Examination of relaxed fibers of C. virginica 

 with phase contrast lenses shows the existence of a 

 distinct diamond lattice pattern shown in figm-e 

 143. In the relaxed dark fibers this doul)le oblique 

 striation is absent and the fibrils are oriented 

 parallel to the axis of the cell. My observations 

 confirm the description made by Hanson and 

 Lowy (1957), who found that in helical configura- 

 tion of myofibrils of the "yellow" part of the ad- 

 ductors of oysters and Ensis ensis the angles be- 

 tween the lielix and the axis of the fiber increased 

 as the muscle relaxed. The so-called diamond 

 lattice pattern of striation is not a permanent 

 feature of the translucent fiber. It becomes visible 

 in a contracted muscle and is usually confined to 

 the cut ends of the fiber. This observation made 

 by Bowden (1958) for Ostrea edulis and C. angulata 

 is in accordance with my observations on C 

 virginica. 



Considerable advance in the understanding of 

 fine structure of bivalve muscle cells was made l)y 

 Philpott, Kahlbrock, and Szent-Gyorgyi (1960), 

 in the work on C. virginica, Mya arenaria, Mer- 



cenaria mercenaria, and Spixiila solidissima. Simi- 

 lar studies of C. angulata were nuide by Hanson 

 and Lowy (1961). 



With respect to the ultrastructure of the fibers 

 of the adductor muscles of these species, the re- 

 sults of the two investigations are in agreement 

 although they present dift'erent theories of the so- 

 called catch mechanism of the adductor, which is 

 discussed later. In both parts of the muscle the 

 fibrils consist of two types of filamentous struc- 

 tures that can be clearly seen on the electron 

 micrograph of the transverse section of the fibril 

 (fig. 144). The thick filaments fonn the largest 

 part of the fibril; their diameter varies from 250 

 to 1,500 A. The thin filaments which occupy the 

 space around the thick ones are about 50 A. in 

 diameter. The thick filaments have the 145 A. 

 periodicity associated with paramyosin. The 

 authors surmise that actomyosin is localized in 

 the thin filaments. Hanson and Lowy (1961), in 

 confirming the presence of two kinds of filaments 

 in the fibrils of C. angvl.ata, assume that the 

 thinner filaments contain mainly actin. Accord- 



'^k.*^^ 



Microns 



Figure 144. — Electron micrograph of a small portion of a muscle fiber of the translucent part of the adductor of C. 



virginica. Courtesy of Philpott and Szent-Gyorgyi. 



THE ADDUCTOR MUSCLE 

 7133-851 O — 64 11 



157 



