The ectoskeletal segments bend to one side by the contraction of the muscles on one side, the point of the outer segmental fold opposite the fixed point becoming converted into the turning-point (C).
The usual result of the arrangement of the locomotive system is the simple curving of the body (C), and then the alternate bending of the body to right and left, which produces the serpentine movements characteristic of the earthworms, the centipede, and many insect larvæ. The most striking example of the wonderful variety of movements which can be made by an insect are those of the Syrphus larva. When feeding amid a herd of aphides, it is seen to now raise the front part of the body erect and stiff, then to bend it down, or rapidly turn it to either side, or move it in a complete circle. (Graber, pp. 23–26.)
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.)
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.
Fig. 19.—Dujardinia rotifera, with jointed tentacles and caudal appendages.—With some changes, after Quatrefages.
The origin of the joints or segments in the limbs of arthropods was probably due to the mechanical strains to which what were at first soft fleshy outgrowths along the sides of the body became subjected. Indeed, certain annelid worms of the family Syllidæ have segmented tentacles and parapodia, as in Dujardinia (Fig. 19). We do not know enough about the habits of these worms to understand how this metamerism may have arisen, but it is possibly due to the act of pushing or repeated efforts to support the body while creeping over the bottom among broken shells, over coarse gravel, or among seaweeds.
It is obvious, however, that the jointed structure of the limbs of arthropods, if we are to attempt any explanation at all of the origin of such structure, was primarily due mainly to lateral strains and impacts resulting from the primitive endeavors of the ancestral arthropods to raise and to support the body while thus raised, and then to push or drag it forward by means of the soft, partially jointed, lateral limbs which were armed with bristles, hooks, or finally claws.