Cellular Organization

Over time, cells began to form groups that functioned together. Some cells retained the ability to live outside a group. Others became dependent on each other for survival.

Colonies

A colonial organism is a collection of genetically identical cells that live together in a connected group. Colonial organisms are not truly multicellular because few cell activities are coordinated.

True Multicellularity

As organisms evolved, their cells became more specialized and eventually were unable to survive independently. Groups of cells took on specific roles within the organism. A group of similar cells and their products that carry out a specific function is called a tissue. Groups of tissues that perform a particular job in an organism are called organs. An organ system is a group of organs that accomplish related tasks. The stomach and liver are organs that are part of the digestive system. Finally, several organ systems combine to make up an organism. This hierarchical organization found in multicellu-lar organisms is shown in Figure 4-9.

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