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. [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.