Porifera, a phylum within the Kingdom Animalia, consists of all species of sponges, about 5,000 total. Sponges are among the simplest animals on the planet, and it is believed that they evolved from the first multi-cellular organisms which eventually evolved into animals. The phylum Porifera represents the only phylum of the Parazoa branch of Kingdom Animalia--those without true tissues, that diverged from the lineage shortly after the evolution of multicellularism from colonial protists.
The majority of sponges are considered to be radially symmetrical. This means that they are divided into two congruent halves by any longitudinal cut through the center of the organism. Actually, no sponges are truly radially symmetrical, because they have no true tissues.
Most sponges also have three "layers" of cells. The outer layer is made up of flattened cells with numerous pores throughout. The layer of cells contains mobile cells as well as the skeletal cells which give the sponge its shape. The skeletal cells are either spongin, a flexible protein, or simply mineral-containing particles. The third, innermost layer is made completely of cells called choanocytes. These cells have a ring surrounding the base of a flagella, which is used to propel water through the pores of the sponge into a central body cavity, and then out of the sponge via a central opening. The second function of a choanocyte is to trap food, mainly bacteria, on its collar, and absorb it by phagocytosis.
The mobile cells in the center layer are called amoebocytes. These cells are able to move by use of pseudopodia. They are constantly moving, and their main function is to pick up food vacuoles from the choanocytes, digest the food inside them, and carry the nutrients to the cells that need them. However, they also function to carry oxygen to other cells, dispose of waste products, maintain the structure of the sponge, and even change into another cell type if they have to.
Sponges have a number of similarities to protists, even though they are definitely animals. Sponges have no digestive tract of any kind (instead, they digest food within cells), they lack the gastrula stage, the three cell layers are not homologous to the body layers of most animals, and they do not have any nerve cells or muscle cells. The probable reason for this similarity to Kingdom Protista is because of their common ancestor. It is believed that the ancient ancestor of sponges is a choanoflagellate, a colonial protist. Choanoflagellates live at the bottom of shallow water, and consisted of a small cluster of flagellated cells incredibly similar to a choanocyte. This similarity means that sponges probably evolved from this type of colonial protist, which would explain the similarity to protists.
Phylum Porifera includes the following classes: