229 



ments on adjacent fibrils may be so arranged that a second helix of 

 fibrillae appears to exist, or, in other words a double oblique striation. 

 In this case the oblique stripes in one direction are due to the fibrillae 

 on the superficial face of the muscle fibre, and those crossing in the 

 other direction are due to the rows of elements of which the same 

 fibrillae are composed. This occurs in the adductor muscle of Anodonta 

 but in Pecten the striping is quite different, being regularly transverse 

 and not due to spirally coiled fibrillae. 



The striped muscle in the mantle resembles the striped muscle of 

 the adductor though the stripes are not so regular. The muscle ap- 

 pears to be made up of band-like fibres which are shown in Figs. 3 a 

 and 4. The fibres readily split however into columns (Fig. 3 b). These 

 columns which form the fibres, are made up of alternately dark and 



Fig. 3. Fig. 4. Fig. 5. 



light staining elements, the dark staining bands being about 3 times 

 as long as the clear stripes. 



The columns are also varicose, each dark band corresponding to 

 a thickening. The transverse striation is produced by several columns 

 lying in juxtaposition with their dark staining elements, all at the 

 same level. If however, as sometimes occurs, the dark elements of 

 adjacent columns are not at the same level, an irregular striping is 

 produced as in Figs. 4 and 5. There are on an average 17 dark bands 

 in a length of 30 ^a. 



It is interesting to note that the muscles which show such an 

 obvious striation are engaged in rapid movements, which are also 

 organised and related to the closing and opening movements of the 

 shell in swimming, another case therefore of the connection between 

 striation and rapid contraction and relaxation. 



The following methods have given good preparations: 1. Fixation 

 in a mixture of equal parts of Hermann's fluid and a saturated 

 aqueous solution of corrosive sublimate. Stain with Heidenhain's iron 

 haematoxylin. 2. Fixation in Zenker's fluid and stain after section 

 cutting (paraffin) in the following manner. I. Twenty four hours in 

 37o solution of potassium bichromate until the sections are yellow. 



