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Page:Popular Science Monthly Volume 18.djvu/97

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THE EVOLUTION OF ORGANIC FORM.

manage to progress slowly, but they depend more particularly on their armor for protection.

In the Mollusk family—another form of reversion from the primitive longitudinal or three-axed form—the conditions of existence necessitate other motive organs. This family of animals, instead of clothing itself in a dermal armor like the Echinoderms, produces a limy covering, a movable house to which it is not anatomically connected, and which principally differs from the stone mansion of the polyp in being movable. Within this house the mollusk preserves his three axed form; having no such strong inducement to yield it as has the Echinoderm. But his contact with exterior nature is but a head and foot contact. He therefore develops head-limbs—tentacular organs—while his slow progress is gained by alternate expansions and contractions of a muscular portion of the ventral surface.

Thus the lower types of animal form are forced, by the necessities of their environment, to evolve certain general anatomical conditions, which, as we shall hereafter see, act as a fatal drag on their subsequent efforts to occupy the higher fields of life.

The worm type has, from the beginning, a marked advantage over them. The creeping forms of this type would naturally tend to develop moving organs at their points of contact with the ground, yielding ventral limbs, extended along the body. Breathing organs might appear on the dorsal surface, in contact with the water; or at the mouth, where inflowing currents would yield the fullest water contact.

In the swimming worms a somewhat different process of limb development would naturally arise. Here, for the freest degree of motion, some form of fin must replace the limb of the creeping worm. There is reason to believe that fins first arose as lateral extensions of the flattened body. This general fin—under the late theory of limb development—in time lost its continuity, and broke up into four separate sections, whence arose the four limbs of the future Vertebrates.

The possession of such longitudinal body-limbs or fins gives much greater rapidity of motion to the worm type than is possible to the head-limbed or radiate body-limbed types. As a consequence of their motive facility they remain naked, rapidity of motion and keenness of sense giving them powers of attack and escape not needed by the tentacled and armored forms.

In fact, the advantage of the longitudinal extension is so patent that we find all the lower types making efforts to attain it, and in this manner reaching their highest limit of progression. This constitutes the next step in the evolution of animal form, and one which presents some exceedingly curious phases.

The phases here referred to are not displayed by the mollusks or the Echinoderms. We shall therefore first speak of their simpler mode of attaining their highest development.

In the mollusk it is attained by a lengthening of the compact body,