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.]