MECHANICAL ORIGIN OF THE SEGMENTS 



31 



me" 



from the worms, being a disposition of the soft parts which is 

 characteristic of the vermian type. This segmentation of the integu- 

 ment 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 reproduc- 

 tive 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 meta- 

 merism is indicated very early in embryonic life 

 by the mesoblastic somites. 



In the un jointed 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 



FIG. 16. Dia- 



cuticle, divided into segments ; hence the worm can ? of the anterior 



... . part of an insect, 



move in all required directions, but only by sections, showing the membra- 



. nous intersegmental 



as seen in Fig. 16, which represents the thickened folds, g. After Gra- 



bcr. 



integument divided into segments, and folded inward 

 between each segment, this thin portion of the skin being the inter- 

 segmental fold. Each segment corresponds to a special zone of the 



subdivided muscular tube 

 (m), the fascia extending 

 longitudinally. The figure 

 shows the mode of attach- 

 ment of the fascia of the 

 muscle-tube to the seg- 

 ment. The anterior edge 

 is inserted on the stiff, un- 

 yielding, inner surface of 

 each segment : the hinder 

 edge of the muscle is at- 

 tached to the thin, flex- 

 ible, intersegmental fold, 

 which thus acts as a ten- 

 don on which the muscle 

 can exert its force. (Gra- 

 ber.) 



" Fig. 17 makes this still clearer. The muscles (m) extend between 

 two segments immediately succeeding each other. Supposing the 

 anterior oue (^1) to be stationary, what do we then see when the 

 muscle contracts ? Does it also become shorter ? The interseg- 



Fio. 17. Diagram of the internment and arrangement 

 of the segments! muscles: A, relaxed; m, muscle; g, 

 membranous articulation ; r, chitinous ring. B, the same 

 contracted on both sides. C, on one side. After Graber. 



