While these organic changes take place in the lower disks, the topmost one, forming the summit of the pile and bearing the tentacles, undergoes no such modification, but presently the first constriction dividing it from the rest deepens to such a degree that it remains united to them by a mere thread only, and it soon breaks off and dies. This is the signal for the breaking up of the whole pile in the same way by the deepening of the constrictions; but, instead of dying, as they part, they begin a new existence as free Medusae. Only the lowest portion of the body remains, and around the margin of this tentacles have developed corresponding to those which crowned the first little embryo; this repeats the whole history again, growing up during the following season to divide itself into disks like its predecessor.
As each individual separates from the community of which it has made a part, it reverses its position, and, instead of turning the margin of the disk upward, it turns it downward, thus bringing the mouth below and the curve of the disk above. These free individuals have been described under the name of Ephyra. This is the third phase of the existence of our Jelly-Fish. It swims freely about, a transparent, umbrella-like disk, with a proboscis hanging from the lower side, which, to complete the comparison, we may call the handle of the umbrella. The margin of the disk is even more deeply lobed than in the Hydroid condition, and in the middle of each lobe is a second depression, quite deep and narrow, at the base of which is an eye. How far such organs are gifted with the power of vision we cannot decide; but the cells of which they are composed certainly serve the purpose of facets, of lenses and prisms, and must convey to the animal a more or less distinct perception of light and color. The lobes are eight in number, as before, with a tube diverging from the centre of the body into each lobe. Shorter tubes between the lobes alternate with these, making thus sixteen radiating tubes, all ramifying more or less.
From this stage to its adult condition, the animal undergoes a succession of changes in the gradual course of its growth, uninterrupted, however, by any such abrupt transition as that by which it began its life as a free animal. The lobes are gradually obliterated, so that the margin becomes almost an unbroken circle. The eight eyes were, as I have said, at the bottom of depressions in the centre of the several lobes; but, by the equalizing of the marginal line, the gradual levelling, as it were, of all the inequalities of the edge, the eyes are pushed out, and occupy eight spots on the margin, where a faint indentation only marks what was before a deep cut in the lobe. The eight tubes of the lobes have extended in like manner to the edge, and join it just at the point where the eyes are placed, so that the extremity of each tube unites with the base of each eye. Those parts of the margin filling the spaces between the eyes correspond to the depressions dividing the lobes or scallops in the earlier stage, and to those radiate the eight other tubes alternating with the eye-tubes, now divided into numerous branches. Along each of these spaces is developed a fine, delicate fringe of tentacles, hanging down like a veil when the animal is at rest, or swept back when it is in motion. In the previous stage, the tubes ramified toward the margin; but now they branch at or near their point of starting from the central cavity, so extensively that every part of the body is traversed by these collateral tubes, and when one looks down at it from above through the gelatinous transparent disk, the numerous ramifications resemble the fine fibrous structure of a leaf with its net-work of nervules.
On the lower side, or what I have called in a previous article the oral region of the animal, a wonderfully complicated apparatus is developed. The mouth projects in four angles, and at each such angle a curtain arises, stretching outwardly, and sometimes extending as far as the margin. These curtains are fringed and folded on the lower edge, so that they look like four ruffled flounces hanging from the lower side of the animal. On the upper side of the body, but alternating in position with these curtains, are the four ovaries, crescent-like in shape, and so placed as to form the figure of a cross, when seen from above through the transparency of the disk. I should add, that, though I speak of some organs as being on the upper and others on the lower side of the body, all are under the convex, arched surface of the disk, which is gelatinous throughout, and simply forms a transparent vaulted roof, as it were, above the rest of the body.
When these animals first make their appearance in the spring, they may be seen, when the sky is clear and the sea smooth, floating in immense numbers near the surface of the water, though they do not seek the glare of the sun, but are more often found about sheltered places, in the neighborhood of wharves or overhanging rocks. As they grow larger, they lose something of their gregarious disposition,—they scatter more; and at this time they prefer the sunniest exposures, and like to bask in the light and warmth. They assume every variety of attitude, but move always by the regular contraction and expansion of the disk, which rises and falls with rhythmical alternations, the average number of these movements being from twelve to fifteen in a minute. There can be no doubt that they perceive what is going on about them, and are very sensitive to changes in the state of the atmosphere; for, as soon as the surface of the water is ruffled, or the sky becomes overcast, they sink into deeper water, and vanish out of sight. When approached with a dip-net, it is evident, from the acceleration of their movements, that they are attempting to escape.
At the spawning season, toward the end of July or the beginning of August, they gather again in close clusters. At this period I have seen them at Nahant in large shoals, covering a space of fifty feet or more, and packed so closely in one unbroken mass that an oar could not be thrust between them without injuring many. So deep was the phalanx that I could not ascertain how far it extended below the surface of the water, and those in the uppermost layer were partially forced out of the water by the pressure of those below.