The sponge central cavity is called the...
1 Asconoids have the simplest, see the drawing above. 2  These sponges are small and tube-shaped. 3  The water enters through tiny ostia into one large internal cavity called a spongocoel, and is expelled through one large osculum.he sponges are an ancient group with a fossil record back as far as the Precambrian. There are about 10,000 known species of marine sponges, and about 150 freshwater sponges. They are relatively abundant in all marine waters at all depths. They vary in size from a few millimetres to over two metres across.