittaker proposed has five kingdoms: Monera, Protista, Fungi, Plantae and Animalia, and is widely used.
These groups are formed on the basis of their cell structure, mode and source of nutrition and body organisation. The modification Woese introduced by dividing the Monera into Archaebacteria (or Archaea) and Eubacteria (or Bacteria) is also in use. Further classification is done by naming
the sub-groups at various levels as given in the following scheme:
Kingdom
Phylum (for animals) / Division (for plants)
Class
Order
Family
Genus
Species
Monera do not have a defined nucleus or organelles, nor do any of them show multi-cellular body designs. Some of them have cell walls while some do not. Of course, having or not having a cell wall has very different effects on body design here from having or not having a cell wall in multicellular organisms.
Protista 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.
These are heterotrophic eukaryotic organisms. Some of them use decaying organic material as food and are therefore called saprotrophs.
Some fungal species live in permanent mutually dependent relationships with bluegreen algae (or cyanobacteria). Such relationships are called symbiotic.
These are multicellular eukaryotes with cell walls. They are autotrophs and use chlorophyll for photosynthesis.
These include all organisms which are multicellular eukaryotes without cell walls.
The first level of classification among plants depends on whether the plant body has welldifferentiated, 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.
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.
The plant body is differentiated into roots, stem and leaves and has specialised tissue for the conduction of water and other substances from one part of the plant body to another. Some examples are Marsilea, ferns and horse-tails.
The plants with welldifferentiated 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.
Gymnosperms 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 and deodar.
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. Plants with seeds having two cotyledons are called dicots.
Animalia which are eukaryotic, multicellular and heterotrophic. Their cells do not have cell-walls. Most animals are mobile.
They are further classified based on the extent and type of the body design differentiation found.
Porifera 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.
These are animals living in water. They show more body design differentiation. There is a cavity in the body. The body is made of two layers of cells: one makes up cells on the outside of the body, and the other makes the inner lining of the body.
The body of animals in this group is far more complexly designed than in the two other groups we have considered so far. 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 called triploblastic.
The body is flattened dorsoventrally (meaning from top to bottom), which is why these animals are called flatworms.
The nematode 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.
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 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 welldefined blood vessels. The coelomic cavity is blood-filled.
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
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.
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:
have a notochord
have a dorsal nerve cord
are triploblastic
have paired gill pouches
are coelomate.
Vertebrates are grouped into six classes.
Cyclostomes are jawless vertebrates. They are characterised 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, unlike the four that humans have.
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 fall in this category
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 scientific name for an organism is thus unique and can be used to identify it anywhere in the world. The system of scientific naming or nomenclature we use today was introduced by Carolus Linnaeus in the eighteenth century. The scientific name of an organism is the result of the process of classification which puts it along with the organisms it is most related to. But when we actually name the species, we do not list out the whole hierarchy of groups it belongs to.
Certain conventions are followed while writing the scientific names:
The name of the genus begins with capital letter.
The name of the species begins with a small letter.
When printed, the scientific name is given in italics.
When written by hand, the genus name and the species name have to be underlined separately.