CIRRIPEDIA.
THE metamorphoses of the Cirripeds, and their resemblance to the lower
Crustacea at each moult, are still more remarkable than those of the
Epizoa. They form two primary groups, the Balanidæ, or Acorn shells, and
the Barnacles or Lepadidæ, which have peduncles or stalks. Both are
parasites, but they do not draw their sustenance from the substances
they adhere to.
[Illustration: Fig. 153. Balanus culcatus.]
The Balanidæ (fig. 153) are grouped in innumerable multitudes, crowded
together on the rocks of the southern and western coasts of England,
like brown acorns. They have an obscurely articulated body, enclosed in
a membrane, and defended by a multivalve conical shell. The base of the
shell is a broad disk fixed to a foreign substance by a cement secreted
by the animal. The walls consist of twelve triangular compartments. Six
rise upright from the edge of the disk, and end in a point at the open
margin of the shell; the other six are inverted and wedged into the
interstices. The whole cone thus constructed is divided into from four
to eight pieces by expansive seams. The mouth of the cone is closed by a
lid formed of four triangular valves, which meet in a point in the
centre, and shut in the creature.
Six pairs of long, slender, curly feet rise from the throat of the
animal, and bend over the prominent mouth, which is placed at the bottom
of a kind of funnel, formed by the divergence of these six pairs of
thoracic feet. It is furnished with a broad upper lip, two palpi, and
three pairs of jaws, of which the outermost are horny and toothed, the
innermost soft and fleshy. Each foot is divided into two similar
many-jointed branches: the shortest pair is nearest to the mouth, the
others increase gradually in length and number of joints to the most
distant (fig. 154). Mr. Gosse estimated that, in a specimen he
possessed, the whole apparatus included nearly five hundred distinct
articulations. Since each joint is moved by its own system of muscles,
the perfection of the mechanism may be conceived. But it is as sensitive
as flexible, for every separate joint is furnished with a system of
spinous hairs, which are no doubt organs of touch, since the whole of
the branches are supplied with nerves. These hairs, which extend at
somewhat wide angles from the axis of the curling filaments, are barbed,
for they have numerous projections, or shoulders, surrounded by whorls
of microscopic hairs.[40]
[Illustration: Fig. 154. Tentacles or feet of the Balanus.]
This beautiful and complicated structure is the fishing apparatus of the
animal, which it is continually pushing out and drawing in through the
valved lid of the shell. When the whole is thrown out it is widely
spread, and the filaments uncurled; then, as they close again, the
innumerable hairs meet and form a sieve through which the water escapes,
but whatever minute particles it may contain are inextricably entangled,
and when the small animals fit for food have been selected, the
filaments curl inwards, and carry them to the mouth; there they are
seized by the jaws and sent through a short gullet to be digested.
The feet and cirri are moved by very strong muscles, the valves of the
lid are opened and shut by muscles attached to the mouth of the shell;
and when the animal wishes to protrude its cirrhated feet, the
longitudinal muscles attached to the lid come into action, and it draws
itself in again by short muscles attached to the base. All the organs of
the animal are supplied with nerves by a double nerve-centre in the
head, and a circle of nerve-centres round the gullet. The ears are
situated at the base of the first pair of cirrhated feet, and consist of
a cavity enclosing a vesicle closed by a nerve, and containing a liquid,
but no otolites.
[Illustration: Fig. 155. Section of Lepas anatifera.]
The common Lepas anatifera, of which fig. 155 is a section, as well as
its allies, have a thick stem and a conical shell closed on the back,
but gaping in front. Their internal structure does not differ
essentially from that of the Balanidæ, and it has been proved by Mr. J.
V. Thompson and others, that there is no material difference between
their transformations during the early stages of their lives.
Each individual cirriped is both male and female, and the eggs are
hatched before they come into the water. Mr. Gosse mentions that he had
seen the Balanus porcatus throw out a dense column of atoms from the
mouth of its shell for several successive days; each column was composed
of thousands of active microscopic creatures, bearing a strong
similarity to the young of the Cyclops Crustacea. A, fig. 156,
represents one of these creatures. Its body is enclosed in a carapace
with a pair of flexible organs like horns, six swimming feet, and a very
black eye deeply set in front. The creature swims and sometimes rests,
but never alights on anything. After some changes this creature takes a
form whose front is represented at B, and its side by C, fig. 156. It is
larger, more developed, and swims with its back downwards.
[Illustration: Fig. 156. Development of Balanus balanoïdes:—A, earliest
form; B, larva after second moult; C, side view of the same; D, stage
preceding the loss of activity; _a_, stomach; _b_, nucleus of future
attachment.]
A new series of transformations changes this embryo into the form
represented by D, fig. 156, which is closely allied to the Daphnia
pulex, or Water Flea. The body is enclosed between two flat oval shells,
united by a hinge on the back, and is capable of being opened in front
for the protrusion of a pair of prehensile limbs; and six pairs of
swimming feet cause the animal to swim by a succession of bounds.
Instead of the single eye, it has now two raised on pedestals, attached
to the anterior part of the body.
[Illustration: Fig. 157. Lepas.]
This animal having selected a piece of floating wood for its permanent
abode, attaches itself to it by the head, which is immovably fixed by a
tenacious glue exuded from glands at the base of the antennæ. The
bivalve shell is subsequently thrown off, a portion of the head becomes
excessively elongated to form the peduncle of the Barnacle or Lepas, and
in that state it is exactly like the Lucifer Stomapod. In the Balanus,
on the contrary, the head expands into a broad disk of adhesion, and the
animal resembles the Mysis or Opossum Shrimp.
From the first segment of the throat a prolongation is sent backwards
which covers the whole body, and the outer layer is converted into the
multivalve shell; and the three pairs of cirrhated feet, which were
formed in the larval state, now bend backwards from the other three
rings of the throat.
Though the Cirripeds lose their eyes in their mature state, they are
sensitive to light. They draw in their cirrhated feet, and the Balanus
even closes the lid of its shell under the shadow of a passing cloud.