ing to their interpretation of the electron micro- 

 graphs which accompany their paper, both types 

 of fihiments are rehitively short in comparison 

 with the length of the fiber; they lie parallel to the 

 fiber axis and are grouped with separate arrays 

 which alternate with each other and appear to be 

 cross-linked by means of transverse projections 

 which belong to the thick filaments. The exist- 

 ence of the projections does not seem to be firmly 

 established, and their connections with the two 

 types of filaments require corroboration. It is 

 obvious from the electron micrographs published 

 by Philpott, Kahlbrock, and Szent-Gyorgyi, 

 (1960) that filaments are randomly distributed 

 throughout the cross-sectioned area of the fibril. 



In tfie relaxed state the muscle cells are stretched 

 and on longitudinal sections of either part of the 

 adductor appear to be arranged in parallel lines 

 separated in places by connective tissue (fig. 145). 



A contracted adductor muscle is strikingly 

 different in appearance from one which is relaxed. 

 Most of the muscle fibers are folded and the 

 entire organ has a herringbone appearance (fig. 

 \AQ). The uniform thickness of the folded fibers 

 indicates that their actual length is not shortened 

 by the contraction; the fibers are compressed to 

 occupy a shorter distance between the valves. 

 Folding implies the existence of a force that acts 

 parallel to the longitudinal axis of the fibers. 

 The question arises as to tlie nature of the force 

 that produces this effect. 1 n an attempt to answer 



200 



Microns 



Figure 145. — Longitudinal section tlirougli a completoly 

 relaxod translucent part of the adductor muscle. Bouin, 

 hematoxvlin-eosin. 



Microns 



Figure 146. — Longitudinal section of the white jiart of the 

 adductor muscle of C. virginica preserved in Bouin with 

 formalin solution. Muscle is in a highly contracted 

 state. Hematoxylin-eosin. Camera lucida drawing. 



this question I examined a series of sections of 

 muscles preserved at various degrees of contrac- 

 tion. Oysters were stimulated to close their 

 valves and were preserved in that state by using 

 a strong and rapidly acting fixative applied through 

 an opening cut in a portion of the shell. In such 

 preparations contracted muscle fibers were found 

 only in the area near the attachment to the valves. 

 In the two photomicrogi'aphs (fig. 147) the con- 

 tracted fibers, nearest to the valve (left side), 

 are short, thick, and deeply stained with eosin. 

 The fibers to the right in the same preparation 

 are narrow and fcilded. 



In a partially closed oyster the contracted fibers 

 may be scattered between the folded fibers 

 throughout the entire cross-sectional area. This, 

 condition, shown in figure 148, is drawn from 

 preparations preserved in osmic acid and stained 

 witii iron hematoxylin. The contracted fibers 

 appear as isolated dark bodies scattered through- 

 out the moderately folded fibers. It may be 

 deduced from the histological picture that only a 

 small number of muscle fibers are in a contracted 

 state. In order to explain the folding of the 

 noncontracted portion of the adductor it is neces- 

 sary to assume tliat a rigidity develops in the 

 contracted fibers in two places— near their contact 



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