CHAPTER III 

 THE LIGAMENT 



Page 



Appearance and structure 48 



Chemical composition 56 



Elastic properties 59 



Bibliograpliy R3 



APPEARANCE AND STRUCTURE 



The significance of the ligament in the pliylDgeny 

 and classification of bivalves was a favored 

 subject in inalacological studies of the past 

 centuiy. Lengthy theoretical speculations about 

 this structure are fcunul in the papers of Bower- 

 bank (1844), Jackson (1890, 1891), TuUberg 

 (1881), Ball (1889, 1895), Reis (1902), Bieder- 

 mann (1902), Stempell (1900), and others. A 

 review of the literature from the earlier years to 

 1929 is adequately presented by Haas (1935). 

 These investigations give little information, how- 

 ever, concerning the microscopic structure, origin, 

 chemical composition, and functif)n of the liga- 

 ment. The latter subjects receive attention in 

 the more recent works of Mitchell (1935) on tlie 

 ligament of Cardium corbi.s, in a series of detailed 

 studies by Trueman (1942, 1949, 1950a, 1950b, 

 1951, 1952, 1953a, 1953b) on the hgaments of Myfilus, 

 Pecfen, Nucula, Osfrea edulis, Tcllina fcuuis, and 

 the Semelidae, and in the paper of Owen, 

 Trueman, and Yonge (1953) on the ligament 

 in the bivalves. 



The ligament of the Atlantic oyster is a narrow 

 band of dark, elastic material situated along the 

 edge of the hinge between the two valves. The 

 ligament does not extend deep into the shell, is 

 not visible from the outside, and is called internal 

 or ligamentum internum by Haas (1935) and 

 "alvincular" by Ball (1889). The latter term is 

 no longer used in nialacological literature. 



The ligament performs a purely mechanical 

 function. Its elastic material, compressed when 

 the contractif)n of the adductor muscle closes the 

 valves, expands and pushes the valves apart when 

 the tension of (ho adductor is released. Tlie 

 extent to which tlie valves may gape de[)ends 

 largely on the shape and size of the beaks. In 



48 



the specimen shown in figure 17 the large, tri- 

 angular space beyond the hinge permits wide 

 excursions of the valves and their gaping may 

 consequently be very broad. 



On the other hand, the narrow and crooked 

 beaks shown in figure 53 greatly restrict the 

 movement of the valves along the pivotal axis 

 regardless of the degree of relaxation of the nmscle. 

 Small pebbles, pieces of lu-oken shell, and other 

 foreign particles often found lodged between the 

 beaks may further limit the opening (jf the valves. 

 The possibility that such purely mechanical 

 ol)structions can impede the movement of the 

 valves should be kept in mind in evaluating the 

 results of physiological tests in which the degree 

 of shell opening is recorded. 



The youngest part of the ligament is that which 

 touches the inside of the valves; the oldest 

 portion, which is usually dried, cracked, and 

 nonfunctional, faces tiic outside. When the 



Centimeters 



I'^iGURE 53. — Longitudinal section through tlic beak and 

 hgament of C. virginica. l.v. — loft valve; r.v. — right 

 valve; Ig.c.p. — functional, compressible part of the 

 ligament; Ig.n.f. — nonfunctional, old part of the liga- 

 ment. 



FISHERY bulletin: VOLUME 6 4, CHAPTER III 



