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Page:Popular Science Monthly Volume 18.djvu/100

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other, giving to the odd, phosphorescent, living tubes a lengthwise movement through the ocean.

The Ascidians—a family of fixed Tunicates—present societies to some extent organically connected. They are grouped by a common connection of their mantles, or rise successively from a common stem, through which an organic unity is established. Yet their individuality continues; for, if one of the Ascidians has its circulation cut off, by a ligature, from the common stem, it continues to exist independently.

In the fixed polyps the subordination of character resembles that of the Ascidians. It is carried further, however. Thus, in some instances, not only is the common stem fed by the efforts of a series of individual mouths, but there seems to be a sensitive connection. If, for instance, one of the expanded animals of an Alcyonium community be touched, not only does this animal contract, but gradually the remaining animals of the community contract also.

Again, in the Hydrozoa, individual members of the community are specialized as reproductive organs, being fed through the common stem by the feeding individuals. In these cases the merging of individuality has extended much beyond the simple case of the Salpæ, certain members of a society being specialized as organs of a compound animal. These reproductive buds, however, in many cases regain their individuality in a very peculiar manner. They separate from the common stem, and continue to exist as free-swimming animals. But their specialized development has produced material modifications in their form and internal organization. They are no longer fixed polyps, but free Medusæ, retaining only a general resemblance to the polyp type, and swimming by means of contractions of their umbrella-like disk.

By this strange modification of the polyp form, to achieve special purposes, a new free animal form is produced, which sometimes follows its new line of development so as to yield an animal markedly distinct from its unspecialized brothers of the same community. Such is one of the many strange modes in which Nature has sought to produce new forms of animal life.

But the greatest subordination of individuality is shown in the Siphonophoræ, a family of Hydrozoa in which a distinct effort seems to be made to attain the elongated, free-swimming form, through combination. In some of these the evolution of a colony into a single animal is almost complete. A large number of individuals are connected by a common stem; but these individuals are so specialized in function as to be no longer capable of a separate existence. They have lost certain powers, and developed others, so that they are reduced to the condition of special organs of a single animal. Some act as food-catching organs, some as mouths, some as reproductive and nursing members, and, by a strange transformation, some have become bell-like organs, which, by successive contractions, expel the water, and force the whole community through the seas.