270 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



fected by the contraction of the longitudinal layer. 

 The external surface of the medicinal leech, for exam- 

 ple, is thrown into a regular series of very fine folds, 

 extending across the longitudinal axis of the body. 

 These folds do not correspond in numbers to the so- 

 mites of the body, which are not indicated externally, 

 five, six, or more of them belonging to one somite. 



When the animal shortens its 

 length, these folds are deepened 

 and the segments thrown closely to- 

 gether ; when extension takes place, 

 the folds are flattened, spread open, 

 although not wholly disappearing, 

 as they are a fixed quantity, so to 

 speak. I believe these folds are 

 due to mechanical action ; by the 

 disposition of the different fibers of 

 the longitudinal series, in being in- 

 serted in a series of planes bounded 

 by the valleys between the folds, 

 this being aided by some of the 

 circular fibers which pass through 

 Fig. 63.-Diagrammatic the longitudinal shcath, and find 



representation of the s>eg- . 



ments of the leech, show- their attachment to the bases of the 



ing tlie folds, valleys, and valleVS 



muscular fibers. _ . , , . 



" Startmg from this point, and 

 supposing the regularity of the folds to have become 

 established from preexisting folds by the regularity and 

 stress of muscular action, we can conceive that when 

 deposits of calcareous matter took place, rings simi- 

 larly formed by a folding of a soft skin would receive 

 that deposit at the most prominent portion of this fold, 

 the convex face, and not in the protected valleys, as 

 there would be more friction or pressure from external 



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