Cirripedes are fascinating models for studying both functional constraints and diversity in larval

development. Adult cirripedes display an amazing variation in morphology from sessile suspension

feeders that still retain many crustacean characters to parasites that have lost virtually all arthropod

traits. In contrast, cirripede larval development follows a common scheme with pelagic larvae

comprising a series of nauplii followed by a cyprid. Variations are mostly concerned with whether

or not the nauplii are feeding and the degree of abbreviation of development, culminating in species

where the larvae hatch as cyprids. The cypris larvae are very similar among the ingroups of the

Cirripedia, but interesting variations occur in structures used for substrate location and attachment.

The cyprid is specialized to both swim through the water and actively explore the substratum by

walking on the antennules and using an array of sensory organs in search for a suitable site to

attach. This unique morphology and behavior of the cyprid have enabled the Cirripedia to colonize

widely different habitats ranging from hard rock to soft animal tissue. Yet, the cyprid can

metamorphose into juveniles as different as a setose feeding barnacle and the vermiform stages of

the parasitic forms. This emphasizes the importance of the cyprid as one of the key features for the

evolutionary success of the Cirripedia.