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"Poriferans" is the scientific term for sponges, members of the animal phylum Porifera, which means "pore-bearer" in Greek. Sponges are the simplest animals known. Unlike all other animal phyla, which have two or three-layered body plans (diploblastic or triploblastic), sponges have only a single body layer (monoblastic), and no true tissues. They have no appendages and no ability to make any movements, lacking muscle tissues. Sponges are exclusively aquatic.
Poriferans get their nutrition by staying in one place, pumping water through themselves, and filtering it for small organisms and bits of food that they digest. Sponges are protected from predators by their low nutrition content as well as irritating spines distributed throughout their bodies, called spicules, which also double as a "skeleton." Over 5,000 species are recognized by science, and new species are discovered regularly. This is partially because the range of sponges is so wide: they are found at every depth, from right up against the shore to down in six-mile deep ocean trenches. The sponge's simple body structure lends itself to survival in ocean pressures equal to dozens of atmospheres.
For a very long time, it was thought that sponges were evolutionarily the simplest organisms, and the first animal phylum to exist. However, a landmark phylogenetics study in 2008 determined that sponges may be secondarily simplified (having evolved from more complex ancestors, probably with true tissues) rather than truly basal. In any case, it is agreed that the modern form of poriferans is the simplest of all animal phyla.
Although poriferans lack true tissues, they do have cell differentiation, and display at least eight types of cells, including choanocytes ("collar cells" with flagella that beat to pump water through the sponge), porocytes (tubular cells which make up the pores through which water is pumped), pinacocytes (which form the outer layer of cells), myocytes (muscle cells which open and close the porocytes), archaeocytes (which can differentiate into other cells), sclerocytes (which secrete defensive spicules), spongocytes (which secrete the structural protein spongin), and collenocytes (which secrete other collagens).