The mobile forms ( Errantia ) tend to have well-developed sense organs and jaws , while the stationary forms ( Sedentaria ) lack them , but may have specialized gills or tentacles used for respiration and deposit or filter feeding , e.g. , fanworms .
The mobile forms tend to have well-developed sense organs and jaws , while the stationary forms lack them , but may have specialized gills or tentacles used for respiration and deposit or filter feeding , e.g. , fanworms .