NCERT Solutions For Class 9 Science Chapter 7
Diversity in Living Organisms

We should choose the basic characteristics among several other characteristics for developing a hierarchy of classification. The basic characteristics that are chosen are free of any other characteristics in their effects on the form and function of the organism. The characteristics in the next level would be dependent on the previous one and would help in deciding the variety in the next level. In this way, the entire hierarchy of commonly related characteristics would be build to be used for classification. This helps in the establishment of hierarchy of characteristics.

Following are the advantages of classifying organisms:

1. Classifying organisms helps us in recognising and establishing the basic arrangement of a hierarchical structure among diverse species.
2. It helps us in establishing relationship and resemblances between various organisms and thus facilitates research of wide variety related with organisms.
3. It helps in understanding the evolution of organisms, as it gives us the information,allowing a reconstruction of phylogeny of life.

All the vertebrates are bilaterally symmetrical, triploblastic, coelomic, and segmented with complex differentiation of body tissues. Following features are common to all chordates:

1. notochord
2. dorsal nerve cord
3. triploblastic
4. paired gill-pouches
5. coelomate

Vertebrates are grouped into following five classes:

Criteria for deciding divisions in plants are:

1. Differentiated or undifferentiated plant body
2. Presence/ absence of vascular tissues
3. With or without seeds
4. Naked seeds/seeds inside fruits

Criteria for deciding sub-groups in animals are:

Kingdom Animalia is divided into two major groups on the basis of the presence or absence of a notochord.

Non-chordates do not possess a notochord, while all members of a phylum chrodates possess a notochord.

Non-chordate is further divided into subgroups on the basis of the following features.

1. Presence or absence of tissues
2. Type of body symmetry (Radial, Bilateral)
3. Types of coelom (Acoelom, Pseudocoelom, eucoelom)
4. Types of true coelom (Enterocoelom and Schizocoelom)
5. Presence of number of germ layers during embryonic development (Diploblastic and Triploblastic)

On the basis of the above features, non-chordates are divided into the following sub-grpups.

Porifera, Coelentrata, Platyhelminthes, Nmatodes, Annilides, Molluscs, Arthropoda and Echinodermata

All members of the phylum chordate possess a non-chordate. However, some animals such as Balanoglossus, Amphioxus, Herdmanina, etc have a notochord which is either absent or does not run the entire length of the animal’s body. Therefore, these animals are kept in a separate subphylum called Protochordata.

The rest of the chordates are included in the subphylum vertebrata. The members of the sub-phylum vertebrata are advanced chordates. They are further categorised into five classes, Pisces, Amphibia, Reptilia, Aves and Mammalia.

The kingdom Plantae is divided into five main divisions: Thallophyta, Bryophyta, Pteridophyta, Gymnosperms and Angiosperms.

The classification depends on the following criteria:

1. Differentiated or undifferentiated plant body
2. Presence or absence of vascular tissues
3. With or without seeds
4. Naked seeds or seeds inside fruits
    

1. The first level of classification depends on whether a plant body well differentiated or not. The plants that do not have a well differentiated body are grouped as thallophyta.   
2. On the basis of the presence or absence of vascular tissues, plants are catogorised as bryophytes (plants without vascular tissues) and tracheophytes (plants with vascular tissues).  
3. On the basis of the absence/presence of seed formation, tracheophyta is sub-divided into two divisions pteridophyta and phenerogams.
4. Pteridophytes do not produce seeds.
5. Phanerogams have well developed reproductive organs that produce seeds.
6. On the basis of the presence of naked seeds and seeds enclosed in fruits, phanerogams are further sub-divided gymnosperms and angiosperms respectively.
7. Gymnosperms are the seed bearing non-flowering plants, whereas angiosperms are flowering plants in which the seeds are enclosed within the fruit.

In 1959, Robert H Whittaker proposed a five kingdom classification of living organisms on the basis of cell structure, mode and source of nutrition and body organisation as main features. The five kingdoms proposed by Whittaker are Monera, Protista, Fungi, Plantae and Animalia.

The basis for grouping organisms into five kingdoms:

1. Complexity of cell structure- based on the presence of membrane bound cell organelles and well-defined nucleus the cells are broadly categorised into: prokaryotic and eukaryotic.
2. Cell structure: Presence or absence of cell wall is another important characteristic
3. Unicellular and multicellular organisms: Based on the number of cells present in the body of the organisms, they are categorised into unicellular and multicellular 
4. Mode of nutrition: Organisms basically have two types of modes of nutrition autotrophic and heterotrophic and they are classified according to that. 
5. Phylogenetic relationship: Phylogeny is the evolutionary history or ancestry of an organism. It is a belief that simple organisms are primitive and complex organisms are advanced. It is also believed that the advanced organisms are evolved from primitive ones. Thus, primitive and advance nature of organisms also helps in the classification of organisms into broad groups.