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Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 Chapter Contents Kingdom Animalia : Basic Features of All Animals Metazoa Basis of Classification Classification of Animals Phylum : Porifera Phylum : Coelenterata (Cnidaria) Phylum : Ctenophora Phylum : Platyhelminthes Phylum : Aschelminthes/ Nemathelminthes Phylum : Annelida Phylum : Arthropoda Phylum : Mollusca Phylum : Echinodermata Phylum : Hemichordata/ Stomochordata Everyday you come across a number of animals. Have a look at your surroundings, you can see a diverse range of animals. You see parrots, cockroaches, dogs, cats, horses, fishes, monkeys, cows, houseflies, mosquitoes, frogs, ants, butterflies, etc. and last but not the least human beings. Some fly, some creep, some jump, some walk and some swim. Just observe them, how different they look, how differently they behave, their habits, their habitats, everything is so diverse and different and yes amazing too. Can you imagine that these organisms, varying from sponges to sharks, ants to whales, earthworms to anacondas, all belong to the same group, i.e., Kingdom Animalia. They all are animals. Now what are animals? What are the basic features which decide that the organism belongs to Kingdom Animalia? Why bacteria, mushroom, plants, etc. cannot be classified as animals? You will be able to answer all these queries after studying and by carefully thinking on the basic features of animals, the criteria of their classification and their differences among themselves. As over a million species have been described till now, the need for classification becomes more important. But before classifying them, let's study the basic features of all the animals that distinguish them from other living organisms. KINGDOM ANIMALIA : BASIC FEATURES OF ALL ANIMALS The kingdom Animalia is characterised by heterotrophic eukaryotic organisms that are multicellular and their cells lack cell walls. Let's elaborate this statement to understand completely the various features of animals. (i) Heterotrophic Nutrition: All animals are heterotrophs. They directly or indirectly depend on plants for food. Now what is the meaning of directly or indirectly depending on plants? As we know animals can be herbivores, carnivores or omnivores on the basis of their nutritional requirements. Those animals which are herbivores, depend directly on plants as they eat only plant products like fruits, vegetables, cereals, pulses etc. Whereas the animals which are carnivores or omnivores eat other animals which obtained their nutrition from either plants or other Chapter 10 Animal Kingdom (General Accounts & Non-Chordates)

NEET Animal Kingdom (General Accounts & Non-Chordates) 117 Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 BASIS OF CLASSIFICATION Inspite of differences in the structure and form of different animals, there are certain fundamental features which are common to various individuals in relation to : (1) Levels of organisation, also called grades of organisation. (2) Patterns of digestive, circulatory or reproductive system. (3) Body symmetry. (4) Number of germ layers during embryonic development. (5) Nature of coelom or body cavity. (6) Presence or absence of segmentation in the body. (7) Presence or absence of notochord. These features are used as the basis of animal classification: (1) Levels of Organisation Organisation means an orderly arrangement. We know that cell is the building block of body of all living organisms. Many specialised cells organised to form a tissue, many tissues an organ, many organs an organ- system and finally many organ-systems an organism. On the basis of how cells are arranged in the body of animals following four levels of organisation seen in animals are : (i) Cellular level of organisation : Cells are the basic unit in this level and perform vital functions of an organism. The cells are arranged as loose cell aggregates but they do not form any tissue. Some division of labour (activities) occur among the cells. This level is exhibited by poriferans. (ii) Tissue level of organisation : This level is somewhat higher and complex to cellular level in which the cells performing the same function or structure and are arranged to form tissues. The tissues perform the various basic functions of animals. This level of organisation is present in coelenterates and ctenophores. (iii) Organ level of organisation : When tissues are grouped together to form organs, this level is called organ level of organisation. Here each organ is specialised for a particular function. This level is exhibited by platyhelminthes. (iv) Organ-system level of organisation : In higher animals, organs are associated to form functional systems and each system concerned with a specific physiological function. For example, various organs like stomach, intestine, gall bladder, pancreas etc. unite to form the digestive system. This level is exhibited by aschelminthes to chordates. (2) Patterns of Complexities of Various Organ Systems Organ and organ system levels began in lower animals like platyhelminthes and aschelminthes but their complexity kept on increasing from lower to higher phyla. These complexities are quite obvious and can be used as a basis of classification of animals. (i) Digestive System : The digestive system of animals can be of two types on the basis of complexity : (a) Incomplete digestive system : When the digestive system has only a single opening to the outside of the body that serves as both mouth and anus, it is called an incomplete digestive system. Coelenterates, Ctenophores and Platyhelminthes possess incomplete digestive system. (b) Complete digestive system : The digestive system has two openings; mouth and anus. Hence the entry of food and exit of waste takes place from separate openings. Aschelminthes to chordates, all possess complete digestive system.
118 Animal Kingdom (General Accounts & Non-Chordates) NEET Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 Mouth Alimentary canal Anus Mouth (A) (B) Fig.: Types of digestive system: (A) Incomplete, (B) Complete (ii) Circulatory System : The circulatory system may be of two types : (a) Open circulatory system : In open type the body cells and tissues are directly bathed in the blood pumped out of the heart as the blood flows in open spaces. Hence there is a direct contact and exchange of materials between the blood and body cells. e.g., Arthropods, non-cephalopod molluscs, hemichordates and tunicates. (b) Closed circulatory system : In this system, the blood circulates through a series of the blood vessels of varying diameters, i.e., arteries, veins and capillaries without ever coming in direct contact with the body cells. A series of these vessels maintain the continuous flow of blood. The exchange of materials between blood and body cells takes place in the capillaries. e.g., Annelids, Cephalopod molluscs, Chordates. (iii) Reproductive system : Lower animals like sponges, coelenterates undergo asexual reproduction along with the sexual reproduction while in higher animals, sexual reproduction becomes the predominating mode of reproduction. (3) Body Symmetry Animals can be classified on the basis of their symmetry also. But before differentiating animals on the basis of body symmetry they possess, let us acquaint ourselves with certain terms. Axis : Axis is an imaginary straight line passing through the centre of the body, e.g., longitudinal axis, transverse axis, etc. Plane : A flat surface which divides the body into corresponding halves, e.g., longitudinal plane, transverse plane, etc. Longitudinal planes divide the body into right and left halves or front and back halves and transverse planes divide it in upper and lower halves. Types of Body Symmetry : On the basis of body symmetry, the animals can be (i) Asymmetric : When any plane passing through the centre does not divide the body into equal halves, the organism is said to be asymmetric. e.g., sponges. Most of the sponges are asymmetrical although some are radially symmetrical also. Fig.: Spongilla (Asymmetrical) (ii) Radially symmetric : When any plane passing through the central axis of the body divides the organism into two identical halves, it is called radial symmetry and the organisms possessing it are called radially symmetric. For e.g., some sponges, coelenterates, ctenophores, adult echinoderms.