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| Animal Organisation |
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| Animals exhibit a complex body organisation which shows a very clear evolutionary trend. The complexity of animal organisation is seen with reference to features like cellularity, germ layers in the body wall, symmetry of the body parts and the nature of body cavity. |
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| One of the main characteristics used in defining grades of animal complexity is the presence or absence of tissues. Tissues are the aggregations of embryologically and morphologically similar cells that become arranged to perform specific functions. The protozoans occur as single cells or colonies of cells and thereby represent unicellular animals. They are described as having an acellular grade of organisation. The single cell performs all the life activities. Beyond this group, all the other animals are grouped under Metazoa, the multicellular animals. Within Metazoa, there is a hierarchy of increasing complexity of body form that can be distinguished into two major levels: |
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 Parazoa: In this group cells do not form tissues. Hence, animals have remained at the acellular grade or organisation. For example, Sponges. |
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 Eumetazoa: In this group, cells group to form tissues and thereafter organs and organ systems. For example, higher multicellular animals. |
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| The embryonic tissue layers formed during developmental stages in Eumetazoa, are called germ layers. All the adult structures develop from these germ layers. Animals in which only two primary germ layers develop (an outer ectoderm and an inner endoderm) are described as diploblastic animals. In these animals, embryonic ectoderm develops into the outer body wall and the embryonic endoderm develops into the living of the inner body wall. Members of the phylum Coelenterata exhibit diplobastic condition. Animals in which three primary germ layers develop (outer ectoderm, middle mesoderm and inner endoderm) are described as triploblastic animals. All higher multicellular animals exhibit triploblastic condition. The evolution of mesoderm has greatly increased the potential for animal complexity. |
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| A fundamental aspect of an animal's body organisation is its body shape, form and the manner in which body parts are arranged. The word symmetry is used to describe the arrangement of body structures with reference to the body axis. Animals which can be bisected or split along atleast one plane so that the resulting halves are similar to each other, are described as symmetrical. Animals which cannot be split into similar halves are described as asymmetrical. There are three basic forms of symmetry. |
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| Spherical Symmetry |
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| It is a type of symmetry seen in animals that have the shape of a sphere, with the body parts arranged concentrically around a central point. A sphere has an infinite number of planes of symmetry that can pass through the centre to divide it into similar halves. In the strict sense, such symmetry is rare in animals and is exhibited only in some protozoans such as radiolarians. In such animals, there is no polarity as in other symmetrical animals. There is no clear differentiation along an axis other than from the centre of the animal towards it's surface. |
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| Radial Symmetry |
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| It is a type of symmetry seen in animals having a cylindrical body shape. The body parts are arranged around an imaginary axis. Such an animal can be cut into two equal halves by any plane passing through the axis. The axis in such animals is described as oral-aboral axis. Nearly perfect radial symmetry is seen in several coelenterates, particularly in their polyps. One modification of radial symmetry is the biradial symmetry where the body parts are arranged in such a way that two planes of sectioning can divide the animal into similar halves. E.g., sea anemones and ctenophores. |
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| Bilateral Symmetry |
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| It is a type of symmetry seen in animals having an elongated body shape. The body parts are arranged on either side of an imaginary axis. Such an animal can be cut into two equal halves by only one plane passing through the axis. In such animals the body axis is described as median-longitudinal or antero-posterior axis. In general, triploblastic animals are bilaterally symmetrical. All higher animals from flat worms to vertebrates exhibit bilateral symmetry. |
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| fig. 10.1 - Spherical, Radial and Bilateral Symmetry |
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| The term body cavity refers to a space in the body wall. The triploblastic, bilaterally symmetrical eumetazoans can be distinguished into three grades based on the nature of body cavity: |
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| Grade Acoelomata |
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| In acoelomata, the body cavity is not seen. The space between outer body wall (skin) and inner body wall (alimentary canal) is occupied by a tissue called mesenchyme. This condition is seen in flat worms (Platyhelminthes). |
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| Grade Pseudocoelomata |
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| In Pseudocoelomata a false body cavity is present. It is called pseudocoelom. It is a body cavity which is not lined by mesoderm. This condition is seen in round worms (Aschelminthes). It develops from the first formed embryonic body cavity called blastocoel. |
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| Grade Eucoelomata |
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| In Eucoelomata a true body cavity is present. It is called coelom. It is a body cavity which is lined by mesoderm (or coelomic epithelium). This condition is seen in the remaining group of animals from annelids to vertebrates. |
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| fig. 10.2 - Diagrams to Show Acoelomate, Pseudocoelomate and Eucoelomate conditions |
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| In Eucoelomate bilaterally symmetrical animals, during the embryonic development, a slit called blastopore appears in the embryonic stage called blastula. The blastopore marks the beginning of antero-posterior elongation of the embryo. Based on the fate of blastopore, Eucoelomata can be distinguished into two groups - Protostomia and Deuterostomia. |
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 Protostomia represents those animals in which the blastopore finally becomes the oral aperture (mouth). Members of phylum Annelida, phylum Arthropoda and phylum Mollusca exhibit this condition. |
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 Deuterostomia represent those animals in which the blastomere finally forms the anus. The oral aperture appears much later as a separate slit. Members of the phylum Echinodermata and Chordata exhibit this condition. |
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