29 



uui.f/jHiis can resist a considerable pull for a shorl time, 

 4,000 grams are not sufHcient to jnill the valves apart 

 unless acting- for some time, when, as is the case with 

 other lamellibranchs, a much less weight siifhces to open 

 them, in fact, as has been shown, starfishes are able to 

 open oysters by a sustained pull. Corresponding to these 

 differences in appearance and function there are 

 differences in the histology of the two regions. The fibres 

 of the large, rapidly contracting part, when seen in 

 sections, show a very obvious striation, the smaller portion 

 of the muscle consists of smooth fibres. This relation 

 between the cross striation of muscle fibres and rapidity of 

 movement is of general occurrence (13 & 15). The 

 striated fibres are very much flattened so that they can be 

 seen either in surface or in edge view (fig. 30, b. and a.). 



If small portions are fixed in osmic or Flemming and 

 stained with iron haematoxylin it is quite obvious that the 

 striping consists of distinct transverse bands ; there is no 

 possibility of its being only an appearance due to fibril lae 

 being thrown into spirals wlien in a contracted state. 



The dark bands are three or four times as long as the 

 light, almost unstained, portions. Moreover, the fibres 

 have a series of constrictions which correspond in position 

 with the light stripe ; this can be seen extremely well if 

 a fibre is observed in edge view, so that the dark portions 

 correspond to the swellings and the light stripes to 

 constrictions. 



The difference in intensity of the stain taken up by 

 the two parts, however, is so great that it would be 

 difficult to affirm that the dark stripes are due to a greater 

 thickness of stained protoplasm, though it is possible that 

 this may be the case (see 14). 



The nuclei of the fibres are not frequent in occurrence, 

 and are pushed rather to one side of the fibre and elongated. 



