The arrangement and mode of working of the muscles, says Lang, is illustrated by Fig. 18, which shows us five segments, one larger (_ct_) and four smaller, in vertical projection. The thicker portion of the integument is marked by strong outlines, the delicate and flexible interarticular membranes (_tg_, _sg_) in dotted lines. The hinges between two consecutive segments are marked _a_. A dorsal muscle (_d_) is attached to the larger segment (_ct_), and runs through the smaller segments, being inserted in the dorsal portion of the crust (_t_) of each by means of a bundle of fibres. A ventral muscle (_v_) does the same on the sternal side (_s_). “The skeletal segments,” adds Lang, “may be compared to a double-armed lever, whose fulcrum lies in the hinges. If the dorsal muscle contracts, it draws the dorsal arm of the lever (the tergal portion of the skeleton) in the direction of the pull towards the larger segments; the tergal interarticular membranes become folded, the ventral stretched, and the four segments bend upward (Fig. 18, _A_). If the ventral muscle contracts, while at the same time the dorsal slackens, the row of segments will be bent downwards (Fig. 18, _C_).” 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.)