Possessing a simple cellular anatomy, sponges are composed of an outer layer of protective surface cells and an inner layer of flagellated cells that help move food and water through its inner cavities. A gelatinous substance, through which amoeboid cells move and carry out a variety of functions, is located between the two layers. The cells aid digestion, transport of nutrients, reproduction, and produce the structures that compose the skeleton of the sponge. The structures, known as spicules, give the sponge its shape and remain intact even after the animal has died.