Porifera, or sponges, are the simplest and oldest of the multicelled animals, with fossils dating back to Precambrian times. They are aquatic and sessile , living permanently attached to submerged objects. More than 5,000 species are known, most of which occur in shallow coastal waters and in the deep sea. About 150 species live in fresh water. Sponges are found at all latitudes, even in polar regions.
Sponges are unique among animals because they lack a brain, nerves, muscles, organs, and specialized tissues. They rely upon highly specialized, but poorly coordinated cells. As the name Porifera ("pore bearers") suggests, the body is perforated. Numerous small pores (ostia) convey water into an internal canal system lined with flagellated collar cells (choanocytes). The flagella of these cells beat synchronously to produce currents that pump water through the sponge. Choanocytes filter water through their sievelike collars to remove suspended food particles (bacteria, protozoans, microscopic algae, organic particles). The particles are digested by wandering amoeboid cells (amoebocytes), which carry nutrients to various parts of the sponge. Filtered water and waste products are expelled through large vents (oscula).
The skeleton supporting these canals and chambers is composed of needlelike spicules and/or elastic protein fibers (spongin). The spicules are made of silica or calcium carbonate and occur in various shapes and sizes characteristic of each species.
Sponges can reproduce both sexually and asexually. Clouds of sperm expelled into the water by one sponge are drawn into other sponges with water currents. Specialized cells (modified choanocytes) carry sperm to the eggs. Zygotes develop into ciliated larvae that are released into the water, where they are planktonic for a short period before settling onto a suitable substrate to become adult sponges. Asexual reproduction occurs by fragmentation and/or budding; for example, freshwater sponges use resistant buds (gemmules) for surviving winter or periods of drought.
Barnes, Robert D. Invertebrate Zoology, 5th ed. Philadelphia, PA: Saunders College Publishing, 1987.
Margulis, Lynn, and Karlene V. Schwartz. Five Kingdoms: An Illustrated Guide to the Phyla of Life on Earth, 3rd ed. New York: W. H. Freeman, 1998.