MECHANICAL ORIGIN OF THE SEGMENTS 



31 



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 nietameric 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 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 FJQ lg _ 

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



_ . part of an insect, 



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



A . nous intersegmeiital 



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

 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- 



FIG. 17. Diagram of the integument and arrangement don Oil which the muscle 

 of the segmented muscles: A, relaxed; m, muscle; g, . 



membranous articulation ; r, chitinous ring. , the same can BXCl't its force. (Grl'a- 

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



ber.) 



" 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 interseg- 





