Part 2 The opening of an acorn barnacle is covered by two pairs, or four, shell plates. Student descriptions of changes in barnacle movements will vary. The barnacles may speed up or slow down. They may extend more or less completely. They may wave their cirri from side to side instead of opening and closing.
Answer depends on experimental results.
In general, barnacles either continue to extend and retract their cirri but at a faster rate when food is present in order to catch more food, or they hold their cirri open for a long time allowing food to accumulate. This latter behavior will result in lower cirri counts. Student responses will vary. Answers will depend on what students observe. Cirri usually create a definite water current.