Ostia are minuscule pores present on the body walls of sponges.
They are formed by porocytes which are tube-shaped cells that function as valves to allow fluid exchange.
The water enters via the Ostia into the spongocoel, the central cavity of the sponge. They also aid in food transport, respiratory exchange, and waste removal.
Osculum:
The osculum is a large opening that forms the excretory tract of the sponge.
The size of the osculum opening is determined by contractile myocytes as the size regulates the volume of water expelled from the sponge.
