Living sponges have traditionally been divided into three or perhaps four classes: Homoscleromorpha, Calcispongiae, Hexactinellida, and Demospongiae. The classification of sponges is based on spicule form and chemical composition. Freshwater sponges tend to be more sedate and not especially colorful. Common coloration of sponges are those of the rainbow such as yellow, orange, green, red, and purple. Most marine sponges are brightly colored due to pigments in their dermal cells. Some sponges may live to be 2,300 years old based on recent studies of size and growth in the Caribbean reef sponge, Xestospongia muta. Sponges vary from a few millimeters to over two meters (6.6 ft.) in diameter. They are paraphyletic because the calcareous “sponges” are more closely related to other animal taxa than they are to siliceous sponges. Molecular analysis clearly showed that sponges are phylogenetically grouped with animals. Because of their simple body plan, zoologists did not completely recognize sponges as animals until well into the nineteenth century, as they were previously thought to be plants. They have a unique water-current system with various degrees of complexity.
Generally, the body of a sponge is an assemblage of cells embedded in an extracellular matrix and supported by a skeleton of minute needle-like spicules and proteins. Sponges are an ancient group of asymmetrical invertebrates with a fossil record preceding the early Cambrian period (541 million years ago), and even today they are the simplest multicellular animals. Seven species of freshwater sponges have been documented in Arkansas. The family Spongillidae is the most speciose and widespread group of freshwater sponges and includes twenty-two genera and more than 130 species from a wide variety of habitats.
The majority are marine, but there are about 150 species of freshwater sponges, including twenty-seven to thirty species found in North America north of Mexico. \)).The phylum Porifera, which contains the sponges, is a highly successful group of metazoan animals that includes about 8,600 living species of marine and freshwater forms as well as some that inhabit brackish waters.
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