All sponges are aquatic, the majority living in the sea where they are found at all depths from the intertidal zone to the deepest abysses. One family, the Spongillidae, contains only fresh water members; they inhabit rivers and lakes, practically all over the world.
In the life history of some sponges, a simple stage known as the Olynthus appears. It is probable that all sponges are modifications of this simple type. The Olynthus is a vase shaped structure attached to the substratum at the narrower end. The lateral surfaces are perforated by microscopic pores called ostia, which lead into a large cavity, the paragaster. At the free end is a single exhalant aperture, the osculum.
The outer surfaces is covered by flattened cells called pinacocytes, closely cemented together. The inhalant pores are perforated through single cells, the porocytes. Lining the internal surfaces are collared-flagellate cells, the choanocytes. The outer dermal and inner gastral layers are separated by a stiff jelly in which are two types of cells. Scleroblasts secrete the tir-radiate spicules which support the body, and frequent amoeboid cells wander through the jelly.
No adult sponge retains the Olynthus type of structure, but all show various modifications which are essentially concerned with increasing the area of surface occupied by choanocytes. The three principal grades of structure are the ascon, sycon and leucon. They are illustrated diagrammatically above, with arrows showing the direction of flow of the water currents.
Skeletal Elements In Sponges
The jelly which gives coherence to the body is strengthened by deposition of various elements by the scleroblasts. In the class Calcarea, the spicules are always made of calcium carbonate and are commonly tri-radiate. The Hexactinellida are characterized by siliceous spicules, of which there is a wide array. The shapes of these spicules are useful in classifying this group. Some members of the Demospongiae have siliceous spicules, but the more common skeletal material is a network of sponging fibres secreted by spongoblasts; this sponging in some genera is impregnated with spicules or granules of silica.