Porifera
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All living sponges are categorized into Phylum Porifera. These are the simplest animals because they are only at a cellular a level of organization. Choanocytes are cells that are nearly morphologically indistinguishable from choanoflagellates and define the phylum. Choanocytes use their flagellum to create a water current and the collar of microvilli that catches microscopic food particles. The "skeleton" of the sponge is made up of a special protein called spongin and inorganic spicules made of either CaCO3(limestone) or SiO2(glass). Archaeocytes are a type of totipotent stem cell that can become other needed cells like choanocytes, pinacocytes(surface cells), and porocytes.