The Subclass Cirripedia is one of the more unusual groups of crustaceans, and includes the barnacles. Barnacles show the gretest departure from the classic crustacean body plan. They are exclusively sedentary animals, living permanently fixed to, or burrowing into, living or non-living substrates. This substrate can be either moving or stationary.

Because of this non-motile mode of living the head has been greatly reduced, the first antennae are reduced and the second are completely absent. In the adult stage eyes are either absent or greatly reduced.

For many years barnacles were classified as molluscs, as their shell is secreted by their "mantle" tissue. However, they were recognised as highly modified crustaceans when their anatomy was studied 150 years ago. The thoracic appendages are modified to form cirri which are used in filter-feeding.

The barnacle shell is composed of several "plates" (see picture below). The rostrum plate represents the side of the shell where the body of the barnacle is attached to the mantle. The scutum and tergum plates are movable; they can move apart to allow the animal to feed using its cirri and can close up when the animal wants to withdraw.

Diagram showing the basic plate morphology of a generalised barnacle, viewed from above. Redrawn from Brusca and Brusca 1990.

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