In fact, it is quite easy to hatch the young from the eggs of either ship's barnacles or acorn-barnacles at the right season of the year. They commence life as do so many Crustacea--in the "nauplius state," with three pairs of jerking limbs (Fig. 9). As they grow the overhanging pair of shells, delicate and transparent, appear; the three pairs of nauplius legs lose their swimming power; the most anterior (always called antennules in all crustaceans) become elongated and provided each with an adhesive sucker, on the face of which a large cement gland opens, secreting abundant adhesive cement; the second pair (antennæ) shrivel and disappear altogether; the third pair lose their long blades for striking the water and remain as simple, but strong, stumps--the mandibles! Two new pairs of little jaw-feet appear behind these, and farther back on the now enlarged body (the whole creature is not bigger than a small canary seed!) six pairs of =Y=-shaped legs appear and strike the water rhythmically, so that the little creature swims with some sobriety. The region to which these legs are attached is marked with rings or segments, and behind it follows a small, limbless, hind body of four segments, or joints, ending with two little hairy prongs like a pitchfork. The right and left movable, shell-like fold, or downgrowth, of the sides of the body encloses the whole creature except the protruding antennules with their suckers. [Illustration: FIG. 12.--Two stages in the growth of the Common Barnacle from the Nauplius stage. Diagrammatic. _cir._, the double legs or cirri; _m_, mouth; _o_, the single eye; _d_, the digestive canal. _a′_, one of the antennules or "feelers" (that of the right side of the head) provided with a sucking disk by means of which the young animal becomes fixed.] In this condition it swims about for a time, and then, once for all, fixes itself by means of the suckers and their abundant cement, on to rock, stone, or floating wood--and there remains for the rest of its life (Fig. 12). It increases enormously in size, the delicate transparent shell develops into hard calcareous plates, opening and shutting on the hinge-line of the back. In the stalked kinds a peculiar elongated growth of an inch or several inches in length takes place between the mouth and the fixed suckers of the antennules (Figs. 10 and 12); in the short, so-called, "acorn" kinds, this stalk does not form, but a separate part of the shell grows into a ring-like protective wall or cone. The creature is thus actually fastened by its head--"upside down, with its legs sticking up" not in the air, but in the water. Those six pairs of =Y=-shaped legs, though no longer enabling the barnacle to swim, increase in relative size, and keep up their active movements. It is they which emerge like a plume when the valves of the shell open and carry on the rhythmic bowing and scraping movement described above.