The chief bulk of the sin gularly formed diphyda is made up of the swimming which are two sub cartilaginous bodies, poly gonal, generally pointed an teriofly, truncated posteri orly, placed one behind the other, and one a little within the other; the posterior por tion being lodged in a little excavation which exists in the anterior. These two parts differ somewhat from one another in form,-but both are hollow, and have large open ings. Their attachment is so slight as to admit of their being separated by agitation of the water. It is at the bottom of the anterior ca vity that the essential parts of the animal are placed. Locomotion is effected by means of the impulse of a current which is kept up by the successive contractions and dilatations of the organs above described. The contractions of the two bodies are not synchronous ; but they succeed one another within a short time, so that a steady progression is maintained ; and in some species it is very rapid.
In the ciliograda, the locomotive organs are large cilia, disposed in longitudinal bands on the surface of the body. These bands are ge nerally eight in number; but in some species, (e. g. axiotoma Gaedii, Esch ,) there are only four. The arches supporting the cilia are of firmer texture, and are less transparent than the rest of the body. In many species they extend from one end of the body to the other ; in some only along a part of the circumference. The structure of the cilia themselves has lately been examined by Dr. Grant,* with his usual care, in the Bera pileus;t and he has found that they are fin-like processes, and that each is composed of several short, transparent, some what curved filaments, placed parallel to each other in a single row, and connected together by the skin of "the animal, like the rays sup porting the fin of a fish. The rays in the middle of the cilium are a little longer than those at the sides. All the rays appear as transparent tubes under high magnifying pow ers. They are so curved that their extremities are directed backwards towards the closed ex tremity of the animal. There are about forty cilia attached to each arch in this species, which is nearly an inch in length. The cilia are so large as to be visible to the naked eye. Most of the ciliograda have their cilia quite exposed; but Pandora is provided with moveable folds of the skin along the cilia-bearing arches, which can be brought over the cilia, in whole or in part, at the animal's pleasure, so as to cover them more or less completely. These cilia are moved nearly in the same manner as the pec toral fins of fishes. But their motion is so rapid, when the animal is vigorous, that the eye cannot follow it. The existence of motion is pointed out, however, by lines of beautiful iridiscent colours playing along the arches, and by the currents which are generated in the cir cumambient fluid. The animal has the power of arresting completely the motion of one, two, or more rows of cilia, while the others are moving. When all are set in motion together,
the animal moves onwards with the inferior or oral surface (inferior in a state of rest) directed forwards. When the motion of some is ar rested, the whole body acquires a rotatory mo tion, and advances in a curvilinear path. The animal has also the power of changing the direction of the currents caused by its cilia, so that it can ascend or descend in the water at pleasure. It can also increase and diminish at will the velocity of the motions of the cilia. Those animals which have the largest cilia, (e. g. Jiledea,) swim with the greatest rapidity. The cilia continue to move for some time after having been separated from the body, in con nexion with part of their arches. Immediately beneath the arches there are vessels conveying a fluid, which is in motion during the vibrations of the cilia. Whether these vessels are destined only for the conveyance of the circulating fluid to the cilia, (which in all probability act as organs of respiration as well as of locomotion,) or carry a stimulus fitted to excite their vibra tions, is not yet determined. Eschscholtz com pares these vessels to those which Tiedemann has described as connected with the feet in the echinodermata. And Dr. Grant is of opinion, with MM. Audouin and Milne Edwards, that it is not improbable that the motions of the cilia are somehow dependent on the movements of the fluids contained in the above-mentioned vessels, seeing that in the actiniee the tentacula are projected by water being forced from below into them. In the other classes of the acaleplice also the same kind of structure prevails. Such of the pulmograda, as have cilia around their margins have also circular vessels running along their bases; and almost all projectile and exten sile tentacules and filaments are provided with sacs and canals, containing fluids, at their roots.
In addition to their cilia, several of the cilio grade acaleplim have other organs of locomo tion in the form of long filamentary arms or ten tacules, with which they can poise themselves in the water without moving their cilia. In Cy dippe these are two in number. They are lodged in two tubes placed alongside of the sto mach, from which they issue near the mouth. They can be extended to four times the length of the animal. They terminate in very fine points, and along their whole course present minute filaments placed at equal distances, which are coiled up spirally, close to the ten tacules, when these are about to be withdrawn into their sheaths. The tentacules are also coiled up in a spirdl form when completely contracted. They are sometimes suddenly sent forth from their tubes to their full length by one impulse, and then their lateral filaments are gradually uncoiled ; a process this of no less interest on account of the gracefulness of the motion than on account of the peculiar mecha nism which it indicates.