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Animal Diversity. All multicellular animals have descended from a common ancestor. They display varying levels of complexity. The earliest had a simple cellular level of organisation, while the more advanced animals developed tissues, then organs and finally systems, which enabled them to become increasingly more complex. The evolution of animals can be seen in the increasing complexity of their body parts. |
Important features of body plans 1. Symmetry 2. Cephalisation 3. Embryonic tissue layers 4. Development of a coelom 5. Number of gut openings 6. Blood systems |
There are 6 |
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What is symmetry and explain the three types |
The way in which body parts are arranged in the body plan is referred to as its symmetry. Three types of symmetry are possible.
Asymmetry : - no symmetry in the body shape. - no pattern to body parts - cannot divide animal into halves that are mirror images - animals usually sessile (attached to substrate) e.g. Amoeba - asymmetrical
Radial symmetry - body parts are arranged around a central axis - mirror images will be seen along a number of planes if sliced longitudinally - radial animals have no front or back end - radial animals are mostly sessile e.g. Starfish - radially symmetrical
Bilateral symmetry - body parts are arranged on either side of a central axis - can be cut in two mirror images in only ONE plane to give a right and left half - bilateral animals are motile - they have a front (anterior) and a back (posterior) end - there are definite dorsal (top) and ventral (bottom) surfaces e.g. Human - bilaterally symmetrical
From Simple to complex ( Increasing complexity) 1) Asymmetry (porifera) - irregularly shaped - no symmetry - simple organisms - sedentary - obtain food from water which flows through them - no need to sense environment for food
2) Radial symmetry (Cnidaria) - sedentary /sessile organisms - senses stimuli from all directions
3) Bilateral symmetry (higher order phyla) - body plan shows two equal halves - mobile organisms - allows for cephalisation, making detection of prey, danger etc much easier |
Think of an amoeba, starfish and human. 4; 4; 5 main points for each respective
Then 5; 2; 3 points for respectively |
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What is cephalisation? N. B. Cephalos means head |
- cephalisation is a characteristic of bilaterally symmetrical organisms - the head forms the anterior end in which there is a concentration of nerve cells. This concentration of nerve cells is a simple brain in primitive animals. In highly advanced animals, the mass of nerve cells in the head form a more complex brain, which receives and interprets stimuli from sensory cells - sense organs are housed in the head region, which places them neat the concentration of nervous tissue or 'brain'. Animals move forward with the head entering the environment first. - the organism is able to detect food, danger and other stimuli and respond quickly. |
4 main points |
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What are the embryonic tissue layers? |
An animal's body plan is laid out during its embryonical stage. An embryo develops layers of cells called germ layers. These germ layers give rise to specific tissue and organs that make up the body of a fully developed animal. There are three living germ layers : - ectoderm (outer layer that forms the skin) - endoderm(inner layer which lines the gut) - mesoderm (middle layer that gives rise to internal organs)
More primitive animals had only two germ layers in their embryos, the ectoderm and the endoderm and are termed diploblastic. The more advanced animals have three germ layers and thus triploblastic. The evolution of a three layer embryo allowed an increase in complexity. Most modern animal groups have organs derived from the mesoderm. Simple animal e.g. Sponges, do not form tissues and organs - they remain at the cellular level of organisation.
Diploblastic animals - organisms have cells arranged in two layers during embryonic development - outer layer is called ectoderm - inner layer is called endoderm (Diagram in picture)
Triploblastic animals - organisms have their cells arranged in three layers during embryonic development - ectoderm, endoderm and mesoderm (diagram in picture)
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Think of Diploblastic and Triploblastic animals. 3-2 main points respectively |
