Motility and Sensation.—The nervous sys tem of the Cirripeds consists essentially of two nervous cords running along the abdominal surface, and swelling out into distinctly formed ganglions, at intervals corresponding to the feet-bearing lobes. The first pair of ganglions is situated above the cesophagus (fig. 339). They are united by a very short nervous cord. — From this supra cesophageal gan glion and the u niting cord, there arise anteriorly three or four nerves, which are distributed to the muscular tunics.
The principal ner vous cords, leav ing the first g,an glion posteriorly, descend to encir cle the cesopha gus. In this course, they give off branches to the salivary glands and other neighbouring parts, and particularly, (as M. St. Ange has pointed out,) a nerve of communication with a. small lateral ganglion (k, k, fig. 339) on either side, situated near the stomach and below the salivary organs. This is connected also with the second pair of ganglions.. From this se cond pair, several branches arise, some of which go to the stomach, and two to the first pair of arms. The other arms receive only one branch each (i, i), which is divided into two, one for each of the jointed processes. In its course along the abdominal surface, the double ganglionic cord—the centre of the nervous,system—lies immediately beneath the skin, between the bases of the arms. The fifth and the sixth pairs of ganglions have the appearance of being closely united. The tu bular process, which terminates the anal ex tremity of the body receives two nerves, one from each of those going to the sixth pair of arms. Dr. Grant directs our attention to the fact that all the anterior parts of this system are very imperfectly developed compared with the posterior parts, and with the same parts in other articulated animals, which have their heads free, and organs of sense more com plete.
The sense of touch is the only one enjoyed by the Cirripeds, so far as we can discover. The ciliated arms of some of the species are acutely sensitive : they are withdrawn imme diately on being touched by any foreign body, and when the surrounding fluid is unfit for respiration. Some observers have also re marked that they shrink from a strong light brought to shine upon them suddenly. In the adult animals, there are certainly no organs which can be regarded as eyes; but, according to Mr. Thompson, what he be lieves to be the free-moving, young have very well developed eyes, like those of some crus tacea.
Some of the littoral Cirripeds, when left dry at ebb-tide, seem to be sensible of certain changes being produced in the state of the sur rounding air by the approach of a living being to the place of their habitation. We have
frequently remarked, on drawing near a spot densely peopled by the small acorn-shells that so abundantly cover most of our rocks on the sea-shore a peculiar faint crackling noise, sud denly pro'cluced, gradually subsiding after the lapse of a few seconds, and not repeated until a movement was made towards another spot ; and, on searching for the cause of this singular sound, we have satisfied ourselves that it is uniformly produced by the sudden closing of the opercules of the Balanids,which seem generally to remain open in ordinary cir cumstances. We have seen this motion again and again follow immediately the movement of the hand towards particular spots, (not, however, nearer the shells than twelve or four teen inches,) so that we could not but con clude that the animal was made sensible, through the medium of the air, of the pre sence of some foreicm body, and, fearing dan ger, closed its shellfor self-protection ; just as the limpet, warned of the approach of hurtful agents by the sliglitest touch of its shell, fixes itself more securely to its rocky footing. What the nature of the sense is which is thus used by the Cirripeds, we have no means of determining.
Digestion.—The minute swimming Crus tacea appear to constitute the principal food of the Cirripeds. Sometimes, however, the shells of minute Mollusca are found in their stomachs, and Burmeister once found part of' an annelid of unknown species. The food is carried towards the mouth by currents pro duced by the rapid motions of the arms, which, in most of the species, are constantly spread out and drawn in, alternately, with great regularity . The mouth is situated just at the bottom of the funnel -shaped cavity formed by the spread arms (b,fig. 340). In the Lepads its position is close to the trans verse adductor muscle. Its jaws form a round protuberance, which presents itself very con spicuously immediate ly on separating the arms. It might al most be reaarded as a c, bead, so prominent is it (fig. 341, b,h) ; but we find it composed only of the lip and jaws, with their muscles. The lip over-arches the jaws ; it is horny, and furnished with minute palpi. There are three pairs of jaws. The first or outer pair are thin horny plates of an oval form, fringed along their opposing sides with long stiff hairs. The other two pairs are curved and deeply serrated on their opposed surfaces.