types is due to the differences in time scale and 

 the condition of stimulation. It was claimed, 

 (Ritchie, 192S, p. 86), althoug:h not proved, that 

 tonus of the adductor muscle is maintained by 

 tetanic contraction. Another view (Winton, 

 19.30), which is more in harmony with the bio- 

 chemical data, explained the locking mechanism 

 as a result of physical changes during contraction, 

 particularly the alteration in viscosity of muscle 

 proteins. Experiments with byssus retractor of 

 Mytilus showed that after stimulation by dii-ect 

 current the viscosity of the muscle was raised 

 and remained high for about 2 hours. Xo such 

 effect was obtained if alternating current was 

 used. These observations suggest that viscosity 

 changes are involved in the contractions of the 

 adductor of bivalves. 



The difference between the white and the 

 translucent parts of the adductor muscle may be 

 primarily of a quantitative character. This 

 suggestion was made by Shukow (1936), who 

 found that in Anodonta and Unio the two parts 

 of locking muscles actively participate in single, 

 spontaneous contractions and in the maintenance 

 of tonus. Shukow's observations indicate the 

 inadequacy of the theory that makes the main- 

 tenance of the tonus the exclusive function of 

 tlie white fibers. 



Studies of the electric phenomena in the smooth 

 adductor muscles of lamellibranchs {Alytilus, AIo- 

 (lioliis, and smooth part of C'hiamys) lead Lowy 

 1953, 1955) to conclude that the hypothesis of 

 "catch mechanism" is unnecessary because, ac- 

 cording to his observations, the tonus in tlie intact 

 muscles of these mollusks is due to a shifting 

 pattern of tetanic stimuli controlled by the 

 nervous system, bringing it in line with the tonus 

 in otlier muscles. Since action potentials were 

 observed in muscles which were isolated from the 

 ganglia, Lowy suggested that they may be of 

 myogenic natiu^e. The question of whether the 

 tonic activity of lamellibranch muscles is neuro- 

 genic or m3'ogenic remains open. Lowy makes an 

 interesting statement that "lamellibrancli muscles 

 maintain a certain level of tension all the time 

 due to the activity of a peripheral automatic 

 system, which works by successive activation of 

 limited areas." ^ This conforms with the histo- 

 logical observations described above which show 

 that in an intact adductor muscle of the 

 oyster preserved in a contracted state only certain 



' — Underscoring is mine. P. .S. G. 



muscle bands are in a true contracted state whOe 

 others are folded. Lowy concludes that further 

 studies are needed before it is decided whether 

 lamellibranch muscles are directly innervated by 

 excitatory and inhibitory nerves or are acted on 

 indirectly via a peripheral ganglionic plexus. The 

 existence of inhibitory axons in Fecten was demon- 

 strated by Benson, Hays, and Lewis, (1942), who 

 found that the relaxation of the adductor of the 

 scallop was considerably accelerated by stimu- 

 lating certain nerve bands going to the msucle. 

 This is in accord with the evidence presented by 

 Barnes (1955) for tlie adductor muscles of Ano- 

 donta. His work implies that the adductor of 

 Anodonta is innervated by three types of nerves: 

 one group of motor fibers supplies the striated 

 muscles and produces phasic contractions which 

 may summate and produce tetanus; another group 

 of activating fibers supplies the unstriated nmscles 

 and produces increased tonus; the third group 

 consists of inliibitory fibers which decrease the 

 tonus. Barnes points out that the nervous mech- 

 anism controlling the adductor activity in Mytilus 

 may be the same as in Anodonta. Mytilus is 

 capable of both phasic and tonic contractions, 

 but there is no obvious differentiation of the 

 muscle into two parts. It nmst be accepted, 

 therefore, either that all nmscle fibers are capable 

 of exhibiting both types of contraction or that the 

 two types of fibers are present but completely 

 interspersed. 



Electrical activities associated with the contrac- 

 tion of the adductor muscle of the oyster have 

 not been studied enough to warrant an evaluation 

 of their role in the locking mechanism of these 

 mollusks. An attempt to solve the paradox of 

 the catch muscle mechanism was made recently by 

 Johnson, Kahn, and Szent-Gydrgyi (1959) and is 

 based on the study of the property of paramyosin. 

 The solubility of this protein was found to be 

 critically dependent upon the pH and ionic 

 strength of the medium. Similar dependence was 

 shown in the glycerinated fibers of the anterior 

 byssus retractor of AI. edulw. The fibers were 

 stretched, and the tension thus developed was 

 measm-ed. To reduce the efl'ect of actomyosin, 

 10"" M Salygi-an and 10"^ m pyrophosphate were 

 added to the medium. Stiffness of the fibers was 

 measured at various values of pH. Below pH 

 6.5 and at low ionic strength of 0.07 m potassium 

 chloride the fibers were relatively stiff. This is 

 a range in which paramyosin crystallizes out of 



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