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Sponges, or poriferans, are animals of the
phylum Porifera.
They are sessile, mostly marine,
waterdwelling, filter feeders, pumping water through their matrix and
filtering out particulates of food matter. Sponges are among the simplest of animals, with partially differentiated tissues but
without muscles, nerves, and internal organs. In some ways they are closer to being a cell colony than multicellular organisms. There are over 15000 modern
species of sponges known, and more are being discovered every day. The fossil record of sponges dates back to the precambrian era. The structure of a sponge is simple: it is shaped like a tube, with
the end stuck to a rock or other object. The open end is called the osculum, and the
interior is the spongocoel. The walls are perforated with microscopic holes to
allow water to flow through the spongocoel.
Sponges have only four types of cells:
- Choanocytes (also known as collar cells), which line the spongocoel
and function as the sponge's digestive system, are remarkably similar to the protistan choanoflagellates. See that article for a
further description.
- Porocytes are tubular cells that
make up the pores.
- Flat epidermal cells
line the outside of the sponge and form its skin.
- Amoebocytes live between the
choanocytes and the epidermis. They carry out many of the sponge's functions, such as transport of nutrients, secretion of the
spicules, and production of gametes.
Sponges have only three body types: asconoid, syconoid, and leuconoid.
It is thought that the earliest animal life on Earth resembled sponges. The earliest
known multicelled animal fossils are sponges from China that are roughly 600,000,000 years old. Sponges have not been as
extensively studied as some other phyla and there may be some surprises still to be found. For example, it has recently been
shown that some sponges are not sessile and can move to more favorable locations as rapidly as few cm a day. Another sponge, the
Venus Flower Basket has some newly discovered uses involving
fiber optics.
Sponges are divided into Demosponges, Hexactinellid sponges, and Calcaerous sponges based on the type of spicules in their skeleton.
Similar fossil animals known as Chancelloria are no longer regarded as sponges. 90% of modern sponges are demosponges. Demosponges are less common than other
sponges, but that is probably because many demosponges have skeletons made only of relatively soft spongin that does not fossilize well. The fossil
Archaeocyantha may also belong here, though their skeletons are solid
rather than separated into spicules. It has been suggested that the sponges are paraphyletic to the other animals. Otherwise they are sometimes treated as their own subkingdom, the Parazoa.
In common usage, the term sponge is usually applied to the skeletons of these creatures alone, from which the
animal matter has been removed by maceration and washing. The material of which these sponges are composed is spongin. Calcareous and siliceous sponges are too harsh
for similar use. Commercial sponges are derived from various species and come in many grades, from fine soft "lamb's wool"
sponges to the coarse grades used for washing cars. They come from the fisheries in the Mediterranean and West Indies. The manufacture of
rubber, plastic and cellulose based synthetic sponges has significantly reduced the commercial sponge
fishing industry over recent years.
Sponges can reproduce either asexually or sexually. During bad times, sponges can form small structures called gemmules, analogous to the endospore of bacteria. The gemmule, made up
of a few amoebocytes surrounded a
tough layer of spicules, can last through conditions that would kill adult sponges.
When conditions are again favorable, the gemmule will resume growing. Sponges also reproduce asexually by budding, where a piece of a sponge falls off and grows into a new one. Sexual reproduction in sponges is relatively simple. Sperm from one sponge swims to the egg of
another, producing a motile larval stage.
Sponge is also the name of a Indie rock band; see: Sponge (band)
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