The principal organ of motion_ in the pulmo grada is the large campanulate, or mushroom shaped, disc, of gelatinous consistence, which constitutes the great mass of the animal. In this, for the most part, no muscular fibres can be seen, and yet the animals move about with some quickness. They have the power of con tracting and dilating their discs at pleasure, in whole or in part. By alternately contracting and dilating their inferior surface, they strike the water in such a manner and with such force as to raise themselves ; when they discontinue this motion, they again sink, being of greater specific gravity than the sea-water. They move onwards horizontally, by acting only with one side of the margin of their disc. Lamarck was of opinion that these isochronous move ments of the disc, by means of which the pul mograda seem to swim, were fitted merely to facilitate the internal vital processes, and not to move the animals through the water ; and he regarded them as dependent entirely on the influence of imponderable agents existing in the circumambient fluid, and alternately enter ing into, and flowing from, the general mass of the animal. He compared the motions with those of the fluid in Franklin's thermoscope, when held in the hand.* In the course of the ordinary progression of the large Medusa aurita of our seas, the contractions of the disc take place from twelve to fifteen times in a minute. The convex surface of the disc always advances foremost.
No fibrous structure has hitherto been dis covered in the general mass of the disc. In ternally, it is cellular, uniform, and very soft. The quantity of solid matter in the disc, and, indeed, in the whole body, is very small. Some medusa), which, when recently taken out of the water, weighed fifty ounces, on being dried, left remains weighing scarcely more than five or six grains. " It is therefore evident, that the sea-water, penetrating the organic tex ture, constitutes the greater part of the volume of these animals!'-j- But in some species there exists a fine muscular membrane, stretched over a certain extent of the lower surface just within its outer margin. Under a lens, this has the appearance of being composed of nu merous fleshy fibres, forming little bundles, arranged in a radiate manner as regards the axis of the animal, and closely adherent to the gelatinous tissue of the disc. When portions of the disc are cut off from living medusa, without any part of this muscular membrane being attached to them, they remain motionless; but when their connexion with the membrane is preserved, even small portions continue their motions of contraction and dilatation for a considerable time.
The tentacula of the pulmograda (which are always pendent from the inferior surface) may be regarded as supplementary organs of loco motion, although they are, in all probability, subservient chiefly to the nutritive function. They are all simple, not branched, generally partly by the movements of the tentacules which hang down from the inferior surface ; but chiefly, perhaps, by the action of the wind on the raised crest, with which most of these animals are provided. Immediately around the mouth are placed numerous small tubular suckers, similar to the feet of many echinoder mata. Exterior to these there are longer tenta cula, for the most part in a single row, and simple ; sometimes branched. Neither of these two kinds of organs is very extensile. The disc from which the tentacules hang, and the crest, are supported internally by a calcareous plate, which is the only organ of the kind in the whole class of acaleplm. It somewhat re
sembles in structure the calcareous axis of retepora, being cellular and porous. Its nu merous cells are filled with air, which renders the whole animal so buoyant that it floats on the surface of the water, and is wafted along by the winds. In velella ( Fig. 11.) there are two plates, one placed horizontally, the other perpen dicularly upon the upper surface of the former. They are marked with lines of growth, enlarg ing from within outwards, like the extravascular shells of the mollusca. The perpendicular plate in velella supports the crest, which stands upright, and exposes a large surface to the wind. Rataria ( Fig. 12.) has its crest provided with strong muscular bands run ning perpendicularly. It lies on the surface of the water, with the crest stretch ed out, so that its whole side touches the water. When it is alarmed, the crest is suddenly contracted, and the centre of gravity is so al tered in consequence, that the position of the body is almost reversed. When the crest is again raised, the body imme diately resumes its former position.
Porpita has a simple plate supporting its disc, without any crest, and long tentacula, which are so delicate as scarcely to bear the slightest touch when the animal is taken out of the water. When the position of the animal is altered by the hand, so as to make the surface covered with suckers the upper one, all the tentacula of one half of the body turn round to the dorsal surface, and all those of the other half stretch over their own surface, and thus the animal very soon regains its old position.
II. Motility and Sensation. — Almost all observers have failed to discover anything re sembling a nervous system in the Acalephm. Even Eschscholtz,* who devoted so much atten tion to their anatomy, could not see nerves in the largest that he examined. But in Cydippe, according to Dr. Grant, f there is a structure which can be regarded only as belonging to the nervous system. It consists of a double transverse filament of a milky white colour, running round the body, near its surface, at a short distance above the mouth. The two cords of which this filament is composed unite in the middle of each of the spaces between hollow; and, when connected with the appen dages of the digestive cavities, or when they have a vesicle at their base, very extensile. Several genera have suckers at the extremities, and along the sides, of their tentacula, by means of which the passing prey is seized. The tenta cula which are extensile seem to be projected by the forcing of water into their internal cavity, by the contractions of the vesicles at their base. The extent to which the filamentary organ is thus lengthened, in some species, is very extra ordinary.* It seems to be shortened again by means of the contractions of circular muscles, which force back the water into the vesicle, and of longitudinal muscles which draw it in. Peron thought that some of the pulmograda were furnished with internal air-bladders ; but Eschscholtz, on directing his attention to this point, satisfied himself that what Peron had taken for air-bladders were merely appendages of the gastric cavities, into which air had acci dentally been introduced during the removal of the animals from their native element.