Tamilnadu Board Class 11 Bio-Zoology Chapter 2
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UNIT I Chapter 2 Kingdom Animalia Chapter Outline 2.1 Basis of Classifi cation 2.2 Classifi cation of Kingdom Animalia 2.3 Non Chordates (Invertebrata) upto Phyla level 2.4 Chordata upto Class level March 20th is celebrated as World Sparrow day to conserve this endangered species, House sparrow (Passer domesticus) Learning Objectives: are invertebrates or animals without backbone. The remaining represents • Justifies the need for vertebrates or animals with backbone. classification. On the basis of the presence or absence • Understands the salient of notochord (vertebral column), animals features of the animal are also categorised into two major groups phyla. and they are non chordates and chordates. Kingdom Animalia comprises millions 2.1 Basis of classifi cation of animal species and studying them without a basic classification may lead Multicellular organisms are structurally to confusion. In addition to this, there and functionally different but yet they are several new species of animals being possess certain common fundamental constantly discovered. Classification is features such as the arrangement of cell very essential for identification, naming layers, the levels of organisation, nature and assigning a systematic position to of coelom, the presence or absence the newly discovered species. Animal of segmentation, notochord and the Kingdom is classified mainly based on the organisation of the organ system. closely resembling characteristic features. Kingdom Animalia is characterised of 2.1.1. Levels of organisation eukaryotic, multicellular, heterotrophic All members of Kingdom Animalia organisms. They include about 35 phyla are metazoans (multicellular animals) of which 11 are considered as major and exhibit diff erent patterns of cellular phyla. Almost 99 percent of animals organisation. Th e cells of the metazoans 18 are not capable of independent existence Organ level of organisation is a further and exhibit division of labour. Among advancement over the tissue level of the metazoans, cells may be functionally organisation and appears for the first isolated or similar kinds of cells may be time in the Phylum Platyhelminthes and grouped together to form tissues, organ and seen in other higher phyla. organ systems. Organ system level of organisation Cellular level of organisation The most efficient and highest level Th is basic level of organisation is seen of organisation among the animals is in sponges. Th e cells in the sponges are exhibited by flatworms, nematodes, arranged as loose aggregates and do not annelids, arthropods, molluscs, form tissues, i.e. they exhibit cellular level echinoderms and chordates. The evolution of organisation. Th ere is division of labour of mesoderm in these animals has led to among the cells and diff erent types of cells their structural complexity. The tissues are functionally isolated. In sponges, the are organised to form organs and organ outer layer is formed of pinacocytes (plate- systems. Each system is associated with a like cells that maintain the size and structure specific function and show organ system of the sponge) and the inner layer is formed level of organisation. Highly specialized of choanocytes. Th ese are fl agellated nerve and sensory cells coordinate and collar cells that create and maintain water integrate the functions of the organ fl ow through the sponge thus facilitating systems, which can be very primitive respiratory and digestive functions. and simple or complex depending on the individual animal. For example, the Animals such as sponges lack nervous digestive system of Platyhelminthes has tissue and muscle tissue, what does this only a single opening to the exterior which tell you about sponges? serves as both mouth and anus, and hence called an incomplete digestive system. Tissue level of organisation From Aschelminthes to Chordates, all animals have a complete digestive system In some animals, cells that perform similar with two openings, the mouth and the functions are aggregated to form tissues. anus. Th e cells of a tissue integrate in a highly coordinated fashion to perform a common Similarly, the circulatory system is function, due to the presence of nerve of two types, the open type: in which cells and sensory cells. Th is tissue level of the blood remains filled in tissue spaces organisation is exhibited in diploblastic due to the absence of blood capillaries. animals like cnidarians. Th e formation of (arthropods, molluscs, echinoderms, tissues is the fi rst step towards evolution of and urochordates) and the closed type: body plan in animals. (Hydra - Coelenterata). in which the blood is circulated through blood vessels of varying diameters Organ level of organisation (arteries, veins, and capillaries) as Diff erent kinds of tissues aggregate to form in annelids, cephalochordates and an organ to perform a specifi c function. vertebrates. 