Final answer:
Choanocytes, also known as collar cells, line the internal cavity of a sponge and facilitate water flow through the ostia. They are crucial for feeding through phagocytosis and are involved in the reproductive process. These cells are embedded in the mesohyl and maintain the sponge's water flow for nutrient and gas exchange.
Step-by-step explanation:
The flagellated cells that line the internal cavity of a sponge and help water flow through the ostia are known as choanocytes, commonly referred to as "collar cells." These cells have a significant role in the feeding mechanism of the sponge, where their flagella beat to draw water into the sponge body, thereby allowing filter feeding. The choanocytes trap and ingest microscopic food particles by phagocytosis and play a crucial role in the sponge's reproductive process by differentiating into sperm for sexual reproduction.
In addition to their feeding function, the collar of the choanocyte, composed of microvilli, acts as a sieve to filter food particles from the water. Embedded in the mesohyl, choanocytes are central to maintaining the unidirectional water flow through the sponge, ensuring that all cells receive nutrients and oxygen while waste products are expelled.