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Glossary entry

is one of three possible body plans for sponges, which form the phylum Porifera in kingdom Animalia and subkingdom Parazoa which means that they do not have a definite body shape. The asconoid body plan is characterized by a stalk-like spongocoel surrounded by a single layer of choanocytes. These choanocytes are cells that have flagella, and the flagella beat to push water through the spongocoel, creating a current. At somewhat regular intervals along the lining of the spongocoel are openings called ostia (singular, ostium). These protrude through the outer layer of the sponge (somewhat like an epidermis, but not entirely like it, since this terminology implies germ layers, which sponges do not have. Finally, at the top of the sponge is the osculum, which is the final exit for water circulating through the sponge.

This single layer of choanocytes limits the size of asconoids, with the largest having a diameter of only 1 mm. This is due to the surface area : mass ratio which would not allow an asconoid sponge of larger size sufficient nutrition.

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