19 2.1.2. Diploblastic and Triploblastic an axis are identical. An animal’s body organisation plan results from the animal’s pattern of development. The simplest body plan is During embryonic development, the seen in sponges (Figure 2.2). They do not tissues and organs of animals originate display symmetry and are asymmetryical. from two or three embryonic germ layers. Such animals lack a definite body plan On the basis of the origin and development, or are irregular shaped and any plane animals are classified into two categories: passing through the centre of the body Diploblastic and Triploblastic. does not divide them into two equal halves Animals in which the cells are arranged (Sponges). An asymmetrical body plan is in two embryonic layers (Figure 2.1), the also seen in adult gastropods (snails). external ectoderm, and internal endoderm are called diploblastic animals. In these animals the ectoderm gives rise to the epidermis (the outer layer of the body wall) and endoderm gives rise to gastrodermis (tissue lining the gut cavity). An undifferentiated layer present between the ectoderm and endoderm is the mesoglea. (Corals, Jellyfish, Sea anemone) Animals in which the developing embryo has three germinal layers are called triploblastic animals and consists of outer Figure 2.2. Asymmetry in sponges ectoderm (skin, hair, neuron, nail, teeth, etc), inner endoderm (gut, lung, liver) and Symmetrical animals have paired body middle mesoderm (muscle, bone, heart). parts that are arranged on either side of Most of the triploblastic animals show a plane passing through the central axis. organ system level of organisation (Flat When any plane passing through the worms to Chordates). central axis of the body divides an organism into two identical parts, it is called radial symmetry. Such radially symmetrical animals have a top and bottom side but no dorsal (back) and ventral (abdomen) side, no right and left side. They have a body plan in which the body parts are organised in a circle around an axis. It is the principal symmetry in diploblastic animals. Cnidarians such as sea anemone Figure 2.1 Germinal layers and corals (Figure 2.3) are radially symmetrical. However, triploblastic 2.1.3. Patterns of symmetry animals like echinoderms (e.g., starfish) Symmetry is the body arrangement in have five planes of symmetry and show which parts that lie on opposite side of Pentamerous radial symmetry. 20 Animals which possess two pairs show bilateral symmetry (Figure 2.5). of symmetrical sides are said to be It is an advantageous type of symmetry biradially symmetrical (Figure 2.4). in triploblastic animals, which helps in Biradial symmetry is a combination of seeking food, locating mates and escaping radial and bilateral symmetry as seen in from predators more efficiently. Animals ctenophores. There are only two planes of that have dorsal and ventral sides, anterior symmetry, one through the longitudinal and posterior ends, right and left sides and sagittal axis and the other through are bilaterally symmetrical and exhibit the longitudinal and transverse axis. (e.g., cephalisation, in which the sensory and Comb jellyfish – Pleurobrachia) brain structures are concentrated at the Animals which have two similar anterior end of the animal (Figure 2.6). halves on either side of the central plane 2.1.4. Coelom The presence of body cavity or coelom is important in classifying animals. Most animals possess a body cavity between the body wall and the alimentary canal, and is Radial symmetry in Pentamerous radial lined with mesoderm. sea anemone symmetry in starfish Animals which do not possess a body Figure. 2. 3 Radial and Pentamerous cavity are called acoelomates. Since there radial symmetry is no body cavity in these animals their body is solid without a perivisceral cavity, this restricts the free movement of internal organs. (e.g., Flatworms) In some animals, the body cavity is not fully lined by the mesodermal epithelium, but the mesoderm is formed as scattered pouches between the ectoderm and endoderm. Such a body cavity is called a pseudocoel and Figure 2.4 Biradial symmetry is filled with pseudocoelomic fluid. in comb jelly Animals that possess a pseudocoel are called pseudocoelomates e.g., Round worms. The pseudocoelomic fluid in the pseudocoelom acts as a hydrostatic skeleton and allows free movement of the visceral organs and for circulation of nutrients. Eucoelom or true coelom is a fluid- filled cavity that develops within the Figure 2.5 Bilateral symmetry in Insects mesoderm and is lined by mesodermal 21 &RHORP %RG\FRYHULQJ IURPHFWRGHUP 7LVVXHOD\HUOLQLQJ FRHORPDQGVXVSHQGLQJ LQWHUQDORUJDQV IURPPHVRGHUP 'LJHVWLYHWUDFW