What Kinds of Organisms Have No Tissue?

Kingdom Protista

• Protists are a diverse group of primarily unicellular organisms. Biologist Gerald Audesirk argues that protists have little in common, and share a classification based on their inability to fit into other animal kingdoms. Although some protists are multicellular or colonial, protists are generally unable to form differentiated tissues; the brown algae Laminaria is a rare exception. Some protists cause diseases in humans, such as malaria and giardia.

Domain Bacteria

• Domain Bacteria comprises single-celled microscopic prokaryotes. Prokaryotic organisms have no cell nucleus. Although people often associate bacteria with disease, relatively few bacteria are dangerous. Many bacteria are actually beneficial to human health. For instance, bacteria help to produce cheese and yogurt, and also aid digestion in the human body. Because bacteria are unicellular, they have no tissue and cannot form tissues. Some bacteria gather together into large colonies. Dental plaque is a common example of a bacterial colony.

Domain Archaea

• Archaea, like bacteria, are microscopic, unicellular prokaryotes that lack tissue. They are similar enough to bacteria that scientists once placed them in the same kingdom. In the late 20th century, however, scientists realized that Archaea have a different evolutionary history from bacteria. Accordingly, they now place these organisms in their own domain. Archaea can exist at remarkable extremes of temperature, pressure and pH, and are important contributors to many ecosystems, because they help to recycle elements like carbon and sulfur.

Phylum Porifera

• The only members of the animal kingdom that don't have tissue belong to the phylum Porifera, and are commonly known as sponges. Scientists have identified approximately 5,000 sponge species. Many sponges feed on tiny organisms like bacteria, while others form symbiotic relationships with other animals. Sponges have some specialized cells, but these cells do not group together to form tissues. Instead, they transform themselves into various types of specialized cells, in order to perform different functions in the animal's body.