Centimeters 



Figure 56. — Three longitudinal sections through hinge and ligament of the shell of C. virginica. (1) posterior portion; 

 (2) central portion or resilium; (3) anterior portion, h. — beaks, Ig. — hganient, l.v. — left valve, r.v. — right valve. 

 Note the arched lines of the resilium (Ig.) in the central drawing (2). Slightly magnified. 



terial they are the visual evidence of these stresses. 

 Since the "springs" (if the resilium do not consist 

 of separate structure parts joined together into a 

 complex unit, the comparison is only superficial. 



The ligament is a nonliving structtu'e secreted 

 at a varying rate by the highly specialized epithel- 

 ium of the subligamental ridge of the mantle (see 

 p. 89). Structurally, the arches, visible at low 

 magnification, represent stages of growth; fimc- 

 tionally and in accentuated form, they reflect 

 compressional deformation in the operating 

 structure of the ligament. 



Under a binocular microscope the lamellae of 

 the resilium, when separated with fine needles, 

 appear slightly bent and zigzagged. A small 

 piece of the resilium cut in the dorsoventral 

 plane and magnified about 250 times (fig. 58) 

 can be seen to consist of fibrillar material and 

 of dark bands of variable width composed of 

 tightly packed, oval, birefringent globules. Pres- 

 sure over the cover slip does not change the shaj)e 

 of the globules, which appear to be firmly em- 

 bedded in the ground substance. The globules 

 contain no acid-soluble material since they are 

 not afl'ected by strong hydrochloric acitl, nor are 

 they soluble in alcoliol or .xylol. Preparations 

 mounted in balsam j)resent the same ajipearauce 

 as nondehydrated sections mounted in glycerin 

 jelly. Besides the globules concentrated in the 

 dark bands within a delicately fibrillar grounii 

 substance, some of them are arranged in longi- 

 tudinal lines at riglit angles to the dai-Jc bands. 

 Some of the horizontal bands (upper part of 



figure 58) are of mucli greater complexity than 

 the others; they consist of oval-shaped light areas 

 surrounded by globules. The two structural 

 elements, namely, the bands of fibrils and the 

 rows of globules, repeat themselves with regularity, 

 the successive layers varying oidy in width and 

 in the concentration and size of globules. The 

 fibrils intersect the arches either perpendicularly 

 or at about 45° (lower part of figure 58) and 

 probably exert additional elastic force under 

 compression. 



The anterior and posterior parts of the ligament, 

 the tensilium of Olsson (19G1) or outer layer of 

 Trueman (1951), are made of tenacious material 

 which withstands considerable stretching without 

 breaking. This can be easily ascertained by 

 trying to tease or to pull apart the dissected 

 parts of the tensilium. In this respect the material 

 of the tensilium differs from that of the resilium, 

 which is weak under tension but strong untier 

 compression. Tiie color of the tensilium dilfers 

 from that of the resilium. In New P^ngland 

 oysters it is usually dark green on tlie surface, 

 wliile the resilium is liglit brown. Tiie tensilium is 

 made of tough lamellae which in a transverse section 

 appear as slender, transparent cylinders of slightly 

 yellowisli sul)stance (fig. 59). Both r.'silium and 

 tensilia jire secreted by highly specialized epi- 

 tlielial cells whieh imderly the ligament. The 

 thickness of eacii kunella corresponds to the width 

 of a ruffle at the edge of the secreting epithelium 

 (See chajiter \, p. ,s9). At low magnification 

 the nniterial of the tensilium appears to be non- 



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FISH AND WILDLIFE SERVICE 



