Crustaceans such as daphnids, brine shrimp, copepods, and ostracods (i.e., those not considered in the class Malacostraca) are excellent examples of filter- or suspension-feeding protostomates. Suspension feeders obtain food by either moving through the water or by remaining stationary. In both cases, bacteria, plankton, and detritus flow through specially designed feeding structures.
Interestingly, because of the large amount of energy required to continuously filter water, there are relatively few protostomes that actually use continuous filtration. A less expensive and also the most commonly employed strategy are to develop specialized filter mechanisms that contain a "sticky" substance such as mucus. An example of this is found in some species of tube-dwelling polychetes that direct water through their burrows and trap food particles in mucus. The mucus is then rolled up into a "food pellet" and manipulated by ciliary action to the mouth where it is consumed.
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