(translucent) fibers are thinner and have a peculiar 

 striation which has been described as oblique, 

 double-oblique, helicoidal, and spiral. Some in- 

 vestigators (Kellogg, 1S92; Orton, 19.35; Hopkins, 

 1936) and authors ot biology textbooks (Borradaile 

 and Potts, 1961) refer to the translucent part as 

 consisting of striated muscles. 



Both types of fibers appear under the light 

 microscope as k)ng cylindrical cells, slightly thick- 

 ened in the middle and tapering toward the ends. 

 An oval-shaped nucleus with one or several 

 nucleoli is near tlie surface, outside the contracting 

 elements which make up the bulk of the cell. 

 Clear homogenous cytoplasm (sarcoplasm) which 

 can be seen under high magnification forms a very 

 thin surface layer of the cell and around the 

 nucleus. The major part of the cell is made of 

 slender fibrils that differ in their orientation in the 

 two types of muscle cells. 



Tlie principal structural elements appear in 

 unstained, isolated fibers examined with phase 

 contrast oil immersion lens under high magnifica- 

 tion. Whole mounts can be made after pieces of 

 muscle are macerated in 20 percent nitric acid 

 and then placed in glycerol. Treatment witli 

 nitric acid apparently does not affect the visible 

 structure of the fibers. Preparations should be 

 made from fibers which have been taken from 

 both a fully relaxed and a completely contracted 

 adductor. The desirable state of relaxation is 

 obtained by narcotizing the oyster in 5 to 10 

 percent magnesium sulfate solution for 48 hours; 

 treating the mantle with a strong solution of 

 hydrocliloric acid causes long-lasting contraction. 

 In opening the oyster, care sliould be exeicised 

 not to damage the visceral ganglion, since injury 

 to this nerve center may cause relaxation of the 

 adductor. 



WHITE MUSCLE FIBERS 



White muscle fibers isolated from a completely 

 relaxed adductor of a full}' narcotized ('. rir<ivuca 

 are from 2 to 3 mm. long and about 10 ;u in diam- 

 eter. Tiie fibers are too short to stretch from one 

 valve to the other and, with the exception of 

 those attached to the shell, end in connective 

 tissue. Occasionally tliey bifurcate but do not 

 anastomose. The body of the fiber consists of 

 many fibrils of variable length and a diameter of 

 only a fraction of a micron. The fibrils are ori- 

 ented parallel to the long axis of the cell and those 

 close to the surface appear to be darker. The 

 arrangement of the fibrils changes somewhat, de- 



pending on the state of contraction. Figure 142, 

 A-D, represents four camera lucitla drawings made 

 of a white auiscle fiber; (A) the fiber is in a com- 

 pletely relaxed state, (B) it is strongly contracted, 

 (C) it is partially contracted, and (D) a noncon- 

 tracted fiber is folded by the contraction of the 

 surroimding fibers. All drawings were made from 

 glycerin-mounted preparations examined with 

 phase contrast lens. The difference between the 

 relaxed and contracted fiber is primarily in the 

 thickness of the fiber, which in B is about three 

 times greater than in A. In both cases the orienta- 

 tion of fibrils is the same. In a partially con- 

 tracted and slightly twisted fiber, C, some of the 

 fibrils are at an angle to the long axis of tlie cell 

 while others retain their original orientation. The 

 fiber D, found in the same preparation with C, is 

 folded but not contracted. Its surface layer of 

 transparent cytoplasm was wider than in the 

 others and the fibrils followed the zig-zag outlines 

 of the fiber. Although the sample was isolated 

 from a contracted adductor, only a few fibers 

 were found in highly contracted state B. The 

 fiber A was separated from a completely relaxed 

 nuiscle. 



DARK MUSCLE FIBERS 



The fibers vi the dark (translucent) part of tlie 

 adductor are from 1 to 2 nun. long and in a relaxed 

 stale are about .5^ in diameter. When isolated in 

 teased preparations, the fibers have a tendency to 

 twist and coil. The connective tissue around them 

 is less tenacious than in the white muscle, and the 

 fibers can be separated easily by fine needles. As 

 early as 1869 Schwalbe showed that the fast ad- 

 ductor nmscle of Ostna is composed of fibers whicii 

 exhibit a clearly defined diamond lattice pattern. 

 Marceau (1909) maintained that double obliquely 

 striated muscles are widely distril)Uted in the fast 

 parts of the shell closing muscles of bivalves, and 

 Anthony (1918) advanced a theory tliat oblique 

 striations are a stage in the evolutionary develop- 

 ment of transverse striation. The fact that true 

 cro.'is striation occurs in the muscles of Pedeii, 

 Lima, Teredo, Spondylu.s, and other bival\-es leads 

 to a widely accepted belief that the dark portion 

 of the adductor muscle, also described by some 

 autliors as yellow, grey, or tinted (Kawaguti and 

 Ikemoto, 1959), consists of cross striated fibers and 

 tliat quick mo\einents of these aninuils are brouglit 

 about by their contraction. 



From their study of the translucent fibers of the 

 adductor of C. angidata, Hanson and Lowy (1961) 



154 



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