Different types of cirral activity of barnacles

From direct observation and from cinematographic records five different types of cirral activity have been recognized in sessile barnacles. They are, arranged in increasing degree of muscular and cirral movement: (1) testing, in which the valves hardly open and the cirri are not protruded; (2) pumping, in which strong rhythmic movements of the operculum occur, but the cirri are protruded only slightly, and not extended; (3) normal beat, a development of pumping, but with the cirri fully extended and withdrawn in rhythm with the opercular movements; (4) fast beat, with less opercular movement, but strong and fast rhythmic cirral movements; (5) extension, in which the cirri are held outside the shell for varying periods without rhythmic movements. Stalked barnacles show simpler cirral activity, without regular rhythmic movements. The internal current which passes through the mantle cavity during normal beat and pumping in sessile species is considered to be primarily respiratory. The flow is shown to be sufficient for this purpose, and the energy expended in driving it through is calculated to be comparatively small. Further respiratory exchange of importance in fast beat and extension takes place via the larger cirri, which are distended and emptied of body fluids during cirral activity. In laboratory experiments under various conditions the balanoids alone showed the full range of movements. The Chthamalidae showed less rhythmic activity and no fast beat, while the stalked barnacles relied mainly on extension. These differences limit the habitats available to the latter groups, as compared with the Balanidae. Micro-feeding, with the smaller cirri used as a filter, can take place during pumping and normal beat, but is less efficient, to judge from rates of filtration, than captorial feeding on larger particles, when the larger cirri are employed. Fast beat is particularly well adapted to captorial feeding in still water, extension to captorial feeding in moving water. A wide range of food particles was found to be ingested, of sizes varying from a few microns to several millimetres; proteinaceous materials, such as planktonic animals and chopped lamellibranch muscle were most readily utilized, phytoplankton less so, while pure fats and starches passed unchanged through the gut. The interrelationship of cirral movements, feeding, and respiratory exchange is traced through the possible course of evolution of cirripedes, to the culmination in the most highly developed group, the Balanidae.