182 THE MORPHOLOGY OF THE COMMON CRAYFISH. 



lighter striae, transversely to the axis of the fibres 

 (fig. 52 A). The distance of the transverse striae from one 

 another varies with the condition of the muscle, from 

 l-4,000th of an inch in the quiescent state to as little as 

 1-30, 000th of an inch in that of extreme contraction. 

 The more delicate muscular fibres, like those of the 

 heart and those of the intestine, are imbedded in the 

 connective tissue of the organ, but have no special sheaths. 



Fig. 53. — Astacus flmiatUis. — A, living muscular fibres very highly 

 magnified ; B, a fibrilla treated with solution of sodium chloride ; 

 C, a fibrilla treated with strong nitric acid, s, septal lines ; sz, 

 septal zones ; is, interseptal zones ; a, transversa line in the inter- 

 septal zone. 



The fibres which make up the more conspicuous muscles 

 of the trunk and limbs, on the other hand, are much 

 larger, and are invested by a thin, transparent, structure- 

 less sheath, which is termed the sarcolemma. Nuclei 

 are scattered, at intervals, through the striated substance 

 of the muscle ; and, in the larger muscular fibres, a layer 

 of nucleated protoplasm lies between the sarcolemma and 

 the striated muscle substance. 



