Cirripedia

Cirripedia, an order of Crustacea, of which the best known forms are the Acorn shell and the Barnacle. They belong to the group of Crustacea known as the Entomostraca, which includes all the small, simply organised forms, with a very variable number of appendages and somites; thus they are allied to Apus on the one hand, and Daphnia, the "water flea," on the other. The Cirripedia are the most clearly-marked off division of the Entomostraca, as their fixed mode of life has led to much degeneration in structure; thus they have lost all organs of locomotion and special sense, and any trace of segmentation. They are fixed by the antennae, which are modified into "cement glands" ; the other appendages of the head, except the mandibles, palps, and maxillae, are lost; there are five pairs of two-branched thoracic appendages, which form a crown of fine long tentacles, by the motion of which are set up currents of water, which bring the animal its food. The shell is composed of many pieces or valves. There is a marked metamorphosis; the first stage after the egg is a "Nauplius," a small unsegmented larva provided with three pairs of cephalic appendages, and a large median eye. The Cirripedia are divided into three orders: the Apoda, the Abdominalia, and the Thoracica; the last includes the three best-known families, the Balanidae (including the Acorn Shells), the Verrucidae, closely-allied forms, and the Lepadidae or Barnacles. Amongst other interesting features in this group is the fact that in some of them the males are dwarfed, degraded forms, parasitic upon the female, whose fertilisation is their sole duty; they are therefore known as "complemental" males. The British species, both recent and fossil, have been monographed by Darwin.