Page:Popular Science Monthly Volume 13.djvu/332

318, and the little fruit-like clusters are persistent jelly-fishes which develop and remain just beneath the row of tentacles (Fig. 13), instead of becoming free jelly-fishes as in Sarsia and in Campanularia.

In the Gulf of Mexico are communities of hydroids so organized that they seem to constitute but one animal. Such is the well-known "Portugese man-of-war" (Fig. 15). This community consists of an elegantly-crested air-sac floating upon the water, and giving off numerous long and variously-constructed appendages. According to Agassiz, the different parts are so many different kinds of members in the community, and fulfill widely-different offices, some catching and eating food for the whole, others producing buds, others being the locomotive or swimming members, and having tentacles that in some cases are twenty or thirty feet long. The air-sac itself is only a few inches in length.

But the most common jelly-fishes are those which are more or less disk-shaped, and hence are called the Discophoræ. The "sunfish" is one of these. This name, we hasten to say, is rather indefinite when used without modification, for it is not only applied to a jelly-fish, but it is also given to our fresh-water bream, and to one of the large marine fishes—orthogoriscus. The "sunfish" of which we now speak attains a diameter of six to twelve inches (Fig. 19). In the early spring it may be seen in large schools near the surface of the water, and at this time is only a small fraction of an inch in diameter. It becomes full-grown by the middle of summer, and great numbers may then be seen swimming slowly by a sort of motion that may be likened to that of partly shutting and opening an umbrella. The motion is indeed effected by the contraction and expansion of the whole umbrella-like disk.

In the study of the sunfish (Aurelia) we are able to see plainly the prominent differences between jelly-fishes as a group and polyps as a group.

The natural attitude of the latter is with the mouth upward, or at least not turned downward, and the body is divided into vertical chambers, by vertical partitions, and the substance of the animal is flesh-like. On the other hand, the typical adult jelly-fishes have their mouth on the under surface, or at least not turned upward, their