BARNACLES AND OTHER CRUSTACEANS
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).
[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."
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.
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. "Monœcious" 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." "Diœcious" 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 monœcious condition has preceded the diœcious
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 monœcious, 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
monœcious 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 monœcious--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 monœcious (bi-sexual)
individuals. These dwarf males were from one-tenth to one-twentieth
the length of the large normal monœcious 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 monœcious individuals was quite
a new thing when Darwin discovered it. That all the males in some
diœcious 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 monœcious 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 monœcious, 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.