272 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



the same manner, since the manner of mutual relation 

 of the segments is the same as in the case of the body- 

 segments. 



" It has been stated that in the leech the folds do not 

 correspond in number to the somites of the body, while 

 they do in the Crustacea. All annelids do not move 

 by means of a muscular system built upon the plan 

 found in the leech. In many the circular layer has to 

 a large extent disappeared, for the longitudino-circular 

 plan is undoubtedly ante-annelidan. The movement 

 of the free medusoid forms, and of the Ctenophora, is 

 the result of a modified arrangement of this plan. 



"With the disappearance of the circular layer, we 

 find a peculiar modification of the longitudinal layer. 

 This layer becomes broken up and the fibers act in 

 moving the setae, which answer to limbs. In a seg- 

 ment of a setiferous annelid, we may observe that the 

 longitudinal muscles of the somite in section at the po- 

 sition of the seta are arranged like the letter 'V in 

 the fork of which the seta lies, the fibers to the left 

 (anterior) pull the seta externally backward, those on 

 the right (posterior) pull the seta forward. The in- 

 troduction of the setae, the origin of which I do not 

 here attempt to explain, has no doubt been, together 

 with the establishment of the external segmentation, a 

 strong factor in causing the breaking up of the muscu- 

 lar tube into sections (myotomes),' which by use and 

 consequent increase have extended each arm of the 

 'V into the segment on each side, while the insertion 

 of the end of the seta has caused a break in the muscle 

 by the formation of an aponurosis. This gives us the 

 peculiar disposition of a myotome to extend across the 

 union of two somites. 



" If we examine the segments of the so-called ab- 



