What is the primary purpose of cilia in clams?

Cilia in clams play a crucial role primarily in the feeding process. These microscopic, hair-like structures are found on the gills of clams and other bivalve mollusks. The movement of cilia helps create water currents that draw in water containing food particles, such as phytoplankton and organic matter, into the clam's mantle cavity. Once the food particles are entrained in the water flow, they are trapped by mucus on the gills and subsequently transported to the mouth for consumption.

In this context, it's important to understand the function of cilia in relation to other processes within clams. While locomotion may also play a role in their overall movement through their habitat, it is primarily accomplished through the contraction of foot muscles rather than ciliary action. Reproduction and defense, while vital biological functions, do not directly involve ciliary movement and are managed by different physiological mechanisms. Thus, cilia are fundamentally adapted for feeding, making this option the correct choice.