After a brief natatory life, the pupa fixes itself by means of the disc-segments of the antennas to some foreign body, such as a rock, a piece of driftwood, the skin of a Cetacean, a Sponge, the carapace of a Turtle, or the colony of an Oceanic Hydrozoon. The "cement-glands," which, as shown by Darwin, are "part of and continuous with the branching ovaria," secrete copiously an adhesive cement, which is poured forth through the central apertures of the antennal discs, and by means of which the animal is firmly and finally fastened down to the object to which it in the first place attached itself. The body now becomes enclosed in a multivalve calcareous "test," produced by a special shell-gland. The organs of the mouth become fully developed, and the lateral eyes of the locomotive pupa disappear altogether. Lastly, the six pairs of natatory limbs of the Cypris-stage are replaced by the six forked and multisegmentate "cirri" of the adult; while the base of the abdomen carries the penis, in the form of a proboscidiform appendage.

The form of the adult, as already said, differs considerably, but the two most important types are those presented respectively by the Sessile and by the Pedunculated Cirripedia.

In the symmetrical Sessile Cirripedes or Baianidae, commonly known as Acorn-shells (fig. 142, C, D), the animal is protected by a calcareous shell, formed by calcifications within the walls of the first three cephalic segments. The animal is placed within the shell, head downwards, and is fixed to the centre of a shelly or membranous plate, which closes the lower aperture of the shell, and which is termed the "basis." The "basis " is fixed by its outer surface to some foreign object, and is sometimes compact, sometimes porous. Above the basis rises a limpet-shaped, conical, or cylindrical shell, which is open at the top, but is capable of being completely closed by a pyramidal lid or " operculum." Both the shell itself and the operculum are composed of calcareous plates usually differing from one another in shape, and distinguished by special names. Within the shell the animal is fixed, head downwards. The thoracic segments, six in number, bear six pairs of limbs, each of which consists of a jointed protopodite and a much-segmented exopodite and endopodite, both of which are bristled, and constitute the so-called "cirri," from which the name of the sub-class is derived. These twenty-four cirri - "the glass hand" of the Balarnus - are in incessant action, being protruded from the opening of the shell, and again retracted within it, constantly producing currents of water, and thus bringing food to the animal. There are no specialised respiratory organs in the family of the Balanidae. Balani sometimes attain a very considerable size, and Balanus psittacus is largely eaten on the coast of Chili.

The remaining family of the Sessile Cirripedes is that of the Verrucidae, comprising only the single genus Verruca. In many respects the Verrncidae approach the Balanidas, but the shell is composed of six valves only, and is unsymmetrical, whilst the scuta and terga (forming the operculum), though movable, are not furnished with a depressor muscle.

In the Barnacles (Lepadidae), the anterior extremity of the animal is enormously elongated, forming, with the prehensile antennae, the cement-ducts, and their exudation, a long stalk or peduncle, whereby the animal is attached to some solid object. The peduncle is cylindrical, of varying length, flexible, and furnished with proper muscles. In some species it is naked, but in others it is furnished with calcareous scales. At its free extremity the peduncle bears the "capitulum," which corresponds to the shell of the Balanoids, and is composed of various calcareous plates, united together by a membrane, moved upon one another by appropriate muscles, and protecting in their interior the body of the animal with its appendages. The thorax and limbs resemble those of the Balamis; but "slender appendages, which from their position and connections are homologous with the branchiae of the higher Crustacea, are attached to, or near to, the bases of a greater or less number of the thoracic feet, and extend in an opposite direction outside the visceral sac" (Owen).

All the Balanidae are hermaphrodite, and this is also the case with most of the Lepadidae, but some extraordinary exceptions occur in this latter order. Thus, in some species of Scalpellum the individual forming the ordinary shell is female, and each female has two males lodged in transverse depressions within the shell. These males "are very singular bodies; they are sac-formed, with four bead-like rudimental valves at their upper ends; they have a conspicuous internal eye; they are absolutely destitute of a mouth, or stomach, or anus; the cirri are rudimental and furnished with straight spines, serving apparently to protect the entrance of the sac ; the whole animal is attached like the ordinary Cirripede, first by the prehensile antennae, and afterwards by the cementing substance. The whole animal may be said to consist of one great sperm-receptacle, charged with spermatozoa; as soon as these are discharged, the animal dies."

Fig. 144.   Lepas anati fera, the common Barnacle.

Fig. 144. - Lepas anati-fera, the common Barnacle.

"A far more singular fact remains to be told; Scalpellum vulgare is, like ordinary Cirripedes, hermaphrodite, but the male organs are somewhat less developed than is usual; and as if in compensation, several short-lived males are almost invariably attached to the occludent margin of both scuta. ... I have called these beings complemental males, to signify that they are complemental to an hermaphrodite, and that they do not pair like ordinary males with simple females" (Darwin).

As regards their distribution, the Balanoids are shallow-water forms, and Balanus itself is cosmopolitan in its range, though, geologically, quite a modern genus. The Lepadoids are not only found attached to floating bodies, dead or alive, but also extend to great depths. Scalpellum, which is a common Cretaceous genus, goes down to 3000 fathoms. Alcippe (which is without a shell and has only three pairs of feet), bores holes in the shells of Gasteropods.