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The Biological Characteristics Of The Biological Complexity Of Animals

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All life has evolved from a single cell, which has since developed into more complex multicellular organisms over time. The biological complexity of an animal can be determined by a number of different characteristics.

Multicellular organisms can be arranged into four different levels of organisation: cells, tissues, organs and organ systems. These range from being the most simple to the most complex.

The cellular level of organisation includes cells, which are the smallest functioning units of an organism, performing a specific function. Porifera are diverse and composed of a loose aggregation of cells. The cell layers of these sponges are not considered to be ‘true’ tissues as there is no basement membrane or intercellular connections – the cells are relatively unspecialised. Sponges are considered to be paraphyletic and to represent the lineage, which is closest to multicellular organisms. This suggests that other animals have evolved and shared a common evolutionary ancestor with sponges.

The tissue level of organisation consists of a group of cells which are similar in function that work together for a specific activity. The higher the complexity of an organism, the more distinctive tissue layers it has. Radiata are an example of a group of organisms which have attained this level of organisation as their highest level. They are characterised by their radial symmetry – they have a top and bottom but no back or front.

The organ level of organisation is composed of
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