hours. To avoid desiccation the oyster in this 

 experiment was surrounded by a small moist 

 chamber. With the increase in weight the time 

 of complete loss of tonus rapidly decreases 

 (fig. 161). 



Muscles which were kept for several hours under 

 a pulling force of about 1.7 kg./cm.^ of cross- 

 sectional area suffered a temporary injury which 

 resulted in abnornuxl shell movements after the 

 return of the oysters to sea water (fig. 162). The 

 two tracings reproduced in this figure are almost 

 identical, although in the case of oyster A a pulling 

 force of 6 kg. was used while S kg. were applied to 

 oyster B. In both instances the pulling force per 

 unit of muscle scar area was the same, 1,676 in A 

 and 1,675 g./cm.' in B. After a few days in 

 running sea water both oysters completely re- 

 covered and their shell movements became normal. 



In oysters kept in sea water the relationship 

 between the weight applied to the valves and the 

 time needed to attain tonus loss is less regular 

 and individual differences are mucli greater tluui 

 for oysters left in the air. Witli a pulling force of 



Figure 162. — Shell movements of C. virginica in sea water 

 after the removal of weight of 6 kg. or 1,676 g./cm.^ of 

 muscle area (upper line) and 8 kg. or 1,675 g./cm.^ of 

 muscle area (lower line) applied to the valves. Weights 

 were removed after complete loss of tonus was attained 

 in 43.52 and 52 hours exposure in air at temperature 

 23° to 24° C. Water temperature 13.5° C. Time 

 interval: A and B, 0.5 hour each. 



about 1.5 kg./cm.^ of muscle area some of the 

 oysters showed tonus loss in less than half an hour 

 wliile others remained closed for many liours. 

 The relationship between the increasing pulhng 

 force and the time required to devek)p loss of 

 tonus is shown in figure 163. 



Changes in the character of sliell movements of 



\'o rs 2 



K I LOG R6MS / CM^ 



Figure 161. — Time in hours reriuired to obtain loss of 

 tonus of the intact adductor muscle kept under constant 

 pull in kg. /cm. 2 of the cross-sectional area of the adductor. 

 Experiments with C. virginica kept in air at tempera- 

 tures between 18° and 24° C. 



10 15 2,0 



KILOGRAM S / CM^ 



Figure 163. — Time in hours required to obtain complete 

 loss of tonus in the adductor muscle of C. virginica kept 

 in water under a constant pulling force expressed in 

 kg. /cm. 2 of the cross-sectional area of the adchictor 

 mu.scle. Temperature 13.9° to 18.0° C. At the pulUng 

 force of 0.59 kg. complete tonus loss was obtained in 274 

 hours (11.5 days). 



178 



FISH AND WILDLIFE SERVICE 



