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“Poriferans” is the scientific term for sponges, members of the animal phylum Porifera, which means “pore-bearing” in Greek. Sponges are the simplest animals known. Unlike all other animal phyla, which have two- or three-layered (diploblastic or triploblastic) body plans, sponges have only one body layer (monoblastic) and no true tissue. They do not have appendages or the ability to perform any movement, since they lack muscle tissue. Sponges are exclusively aquatic.
Porifera 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 nutritional content, as well as irritating spines distributed throughout their bodies, called spicules, which also function as a “skeleton.” Science recognizes more than 5,000 species and new species are discovered periodically. In part, this is because the range of sponges is so wide: they are found at all depths, from the coast to ocean trenches six miles deep. The simple structure of the sponge body lends itself to survival in ocean pressures equal to tens of atmospheres.
For a long time, sponges were thought to be evolutionarily the simplest organisms and the first animal phylum to exist. However, a landmark phylogenetic study in 2008 determined that sponges may be secondarily simplified (having evolved from more complex ancestors, likely with true tissues) rather than truly basal. In any case, the modern form of Porifera is accepted to be the simplest of all animal phyla.
Although porifera do not have true tissues, they do have cellular differentiation and exhibit at least eight cell types, including choanocytes (“neck cells” with flagella that beat to pump water through the sponge), porocytes (tubular cells that form pores through which water is pumped), pinacocytes (which form the outer layer of cells), myocytes (muscle cells that open and close porocytes), archaeocytes (which can differentiate into other cells), sclerocytes (which secrete spikes defensive cells), spongocytes (which secrete the structural protein spongin) and colenocytes (which secrete other collagens).