All the Cirripedes, when adult, live either a fixed or parasitic existence, and as so frequently happens with animals of this kind, they have departed widely from the ordinary structure of the class to which they belong. Their anomalous appearance and the mystery surrounding their propagation gave rise, probably, to the old legend that the Barnacles (Lepadidae), which live attached to pieces of floating timber, hatched out into Barnacle geese[63]; and even so late as 1678, in the Royal Society’s _Transactions_, Sir Robert Moray describes what he takes to be little birds enclosed in Barnacle shells, washed ashore on the coast of Scotland: “The little Bill like that of a Goose, the Eyes marked, the Head, Neck, Breast, Wings, Tail, and Feet formed, the Feathers everywhere perfectly shaped and blackish coloured, and the Feet like those of other Water-fowl, to my best remembrance.” Cuvier in his classification of the animal kingdom included them in the Mollusca; and it was not until 1830 that J. V. Thompson described their larval stages, and showed conclusively that they belonged to the Crustacea. Since the work of this naturalist a number of observers have securely founded our knowledge of the group, but we may especially mention the epoch-making works of Darwin,[64] Hoek,[65] and latterly of Gruvel.[66] The young Cirripede is hatched out from the maternal mantle-cavity as a free-swimming Nauplius, a larval form common to most of the Entomostraca and to some Malacostraca; the Cirripede Nauplius (Fig. 49) is characterised by the presence of well-developed frontal horns, and usually by the long spiny processes which spring from various parts of the body. As an introduction to the study of the group, it will be well to follow the transformations of this larva in _Lepas_ up to the period when it begins its sessile existence. The liberated Nauplii swim freely near the surface of the sea, and remaining in this condition for several days are dispersed widely from their birthplace; they are then transformed by the process of moulting into the second larval stage, known as the Cypris (Fig. 50), from its resemblance to a bivalve Ostracod. The Cypris larva continues to swim about by means of the six pairs of biramous thoracic legs until it finds a suitable place on which to fix; in the case of _Lepas_ fixation usually takes place on loose floating logs; the Cypris fixes itself by means of the first antennae, at the bases of which a large cement-gland secretes an adhesive substance. The biramous swimming legs are cast off, and six pairs of biramous cirri characteristic of the adult take their place; at this stage the body has the appearance shown in Fig. 51. The region of the head at the base of the antennae now becomes greatly swollen and elongated to form the peduncle or stalk of the adult; the larval bivalve carapace is cast off and on the external surface of the mantle the calcifications begin which will give rise to the exoskeletal plates of the adult.