THE ship's barnacle looks at first, when you see one of a group of them hanging from a piece of floating timber, like a little smooth, white bivalve shell, as big as your thumb-nail, at the end of a thickish, worm-like stalk, from one to ten inches long (Fig. 10). But you will soon see that there are not only two valves to the white shell, but three smaller ones as well as the two principal ones. This does not separate them altogether from the bivalve-shelled molluscs (mussels, clams, oysters), for the bivalve molluscs, which bore in stone and clay, have small extra shelly plates, besides the two chief ones, whilst the Teredo, or ship's worm—a true bivalve mollusc—has an enormously long, worm-like body which favours a comparison with it of the long-stalked barnacle. If a group of barnacles is floating attached to a piece of timber undisturbed in a tank of sea-water you will see the little shells gape, and from between them a bunch of curved, many-jointed feelers will issue and make a succession of grasping or clawing movements, as though trying to draw something into the shell, which, in fact, is what they are doing—namely, industriously raking the water on the chance of bringing some particle of food to the mouth which lies within the shell (Fig. 10).

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.

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."

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.

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.

The barnacles have, in fact, undergone a transformation which may be compared to that experienced by a man who should begin life as an active boy running about as others do, but be compelled suddenly by some strange spell or Arabian djin to become glued by the top of his head to the pavement, and to spend his time in kicking his food into his mouth with his legs. Such is the fate of the barnacles, and it is as strange and exceptional amongst crustaceans as it would be amongst men. Indeed, to "earn a living" human acrobats will submit to something very much like it. It is this change from the life of a free-living shrimp to that of a living lump, adherent by its head to rocks or floating logs, that Vaughan Thompson in 1830 discovered to be the story of every barnacle, and so showed that they were really good crustaceans gone wrong, and not molluscs. It is a curious fact that the young ascidian or sea-squirt which swims freely and has the shape of a tadpole, also when very young fixes itself by the top of its head to a rock or piece of seaweed, and remains immovable for the rest of its life. Though agreeing in their strange fixation by the head, the barnacle and the ascidian are very different kinds of animals. (For some account of the Ascidian the reader may consult the chapter "Tadpoles of the Sea" in "Science from an Easy Chair," Second Series. Methuen, 1912.)

The name "Cirripedes" is commonly used for the order or group formed by the barnacles—in allusion to the plume-like appearance of their "raking" legs. Stalked barnacles often are found in the ocean attached to floating pumice-stone, and one species has been discovered attached to the web of the foot of a sea-bird. They, like many other creatures, benefit by being carried far and wide by floating objects. Whales have very large and solid acorn-barnacles peculiar to them, fixed deeply in their skin. Others attach themselves to marine turtles.

With few exceptions the crustaceans are of separate sexes, male and female. But in nearly all classes of animals we find some kinds, even whole orders, in which the ovaries and spermaries are present in one and the same individual. "Monoecious" or "one-housed"—that is to say, possessing one house or individual for both ovaries and spermaries—is the proper word for this condition, but a usual term for it is "hermaphrodite." "Dioecious" is the term applied to animals or plants in which there are two kinds of individuals—one to carry the spermaries, the male, and the other to carry the ovaries, the female. It is probable that the monoecious condition has preceded the dioecious in all but unicellular animals. In vertebrate animals as high as the frogs and the toads we find rudimentary ovaries in the male, and in individual cases both ovaries and spermaries are well developed. Such a condition is not rare as an individual abnormality in fishes. In some common species of sea-perch (Serranus) and others it is not an exception but the rule.

Many groups of molluscs are monoecious, and it is not in any way astonishing to find a group of crustaceans which are so. The Cirripedes or barnacles are an example. It is probable that the presence of ovaries and spermaries in the same individual—the monoecious condition—is an advantage to immovable fixed animals. During the voyage of the "Beagle," and making use on his return of the collections then obtained, Darwin carried out a very thorough study of the Cirripedes of all kinds from all parts of the world. He worked out their anatomy minutely, classified the 300 different kinds then known, and described many new kinds. The stalked barnacles often occur in groups, the individuals being of different ages and sizes, the small young ones sometimes fixing themselves by their sucker-bearing heads to the stalks of their well-grown relatives. In all the varied kinds studied by Darwin he found that the full-grown individuals were monoecious—that is, of combined sex—as was known to be the case in those studied before his day. But Darwin made the remarkable discovery that in two kinds of stalked barnacles (not the common ship's barnacles), comprising several species, "dwarf males" were present perched upon the edge of the shell of the large monoecious (bi-sexual) individuals. These dwarf males were from one-tenth to one-twentieth the length of the large normal monoecious individuals, but usually possessed the characteristic details of the shell-valves and other features of the latter.

This existence of a sort of supernumerary diminutive kind of male as an accompaniment to a race of normal monoecious individuals was quite a new thing when Darwin discovered it. That all the males in some dioecious animals are minute as compared with the females was known, and has been established in the case of some parasitic crustaceans, in some of the wheel-animalcules, and in the most exaggerated degree in the curious worms, Bonellia and Hamingia. But the existence of "complemental males," as Darwin called them, existing apparently in order to fertilize the eggs should they escape fertilization by the ordinary monoecious individuals, was a new thing. And it was doubted and disputed when Darwin described his observations fifty-six years ago. They were, in fact, by many regarded as a distinct species parasitic upon the larger barnacles on which they were found until Darwin's conclusion as to their nature was confirmed by the report of Dr. Hoek, on the barnacles brought home by the "Challenger" expedition.

It is an interesting fact that recent studies have shown that in some of the barnacles with dwarf males (species of Scalpellum) the large individuals are no longer monoecious, but have become purely females, whilst in some other species dwarf males have been discovered which have rudimentary ovaries. Thus we get gradations leading from one extreme case to the other. Darwin always felt confidence in his original observations on this matter, and was proportionately delighted when, after thirty years, his early work was proved to be sound. In the Natural History Museum at the Darwin centenary in 1909, a temporary exhibition of specimens, note-books, and letters associated with Darwin's work, was brought together. His original specimens and drawings of Cirripedes and of the wonderful little "complemental males" of the barnacles were placed on view.