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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