valves are forcibly separated, the ligament breaks 

 approximately along the pivotal axis of the shell 

 (fig. 54, piv. ax.) and the two parts remain 

 attached to the respective valves. 



The three parts of the ligament at the edge of 

 the valves differ in color, size, and shape. The 

 usually brownish central (inner) part called 

 resilium forms a bulging ridge marked by fine 

 striations visible to the naked eye or under a low 

 magnification. The resilium is attached to a 

 groove called resilifer or chondrophore (figs. 

 54, 2, 16). The dark olive anterior and posterior 

 portions of the ligament called by Olsson (1961) 

 tensilia are attached to the edges of the valves 

 (nymphae). 



The resilium consists of tightly packed lamellae 

 arranged at about right angles to the longitudinal 

 axis of the ligament; they can be seen on the ex- 

 posed surface of the central part. These lamellae 

 are intersected by fine striations visible on the side 

 of the resilium after the removal of the adjacent 

 lateral part (fig. 55). 



When, the valves are closed the resilium is com- 

 pressed because of its considerable thickness while 

 both lateral parts (tensilia) are slightly stretched. 

 It can be seen in a series of cross sections of the 

 hinge nuide at right angles to its pivotal axis (fig. 

 56) that the curved lines of the compressed resilium 

 (2) are deeply arched, while those in the lateral 

 parts arc almost straight. This observation 

 agrees with the description of the operation of the 

 ligament of 0. edulis by Trueman (1951). Since 

 the beaks of the oyster illustrated are asynunetri- 

 cal, the distance between the two valves is greater 

 at the anterior than at the posterior end (fig. 56, 

 3, and 1) and, conseciuently, the anterior portion 



-PIV, AX. 



Cent I meters 



Figure 54. — Ligament of large C. virginica attached to 

 the right valve. View from the inside, piv. ax. — 

 pivotal axis. The resilium occupies the central position 

 and on both sides is flanked by tensilium. Slightly 

 magnified. 







Millimeters 



Figure .5.5. — Central portion of the ligament (resilium) 

 attached to the groove (bottom) of a valve. Note the 

 lamellar structure and fine striations visible on the right 

 side of the figure. C. virginica. 



of the ugament stretches more than its posterior 

 part . 



The ligament effectively seals the space between 

 the dorsal edges of the valves and forms an elastic, 

 watertight joint that prevents the entry of water 

 and organisms which otherwise could easily invade 

 the mantle cavity. 



The spring-like action of the ligament is a func- 

 tion (if the elasticity of its component parts. 

 Examination of transverse and longitudinal sec- 

 tions of fresh ligament under low power discloses 

 its amazingly complex structure. A cross section 

 made witii a razor blade at a right angle to the 

 pivotal axis of the valves shows a series of well- 

 defined curved lines extending from tiie right to the 

 left valve, and a complex system of lamellae ar- 

 ranged perpendicularly to the curves. Both sys- 

 tems are clearly seen in unstained preparations 

 mounted in glycerin jelly or in balsam (fig. 57). 

 The pivotal axis of the ligament lies in the center 

 of the drawing, perpendicular to the plane of the 

 paper; tlie valves (not shown in the figure) are on 

 the right and left sides, and the newly deposited 

 portion of the ligament lies at the bottom of the 

 drawing. The most conspicuous arches extend 

 almost without interruption from one side to an- 

 other; the lighter ones can be traced only for short 

 distances over the cross-sectional area. The struc- 

 tiu-e of the resilimn resembles a leaf plate of an 

 old-fashioned automobile spring, suggesting that 

 the arches are the lines of stresses corresponding 

 to the deformation of the ligament under com- 

 pression. Within the mass of the ligainental ma- 



THE LIGAMENT 



49 



