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Multicellular organisms are organisms that consist of more than one cell, and which have differentiated cells that perform specialized functions. Most life that can be seen with the naked eye is multicellular, as are all animals (i.e. members of the kingdom Animalia) and plants (i.e. members of the kingdom Plantae).
Multicellular organisms exhibit organization at several levels:
The simplest extant (currently living) multicellular organisms, sponges, consist of multiple specialized cellular types that cooperate together for a common goal. These cell types include Choanocytes, digestive cells; Sclerocytes, support-structure-secreting cells; Porocytes, tubular pore cells; and Pinacocytes, epidermal cells. Although the different cell types create an organized, macroscopic multicellular structure—the visible sponge—they are not organized into true interconnected tissues. This is illustrated by the fact that a sponge broken up in a blender will reaggregate from the surviving cells. If individually separated, however, the particular cell types cannot survive alone. Simpler colonial organisms, such as Volvox, differ in that their individual cells are free-living and can survive on their own if separated from the colony.
More complex organisms such as jellyfish, coral and sea anemones possess a tissue level of organization, in which differentiated, interconnected cells perform specialized functions as a group. For instance, jellyfish tissues include an epidermis and nerve net that perform protective and sensory functions, along with an inner gastrodermis that performs digestive functions. The overall spatial organization of differentiated cells is a topic of study in anatomy.
Organs and organ systems
Even more complex organisms, while also possessing differentiated cells and tissues, possess an organ level of development, wherein multiple tissues group to form organs with a specific function or functions. Organs can be as primitive as the brain of a flatworm (merely a grouping of ganglion cells), as large as the stem of a sequoia (up to 90 meters (300 feet) in height), or as complex and multifunctional as a vertebrate liver.
The most complex organisms (such as mammals, trees, and flowers) have organ systems wherein groups of organs act together to perform complex related functions, with each organ focusing on a subset of the task. An example would be a vertebrate digestive system, in which the mouth and esophagus ingest food, the stomach crushes and liquifies it, the pancreas and gall bladder synthesize and release digestive enzymes, and the intestines absorb nutrients into the blood.
The oldest known taxonomically resolved multicellular organism is a red algae, Bangiomorpha pubescens, found fossilized in 1.2 billion year old rock from the Ectasian period of the Mesoproterozoic era.
To reproduce, true multicellular organisms must solve the problem of regenerating a whole organism from germ cells (i.e. sperm and egg cells), an issue that is studied in developmental biology. Therefore, the development of sexual reproduction in unicellular organisms during the Ectasian period is thought to have precipitated the development and rise of multicellular life.