These swimming bells are not unlike the Medusæ in this particular, but have become far more specialized than the Medusæ, as they possess none of the organs requisite to individual life.
In the Physalia, or Portuguese man-of-war, the connecting body is developed into a floating bladder, moving by force of the winds, and with its variously modified polyps beneath it.
Such are some of the modes adopted by Nature to produce free motion in the lower types of animal life. The animals produced by this social subordination of function are imperfect because the subordination is indefinite. There is not a single organ adapted to each function, but a variable number. And the very means by which propulsion through the water is gained renders this imperfection necessary. For, if a single individual constituted each organ, the animal would become compact, and be moved by a single contracting bell. Its powers of motion would be reduced to those of the Medusæ, and its organization retrograde toward the original compact stage.
This line of progress, with its necessarily imperfect specialization, is evidently incapable of attaining the level of the Echinoderm, much less of the mollusk.
But another line—that of the segmented animals—seems much better adapted to attain a high grade of evolution. Not but that its segments possess anatomical characters as stubborn as those of the Radiates, but that these are less restrictive to a high evolution.
It is, of course, not the usual view to consider the Articulates as the result of an original social organization. The segments, in the higher genera, are so specialized that they now exist but as organic parts of a single animal. And yet, if we consider the lower articulated worms, evidences of such an origin may be discovered.
In these lowest Articulates scarcely any difference is to be traced between the segments. The anterior, from its position, acts as a mouth, but otherwise they are as similar as the individual Salpæ. But the most significant feature is that in many cases each of them possesses the organization of an individual. Each segment still retains its separate nervous ganglion, its separate muscles, its separate limbs, frequently its separate breathing organs, and, in a partial degree, its separate circulation. These are only subordinated to the extent of being joined by connecting links, while the intestine of each becomes continuous as a common intestine.
In fact, this organic individuality is carried, in certain cases, to a yet more significant extent. The organs of special sense the most highly specialized of animal organs—are, in some instances, retained by the separate segments. There are not only existing worms with eyes at each extremity of the body, but others which possess eyes in each separate segment.
Thus we are led not alone to the conception of an original animal which became associated into the Articulates, but even to some idea