Final answer:
Choanocytes, or collar cells, are the flagellated cells that line the sponge's internal cavity and are critical for driving water flow through the ostia, filtering food particles, and facilititating digestion within sponges.
Step-by-step explanation:
The flagellated cells that line the internal cavity of a sponge and help to facilitate water flow through the ostia are called choanocytes, also known as collar cells. Choanocytes have a unique structure; the cell body is embedded in a jelly-like substance called mesohyl, and a mesh-like collar composed of microvilli surrounds a single flagellum. The flagellum beats to move water through the sponge, enabling it to filter out food particles like bacteria and microscopic organisms. These cells play a crucial role in the sponge's feeding process through intracellular digestion, wherein food particles are trapped in the mucus of the choanocytes' collar, ingested by phagocytosis, and subsequently digested within the cell itself. Amoebocytes transport the nutrients to other cells of the sponge.