Phylum Porifera
The phylum porifera contains three classes in it: Calcarea, Demospongia, Hexactinellida. Sponges are among the simplest creatures in the world, and it is believed that they had evolved from the first multicellular organisms in the planet. Sponges, unlike most animals, are not radially symmetrical, mainly due to the fact that they are not made of tissue. Sponges lack many cells, which make them different than any other living organism in the world. They lack a digestive tract, nerve cells, and muscle cells.