Multicellular organisms are
organisms consisting of more than one
cell, and having
differentiated cells that perform specialized functions. Most life that can be seen with the naked eye is multicellular, as are all members of the
kingdoms Plantae and
Animalia (except for specialized organisms such as
Myxozoans in the case of the latter).
Multicellular organisms such as sponges consist of multiple specialized cellular types cooperating 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. Though 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 using cheese cloth and a very specific ion cocktail (the classical blender experiment does NOT work) 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.
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.
