[Illustration: FIG. 10.--The Common Ship's Barnacle, Lepas anatifera, natural size. The name "Anatifera," the "goose-bearer," was given to this species by Linnæus in reference to the legend of its giving birth to young geese. _st._, stalk. _cir._, cirri, or double hairy legs. _pe._, opening of the seminal duct. _sc._, scutum; _t_, tergum, the two plates or shells of the left side; _c_, the middle piece or shell called the "carina."] It is not every one who has the chance of seeing living ship's barnacles (Lepas), but anyone can pick up a stone or bit of rock on the seashore with live sea-acorns or acorn-barnacles (Balanus) adherent to it. Each is like a little truncated volcano (Fig. 11), the sides of which correspond to the pair of larger shells of the ship's barnacle, fused together and grown into a cone-like wall. The acorn-barnacle has no stalk, but adheres by its broad base to the stone. Just within the shelly crater are four small hinged plates or valves in pairs, identical with the smaller shelly bits of the ship's barnacle. When you first see your specimen, the valves are tightly closed. After a few minutes in a glass of sea-water they open right and left, and up jumps--jack-in-the-box-wise--a tuft of bowing and scraping feelers or tentacles, like those of the ship's barnacle. If disturbed, they shoot inwards, and the valves close on them like a spring trapdoor. [Illustration: FIG. 11.--A large British Sea-acorn, Balanus porcatus, allied to the Ship's Barnacle. _l_, the feather-like legs issuing from the shell. Drawn of the natural size.] Now, these clawing, feathery little plumes are found, when we examine them with a hand-glass, to be six pairs in number, and each of them is =Y=-shaped, like the swimmerets of a lobster. The arms of the =Y= are built up of many little joints and covered with coarse hairs. As a result of the study of the young condition of the ship's barnacle and the sea-acorn, we find that these six pairs of =Y=-shaped plumes are six pairs of legs corresponding to those of the mid-body (some of the walking legs and some of the foot-jaws) of the lobster, and that the shelly hinged plates of the barnacles correspond to the overhanging sides of the "head" of the lobster and prawn, which one can imagine to be hinged along a line running down the back so as to open like the covers of a book. There are very common little, free-swimming "water-fleas" (minute crustaceans) of many hundreds of kinds which have hinged shells of this description when in the full-grown condition, and it is found that the young barnacles and sea-acorns pass through a free-swimming phase of growth (the Cyprid stage), in which they greatly resemble these "water-fleas."