ASYMMETRY AS A CREATIVE FACTOR. 459 



tative difference between corresponding right and left organs, the other parts of 

 the body promptly respond by a change of form, or position, which tends to restore 

 the lost equilibrium, producing thereby a more obvious deformity or asymmetry. 



It is often assumed that asymmetry is the result of a sessile or parasitic mode of 

 life. But it is altogether more probable that the reverse is true, for comparatively 

 few sessile or parasitic animals are asymmetrical, and extremely asymmetrical 

 forms would be likely to find a sessile or parasitic mode of life the only one open to 

 them, because these conditions are less exacting. 



The initial cause of asymmetry is usually some event that occurs within the 

 egg itself, probably at a very early period. It has been shown, for example, that 

 under normal conditions, such an ancient and stable type as Limulus produces a 

 surprisingly large number of asymmetrical embryos. When one side is entirely 

 absent, the remaining side is thrown out of its original straight line into the form 

 of a bow, a half circle, or a semi-spiral. These forms may live several months 

 and appear to be perfectly healthy, but apparently they never develop beyond 

 the larval stages. 



Extreme asymmetery in the embryonic development of some cirriped-like 

 arthropod probably initiated the great class of echinoderms, for the echinoderm 

 larva, which in its structure, metamorphosis, and mode of attachment, resembles 

 that of a cirriped, is at first quite symmetrical. When the organs on one side 

 degenerate, or fail to develop, the remaining side bends till its two ends meet and 

 form a ring; the segmental organs are then arranged along the radii of a circle, 

 instead of in parallel lines. These changes are similar to those that occur in 

 Limulus embryos, except that they are carried farther and give rise to animals 

 capable of surviving in their new form. 



Thus such a negative character as the absence of one side of the body has 

 created a new condition, a new organic environment, that has in turn created a 

 new type of radiate structure, and at a single stroke initiated the evolution 

 of a new class of animals. Organic readjustment, during this crisis, was no doubt 

 extremely rapid. But after the essential changes took place, elaboration toward 

 a more active neuro-muscular existence practically ceased. The history of this 

 ancient phylum indicates that it never completely recovered from the effects of 

 this radical metamorphosis, for no other large group of animals, with an equal 

 grade of organic complexity, shows such a low grade of neuro-muscular adjust- 

 ment to its environment. 



C. Chiten and the Exoskeleton as Creative Factors. — Chiten is one 

 of the most characteristic features of the arthropods, and like the cellulose wall 

 of the plant cell is a creative factor of very great significance. 



It is tough and flexible, but inelastic; it is capable of great hardness and 

 forms for the entire external surface of the body a water- and air-proof covering, 

 unaffected by any chemical changes likely to occur in the surrounding media. 

 More perhaps than any other factor it controls the form of the body, its method 

 of growth, the distribution and attachment of muscles, the character of the appen- 



