182 THE MORPHOLOGY OF THE COMMON CRAYFISH. 
lighter strie, transversely to the axis of the fibres 
(fig. 52 A). The distance of the transverse strize from one 
another varies with the condition of the muscle, from 
1-4,000th 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 fluviatilis.—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, transverse 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. 
