observed that the white part of tlie adductor con- 

 tracts very sh>wly and can remain in a contracted 

 state for a hmg time. Marceau (1904a, 1904b) 

 confu-med these results by a series of experiments. 

 He cut off either white or translucent portions and 

 found that in 0. ednlis the rapid closing of the 

 valves is accomplished by the contraction of the 

 translucent part of the muscle wliile the elasticity 

 and tonus of the white part counteract the pullino- 

 force of the ligament. Useful reviews of many in- 

 vestigations dealing with the muscle physiology 

 of bivalves and other invertebrates are found in 

 the papers of Ritchie (1928), Jordan (19.38), 

 Evans (1926), and others. 



It is a well-established fact that the two parts of 

 the adductor muscle contract at different speeds. 

 In scallops the isolated striated (translucent) por- 

 tion contracts in about 100 microseconds in sec); 

 its rela.xation time is about 0.1 second (sec.) 

 (Bayliss, Boyland, and Ritchie, 1930). In the 

 slow part of the adductor the contraction time 

 varies from 500 m sec. to 2.5 sec. and the relaxation 

 time is from 10 to 45 sec. The contraction of the 

 adductor muscle of oysters is always several times 

 faster than its relaxation, the ratio var^ving 

 according to the type of muscular reaction. Mar- 

 ceau (1909) published a number of tracings of the 

 spontaneous movements of the valves of 0. edulis 

 in which only the white (slow) part of the muscle 

 was left. The time of relaxation was from 15 

 minutes to 1 hour. 



In many bivalves the adductor muscle can re- 

 main contracted, keeping the valves closed 

 tightly, for a long time. This behavior varies, 

 however, in different species. For instance, com- 

 mon scallops of the American and Eiu-opean 

 coastal waters, Astro-peden inadians and Chlamys 

 opercularis, close their valves for only a short 

 time. Soon after being taken out of water the_y 

 gape, lose shell licjuor, and perish. My observa- 

 tions on pearl oysters of the Hawaiian Islands 

 and Panama {Pinctada (jaltmff, P. mazatlanica) , 

 .show that shortly after being taken out of water 

 their shells gape and the muscle fails to contract. 

 These species cannot be transported over long 

 distances unless they are kept in frequently re- 

 newed water all the time. On the other hand, the 

 bivalves in which the adductor muscle remains 

 contracted for a long time can survive long ex- 

 posure and can be shipped alive over great 

 distances. 



Oysters living within the tidal range on flats 

 thrive in this situation because they can keep 

 their valves closed during the time of exposure. 

 It is obvious that this ability provides a great 

 survival value for those sedentary animals that 

 can withdraw within their heavy shells to avoid 

 desiccation and remain protected against un- 

 favorable conditions or attacks of predators. 



The abOity of bivalve muscles to keep the shells 

 closed is frequentl}' called a "catch" or locking 

 mechanism. The idea originated from observa- 

 tions made by Uexkiill (1912) on the scallop; if a 

 piece of wood is pushed between the valves the 

 adductor contracts with such force that the edges 

 of the shells may be splintered. The wooden 

 wedge is held as firmly as if it were in a vise and 

 can be removed only by twisting and pulling. 

 The valves, however, remain motionless, and the 

 muscle that holds them in their position shows 

 no elasticity. The muscular fibers seem to be 

 frozen solid. The shell cannot be opened, but if 

 the valves are pressed on both sides they may be 

 brought nearer together and remain fixed in their 

 new position. This ability Uexkiill called "Sper- 

 rung", which in English means "locking." Bay- 

 liss (1924) interpreted Uexkiill's expression using 

 the word "catch," probably influenced by Griitz- 

 ner's (1904) suggestion that the muscle fibers of 

 the bivalve adductor must somehow be "hooked 

 up" by a mechanical arrangement similar to a 

 ratchet consisting of two pieces with teeth facing 

 each other. In his proposal the upper piece 

 could be pushed only in one direction, shortening 

 the total length of the model, and the upper 

 teeth could not move back unless the two pieces 

 were separated from each other by the depth of 

 the teeth. There is nothing in the struct ui-e of the 

 muscle fiber which even remotely suggests the 

 existence of such a mechanism. The expression 

 "catch mechanism" imphes some mechanical de- 

 vice and is, therefore, misleading. It has been 

 used, however, for such a long time that the 

 literary meaning of the words has been lost and 

 the term simply refers to the continuous state of 

 contraction of the closing muscle of bivalves. 



Several theories have been proposed to explain 

 the locking or catch mechanism of the adductor 

 muscles. Some investigators assumed that the 

 muscle twitch (i.e., the contraction in response 

 to single brief stimulus) is common to all muscles 

 and the difference between the behavior of the 

 adductors of bivalves and of the muscles of other 



THE ADDUCTOR MUSCLE 



165 



