Cirripedes are crustaceans, where the adult forms are so structurally and biologically diverse that it would be impossible to argue from these forms alone that they belong to this group of organisms (Fig. 5.1; Anderson 1994). Yet, cirripede monophyly is assured by a similar and unique ontogeny and in recent years also by robust molecular evidence (Pérez-Losada et al. 2009). The taxon comprises three major subdivisions, the Thoracica (stalked and acorn barnacles), the Acrothoracica (burrowing barnacles), and the Rhizocephala (parasitic barnacles) (Fig. 5.2). They all have a larval development starting with a number of free-swimming naupliar instars and terminating in a cypris larva (Fig. 5.3). The nauplii serve in dispersal and growth, while the cyprid is a stage entirely specialised to settle and by a process of metamorphosis initiates the permanently juvenile and adult phase. Cirripede larvae are always easily recognisable from those of other crustaceans, but they also vary across the taxon. This is especially true for the nauplii and probably relates to differences in habitat and development. Naupliar development can be planktotrophic or lecithotrophic, and each of these schemes can be either very short or very long lasting. In addition, the larvae of different species can live and disperse at different depths, from the surface layers to the deep sea and in some species even in freshwater (Yorisue et al. 2013; Martin et al. 2014). Compared to this, the cyprid has an almost stereotyped structure, at least when studied with low-power microscopy (Høeg et al. 2004). The reason is that this larva is so specialised to its task of settlement that there is little room or need for any structural change. Yet, cyprids do differ in precisely the organs related to the very different habitats of the adults, viz. the structure of the antennules used for surface exploration and attachment, and in the sensory structures employed in locating the settlement site. But from such stereotyped cyprids, the ensuing metamorphosis leads to organisms as different as intertidal suspension feeders, commensals in the tissues of corals, or parasites so specialised they have lost any similarity to crustaceans or even to arthropods altogether.