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Page:Popular Science Monthly Volume 18.djvu/99

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this line of animal evolution has reached its ultimate at a much lower level than that attained by the Mollusca.

But, by this review of what we may, in a figurative sense, call Nature's failures in animal evolution, we begin to perceive the requisites to success. The retrograde forms, after again developing into the lengthened type, are constitutionally restricted from gaining certain structural advantages which are primitive possessions of other types.

These advantages we may classify as body-limbs, adapted to walking or swimming; and an articulated body, capable of a flexibility not possible to the compact, single-sectioned animals. All the other animal types, besides those we have considered, have made an effort to attain this articulated structure, sometimes by a very curious process. The success attained in this effort is closely dependent upon the primitive structure of the articulated animal, which has placed impassable restrictions in the path of some types.

In the polyps and in the articulates the end seems to have been attained by the linking together of a colony of animals, forming a structure, originally compound, which has become simple by a division of functions between the successive sections.

In the Vertebrata alone has it been attained by the articulation of an originally single animal. The vertebrates thus possess special structural advantages denied to the other articulated forms, the compound origin of these latter curiously limiting their powers of evolution.

In this merging of societies into single animals, Nature presents us instances of every step of the process, from those in which individuality remains intact, to those in which it is subordinated to the requirements of the compound animal.

A first step in the process is displayed by the Tunicate mollusks. The Salpa—one form of these shell-less creatures—is a free-moving animal, progressing by the aid of water, which is drawn into one end of its straight intestine and expelled at the other. They exist in two conditions, the single and the compound. In the latter they unite into long chains, not organically connected, but apparently adhering by little suckers.

This primitive combination seems assumed for one advantage only, that of aiding their motion. The animals in the chain contract and expand simultaneously, the whole chain moving like one lengthened animal.

The same end is achieved in a still more curious way in the Pyrosoma, another of the Tunicata. These little creatures so group themselves as to form a hollow tube, open at one end and closed at the other. The minute animals which compose the walls of this tube have one gill-opening extended outward, the other inward. Thus they draw water from outside and discharge it into the interior of the tube. This being closed at one end, the water is necessarily driven from the