A stationary marine filter-feeder found in all the world’s oceans
The sponges are simple sessile aquatic animals of the phylum Porifera with a porous baglike body and a rigid or elastic internal skeleton. They constitute one of the most basic multicellular members of the animal kingdom, having bodies full of pores and channels allowing water to circulate through them. Unlike higher animals, they lack true tissues and organs and have no body symmetry. Their shapes are adapted to maximize the flow of water through their bodies for extraction of nutrients. Water and waste products are eliminated primarily through an osculum or central hole. The vast majority of sponges are marine animals, although a few freshwater species are known. They have no nervous system. Adult sponge habitats range from the tidal zone to depths of over five miles and from the polar regions to the tropics. Fertilized eggs swim and drift with the current until sufficiently mature to affix themselves to a solid object on the seafloor. They feed primarily on bacteria and other food particles in the water that passes through their bodies. There are a few species that have formed symbiotic relationships with photosynthesizing microorganisms that take up residence within their exterior cells, with the sponges providing shelter and the microorganisms providing nourishment. Sponges are classified primarily according to the composition of their skeletons or spicules (small hard internal bodies capable of forming a primitive framework). Calcareous sponges lack skeletons but have spicules composed primarily of calcium carbonate. Glass sponges also lack skeletons but have spicules composed primarily of silica. Demosponges have both skeletons and spicules composed primarily of silica. The skeletons or spicules of most sponges make them too rough and inflexible for general use by humans, but two species, Hippospongia and Spongia, have soft skeletons. Until they were largely fished out, these species were harvested commercially and used for functions such as padding for helmets and drinking water storage. Roman legionnaires frequently affixed these sponges on sticks and used them as a form of toilet paper. With the elimination of commercial quantities of these natural sponges, synthetic sponges have been developed to take their place. Science is now investigating the medical and biological properties of compounds found in sponges.