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L. B. Sharp suggests, that in the Crustacea the rings formed by “the regularity and stress of muscular action” would be hardened by the deposition of lime at the most prominent portion, i.e. between what we have called the intersegmental folds. (American Naturalist, 1893, p. 89.) Cope also states that “with the beginning of induration of the integument, segmentation would immediately appear, for the movements of the body and limbs would interrupt the deposit at such points as would experience the greatest flexure. The muscular system would initiate the process, since flexure depends on its contractions, and its presence in animals prior to the induration of the integuments in the order of phylogeny, furnishes the conditions required.” (The Primary Factors of Organic Evolution, p. 268, 1895.)

It is apparent that the jointed or metameric structure of the bodies of insects and other arthropods is an inheritance from the segmented worms. In the worms the body is a continuous dermo-muscular tube, while in arthropods this tube is divided into regions, and the cuticle is thicker and more resistant. To go back to the incipient stages in the process of segmentation of the body, we conceive that the worms probably arose from a creeping gastrula-like form, the gastræa. The act of creeping gradually induced an elongated shape of the body. The movement of such an organism in a forward direction would gradually evolve a fore and aft, dorsal and ventral, and bilateral symmetry. As soon as this was attained, as the effect of creeping over rough irregular surfaces there would result mechanical lateral strains intermittently acting during the serpentine movements of the worm. The integument would, we can readily suppose, tend to bend or yield, or become permanently wrinkled, at more or less regular intervals. The arrangement of the muscles would gradually conform to this habit of creeping, and finally the nervous system and other organs more directly connected with the creeping movements of the organism would tend to be correlated in their arrangement with that of the segments. In this way the homonomous segments of the annelid body probably became developed, and their relations and shapes were eventually fixed by inheritance. After this stage was reached, and limbs began to appear, the segments would tend to become heteronomous, and to be grouped into regions.