A

Sponge belongs to the phylum Porifera. Poriferans have numerous small openings in their body called ostia (singular ostium) and a large opening called the osculum. Water enters the sponge through ostia into the spongocoel. From there water leaves the body through the osculum. Flow of water is unidirectional and helps in different processes like food accumulation, exchange of respiratory gases, excretion etc.