Numerous minor contractions (fie;. 153, A-2) that 

 occur between the rejection reactions only slightly 

 reduce the opening between the valves and are 

 more difficult to interpret because they are not 

 accompanied by the discharge of any material. 

 Possibly they represent the fine adjustments made 

 by the oyster in maintaining a steady flow of water 

 through the gills. On the other hand, it is also 

 possible that they are responses to minor physical 

 disturbances such as vibrations of laboratory 

 floors and slight changes in illumination. None of 

 the existing laboratories in the United States 

 have the shockproof floors and walls that would 

 assure complete elimniation of the outside dis- 

 turbances caused by street traffic and footsteps 

 within the building. 



Type B 



Type B shell movement is characterized by the 

 increased frequency and well-pronounced perio- 

 dicity of contractions and corresponds to the state 

 of increased excitability (fig. 154). Curve B-1 

 was observed in oysters which were exposed to a 

 rapid rise of temperature from 13° to 25.6° C. 

 B-2 represents the behavior of oysters aftected by 



the metabolites accumulated in stagnant and 

 unaerated water. The uniform and rapid con- 

 traction shown in B-2 stopped mimediately when 

 the water was changed. 



Cm've B-3 represents a similar activity recorded 

 on a rapidly moving drum. The relaxation peri- 

 ods are much shorter, but the level of the muscle 

 tonus remains constant. Shell movements of this 

 type were frequently observed in oysters which 

 were left after spawning in water containing large 

 quantities of oyster eggs and sperm. Normal 

 movements of the type A-1 were resumed as soon 

 as the water was changed. 



TypeC 



The cm've of type C shell movements (fig. 155) 

 illustrates periods preceding or following changes 

 in the degi-ee of opening and closing of the valves. 

 Both periods are characterized by a series of minor 

 contractions and relaxations until the final tonus 

 level is reached. The type shown in C-2 (left 

 part of the curve) is a typical "staircase" or 

 "Treppe" reaction of the adductor muscle, which 

 contracts in several distinct stages. This reaction 

 is the response to an irritating substance added 



B-2^ 



B-3 



^'WA/V\N^VvAA^/VY^^^^^^^ 



Figure 154. — Shell movements of type B are typical for the state of increased excitability frequently caused by the 

 accumulation of metabolites in sea water or rapid rise of temperature. Vertical magnification in B-.3 is about one- 

 fourth of that ii B-1 and B-2; uppermost points correspond to closed shells. B-1 temperature increased from 1.3° C. 

 at the start to 25.6° C. at the end. B-2, B-3 increased muscular activity due to the accumulation of metabolites. 

 Time interval: B-1, 1 hour; B-2, 1 hour; B-3, 1 minute. 



THE ADDUCTOR MUSCLE 



171 



