The segments are merely thickenings of the skin connected by folds or duplications of the integument, and not actually separate or individual rings or segments. This is shown by longitudinal (sagittal) sections through the body, and also by soaking or boiling the entire insect in caustic potash, when it is seen that the integument is continuous and not actually subdivided into separate somites or arthromeres, since they are seen to be connected by a thin intersegmental membrane (Fig. 16). But this segmentation or metamerism of the integument is, however, the external indication of the segmentation of the arthropodan body most probably inherited from the worms, being a disposition of the soft parts which is characteristic of the vermian type. This segmentation of the integument is correlated with the serial repetition of the ganglia of the nervous system, of the ostia of the dorsal vessel, the primitive disposition of the segmental and reproductive organs, of the soft, muscular dissepiments which correspond to the suture between the segments, and with the metameric arrangement of the muscles controlling the movements of the segments on each other, and which internal segmentation or metamerism is indicated very early in embryonic life by the mesoblastic somites.
Fig. 16.—Diagram of the anterior part of an insect, showing the membranous intersegmental folds, g.—After Graber.
In the unjointed worms, as Graber states, the body forms a single but flexible lever. In the earthworm the muscular tube or body-wall is enclosed by a stiffer cuticle, divided into segments; hence the worm can move in all required directions, but only by sections, as seen in Fig. 16, which represents the thickened integument divided into segments, and folded inward between each segment, this thin portion of the skin being the intersegmental fold. Each segment corresponds to a special zone of the subdivided muscular tube (m), the fascia extending longitudinally. The figure shows the mode of attachment of the fascia of the muscle-tube to the segment. The anterior edge is inserted on the stiff, unyielding, inner surface of each segment: the hinder edge of the muscle is attached to the thin, flexible, intersegmental fold, which thus acts as a tendon on which the muscle can exert its force. (Graber.)
Fig. 17.—Diagram of the integument and arrangement of the segmental muscles: A, relaxed; m, muscle; g, membranous articulation; r, chitinous ring. B, the same contracted on both sides. C, on one side.—After Graber.
“Fig. 17 makes this still clearer. The muscles (m) extend between two segments immediately succeeding each other. Supposing the anterior one (A) to be stationary, what do we then see when the muscle contracts? Does it also become shorter? The intersegmental fold is drawn forwards, and hence the entire hinder segment moves forward and is shoved into the front one, and so on with the others, as at B. Afterwards, if the strain of the muscle is relieved by the diminishing action of the tensely stretched, intersegmental membrane, it again returns to a state of rest.” (Graber.)
Fig. 18.—Diagrams to demonstrate the mechanism of the motion of the segmented body in the Arthropoda: One larger segment (cf) and 4 smaller. The exoskeleton is indicated by black lines, the interarticular membranes by dotted lines. The hinges between consecutive segments are marked at, tergal (dorsal) skeleton; s, sternal (ventral) skeleton; d, dorsal longitudinal muscles = extensors (and flexors in an upward direction); v, ventral longitudinal muscles = flexors. In B, the row of segments is stretched; in A, by the contraction of the muscles (d) bent upward; in C, downward; tg, tergal; sg, sternal interarticular membranes.—After Lang.
While we look upon the dermal tube of worms as a single but flexible lever, the body of the arthropods, as Graber states, is a linear system of stiff levers. We have here a series of stiff, solid rings, or hooks, united by the intersegmental membrane into a whole. When the muscles, extending from one ring to the next behind contract, and so on through the entire series, the rings approximate each other.