Animal Kingdom - Porifera

Sponges of phylum Porifera, described in NCERT Class 11, Chapter 4 (Animal Kingdom), are primitive multicellular animals possessing a unique water transport or canal system. Water enters through minute pores called ostia, passes through the central spongocoel, and exits via a large opening, the osculum. This flow is driven by specialised collar cells called choanocytes, which line the spongocoel and canals; their beating flagella create the current that helps in food gathering, respiration and the removal of wastes. Hence choanocytes are the correct answer. Cnidoblasts are incorrect because they are stinging cells found in Cnidaria, not in sponges. Flame cells belong to the excretory system of flatworms, the Platyhelminthes, so they are unrelated to the sponge water current. Nematocysts are the stinging organelles housed within cnidoblasts of coelenterates and are entirely absent in Porifera. Sponges show only cellular-level body organisation, lacking true tissues, and they are supported by a skeleton made of spicules or spongin fibres. A consistency check confirms the reasoning: only choanocytes possess the flagella positioned along the internal canals that could physically pump water through the sponge, making them the structures responsible for maintaining the current.