This region is known as the “capitulum” of the adult, as opposed to the “peduncle.” The young Cirripede is now known as a pupa, and from this stage the adult form is reached by a gradual transition. [Illustration: FIG. 49.—Nauplius larva of _Lepas fascicularis_, × 12. _A_{1}_, _A_{2}_, 1st and 2nd antennae; _B_, brain; _E_, eye; _H_, fronto-lateral horn; _M_, mandible; _S_, stomach. (After Groom.) ] [Illustration: FIG. 50.—Cypris stage in the development of _Lepas australis_, × 15. _A_, Peduncle; _A.M_, adductor muscle; _C_, caecum of oesophagus; _C.g_, cement-glands; _Cr_, cirri (thoracic appendages); _E_, compound eye; _E^1_, simple eye; _G_, ventral ganglia; _I_, intestine; _M_, mouth; _M.C_, mantle-cavity; _O_, ovary; _S_, stomach. (After Hoek.) ] [Illustration: FIG. 51.—Pupa of _Lepas pectinata_, × 8. _A_, Antenna; _C_, carina; _M_, adductor muscle; _S_, scutum; _T_, tergum. (After Gruvel.) ] The body of the adult _Lepas_ is retracted into the mantle, and lies free in the mantle-cavity, but is continuous anteriorly with the tissues of the peduncle, into which the mantle does not extend. The thorax, with its six pairs of legs, can be protruded from the mantle-cavity through the slit-like opening which separates the two valves of the mantle along the ventral middle line; and when the animal is feeding, the thoracic legs are so protruded, and by their concerted waving action they drive the food-particles in the water along the channel between them, until the particles reach the oral cone, where they are masticated by the mandibles and two pairs of maxillae, and so passed into the alimentary canal. When the animal is disturbed it rapidly retracts its limbs, the valves of the mantle are closed by means of a strong adductor muscle in the head, and the animal is protected from all external influences. In the acorn-barnacles (Operculata), which live in great numbers attached to rocks and other objects between tide-marks, the body is constructed on a similar plan, save that there is no stalk, and the body is completely enclosed in a hard calcareous box formed from the mantle, which, when the valves are closed, as they always are during low tide, completely protect the animal inside from desiccation or danger of any kind. Besides the cement-glands situated in the peduncle, we can distinguish the generative organs, consisting of a pair of ovaries and testes, the majority of Cirripedes being hermaphrodite. The testes open at the end of an elongated median penis behind the thoracic limbs, while the ovaries, situated in the peduncle, have paired openings into the mantle-cavity on either side of the head. A pair of maxillary glands or kidneys are present, and the alimentary canal is provided with various digestive glands. Special branchial organs are not present in the Pedunculate Cirripedes, but in the Operculate genera two branchiae are formed from the plications of the internal surface of the mantle. There