CIRRIPEDIA, a sub-class of the Crustacea (q.v.), compris ing the barnacles and acorn shells and certain parasitic forms related to them. All Cirripedes are completely sedentary in the adult state and with the loss of the power of locomotion they have become so modi fied as to show, at first sight, little trace of resemblance to the more ordinary Crus tacea.
The common goose-barnacle, Lepas an atifera, is found adhering to the bottom of ships and to floating timber. It has a fleshy stalk, fixed at one end to the supporting object and having at the other end a shell of five separate plates enclosing the body of the animal. The stalk is formed from the front part of the head, and the body, when exposed by opening the shell, is seen to be bent nearly at right angles to it. Em bedded in the cement which fastens the end of the stalk to the support may be found the remains of the antennules which served for the attachment of the larva.
The antennae are wanting, but within the shell the mouth is surrounded by mandi bles, maxillulae and maxillae, and these are followed by six pairs of long, curled, two-branched appendages fringed with hairs. These appendages can be protruded from the slit-like opening of the shell, forming a kind of casting net for the capture of minute floating particles of food. The acorn-shells which are abundant in most seas at tached to rocks in shallow water, differ from the goose-barnacle and its allies in having no peduncle. The shell is conical, cemented directly to the rock, and has an opening at the top closed by four movable valves. The shell has a more or less perfect radial sym metry. Allied to the more normal Cirripedia just described are a number of more degenerate and, for the most part, parasitic forms. The most degenerate are the members of the order Rhizocephala, of which the genus Sacculina, a common parasite of Crabs, is a familiar example. It has a simple sac-like body attached under the abdo men of the crab by a short stalk from which root-like processes ramify throughout the internal organs of the host. The body is enveloped by a fleshy mantle, with a small opening, and is without appendages, mouth or alimentary canal, the only organs which are well developed being those of the reproductive system. There is no trace of Arthropodous structure and only the larval develop ment allows Sacculina to be referred to the Cirripedia. For the effect of Sacculina on its host, see SEX.
Unlike the majority of Crustacea and, indeed, of Arthropoda in general, the Cirripedia are nearly all hermaphrodite, but in a few species dwarf and degenerate male individuals are found attached at, or within, the opening of the shell of the normal indi viduals. The latter may be, as usual, hermaphrodites, in which case the males were termed by Darwin "complemental males" but in a few instances the large individuals are purely female and the separation of the sexes is complete.
Most Cirripedes are hatched from the egg in the nauplius stage, but the earliest larvae are characterized by the development of spines and processes from the body and in some oceanic species these spines are of relatively enormous length. At a later stage the larva is enclosed in a bivalve shell superficially resembling that of an Ostracod and on this account it is known as the "Cypris" stage. Like the nauplius stages the cypris swims freely but ulti mately it attaches itself by the antennules, which are provided with glands producing an adhesive secretion. The shell is cast off, leaving a membranous mantle within which calcareous plates appear, at first, apparently, always five in number.
The Cirripedia are almost exclusively marine animals, only one or two species penetrating a little way into estuaries and the like. Certain species attach themselves to whales, marine turtles, or the larger Crustacea, and truly parasitic forms are found on Echinoderms and Crustacea. The earliest fossils that can be certainly re ferred to the Cirripedia occur in the Rhae tic formation and belong to the Lepado morpha.
The classification of the group is still somewhat unsatisfactory. In particular, the relationship of the parasitic forms to the more normal Cirripedes is obscure. The following scheme is the most generally adopted.
Order Lepadomorpha. The stalked barnacles. Lepas, Scalpel lum, Mitella.
Order V errucomorpha. Asymmetrical sessile forms. Verruca. Order Balanomorpha. The symmetrical sessile acorn shells. Balanus, Chthamalus, Coronula.
Order Acrotlioracica. Somewhat degenerate forms, boring in molluscan shells and corals. Alcippe.
Order Ascothoracica. Parasitic on Coelenterates and Echino derms. Affinities obscure. Laura, Dendrogaster.
Order Apoda. Known only from a solitary specimen described by Darwin. Parasitic on a stalked Cirripede. Proteolepas.
Order Rhizocephala. Extremely degenerate parasites on Deca pod Crustacea. Sacculina, Peltogaster.
See also BARNACLE. (W. T. C.)