MECHANICAL ORIGIN OF THE SEGMENTS 33 



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 

 (c) 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 (c), and runs through the 

 smaller segments, being inserted in the dorsal portion of the crust 

 (f) 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, 6')." 



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 indura- 

 tion 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 con- 

 tractions, and its presence in animals prior to the induration of the 

 integuments in the order of phylogeny, furnishes the conditions re- 

 quired." (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 con- 

 ceive that the worms probably arose from a creeping gastrula-like 

 form, the gastraea. The act of creeping gradually induced an elon- 

 gated shape of the body. The movement of such an organism in a 



