What structure in clams is primarily used to pump water for feeding?

The siphon in clams primarily serves the function of pumping water for feeding. Clams are bivalve mollusks, and they have specialized structures that allow them to filter feed efficiently. The siphon is a tubular structure that extends from the body and is used to draw in water. As water flows in through the siphon, it passes over the gills, where the clam extracts food particles such as phytoplankton and detritus for nourishment.

The mantle is a significant part of a clam's anatomy that secretes the shell and can aid in respiration, but it is not directly responsible for the flow of water for feeding. Gills play a crucial role in gas exchange and also aid in filter feeding, but they do not actively pump water themselves; rather, they catch food particles from the water that enters through the siphon. The shell provides protection for the clam but does not have any active role in the feeding process or water movement. Thus, the siphon is the key structure for the actual pumping mechanism involved in feeding in clams.