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.

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]

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.

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.

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.

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.