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Page:Popular Science Monthly Volume 18.djvu/102

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THE POPULAR SCIENCE MONTHLY.

of the organization of this primitive animal. If we assign to it the organs still possessed by the segments of the Annelides, we find it to have had an intestine separate from the circulation, being thus superior to the polyps. It had also simple nervous and muscular systems, and immature eyes, a chitinous armor, a water-vascular system, and possibly distinct exterior breathing organs and feet. It may, indeed, have been the primitive form from which other animal types besides the Articulates originated—through a diverse process of evolution.

It is not improbable that the Articulate condition was reached, not by a combination of free individuals, but by a continued adherence of longitudinal buds. The increase in number of segments by division is still common in Articulates. The minute fresh-water worm called the Nais, is separated into two sections by a bud which appears in the center of the body. One section develops a head and the other a tail, at the ends adjoining the bud. But the bud itself again and again divides, each division becoming a young Nais, so that finally a chain of worms is formed, all organically connected, and fed by the mouth of the anterior Nais. Eventually they separate, each becoming a free individual.

The question now arises as to how a developing force would act on such an articulated society. The highest results of evolution are reached through concentration of function. Such specialization is opposed to a continuous increase in the number of segments, and must tend to the production of a definite organism, of limited extent. The activity of this organism is increased by its gaining limbs more useful than the bristle-like setæ of the Annelides. Its range of food expands when its fore-limbs are changed into food-getting organs. Its powers of motion increase when the body is compacted, and the number of joints decreased, by a welding of several segments into one.

But whence come such new limbs? A consideration of their character leads us to the idea that they may proceed from a simple continuance of the budding process, acting, in this case, in a lateral direction instead of lengthwise. For the limbs are hollow, jointed segments, covered with chitine like the body-segments. They seem, indeed, to be specialized side-segments which have lost their internal organs through disuse, retaining only their chitinous armor, their muscles, and their intestinal cavity. And the successive joints of the limbs appear to be formed by a continuance of the budding process. One evidence of this is the fact that they may be reproduced by budding when broken off at the joint; and also that lateral budding again takes place at the extremity of the limbs, yielding double tarsi or pincers in the head-limbs.

Such is the character of the articulated animal; and it appears as if this persistent partial individuality of the segments must prevent that complete localization of function which seems necessary to the greatest animal development.