[Illustration: Aurelia flavidula, seen in profile]
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.
It is not strange that the relation between the various phases of this extraordinary series of metamorphoses, so different from each other in their external aspects, should not have been recognized at once, and that this singular Acaleph should have been called Scyphostoma in its simple Hydroid condition, Strobila after the transverse division of the body had taken place, Ephyra in the first stages of its free existence, and Aurelia in its adult state,--being thus described as four distinct animals. These various forms are now rightly considered as the successive stages of a development intimately connected in all its parts,--beginning with the simple Hydroid attached to the ground, and closing in the shape of our common Aurelia, with its white transparent disk, its silky fringe of tentacles around the margin, its ruffled curtains hanging from the mouth, and its four crescent-shaped ovaries grouped to form a cross on the summit. From these ovaries a new brood of little embryos is shed in due time.