All barnacles are marine inhabitants, who lead a sedentary or parasitic way of life in the adult state and, in this connection, possess a very peculiar and simplified organization.
About 800 species of these crustaceans are known. Nonparasitic barnacles – sea acorns and sea ducks – attach to all kinds of underwater objects – to rocks and rocks, piles, bottoms of ships, to other animals. Their body is not clearly segmented and is surrounded by a fold of the skin – a mantle corresponding to the carapace of other crustaceans. The mantle distinguishes a different number of lime impregnated plates. These plates are connected to each other so that they form a house or shell, which completely covers the body of the animal. Two or four plates are interconnected and movable and equipped with driving muscles. Due to this, the shell can be opened slightly and water enters the mantle cavity of the animal.
The head part of the body, with which the animal is attached to the substrate, is very strongly modified. Small antennas are connected to the substrate, there are no antennas. The mouth is placed on the end of the oral cone and is surrounded by oral appendages of a simplified structure. The breast carries six pairs of bifurcated legs with multi-segmented branches. These legs perform at the same time characteristic swings, and the water is forced into the mantle cavity.