16 



BULLETIN OF THE BUREAU OF FISHERIES 



At this temperature the rate of flow of water varies in the different oysters from 0.5 

 to 3.9 liters per hour; the majority of the oysters, however, filter the water at a rate 

 of from 2.5 to 2.9 liters per hour. With the cooling of the water comes a gradual 

 retardation, of the ciliary activity of the gills, and, as can be seen in Figure 4, the 

 peak of the frequency polygons moves toward the left, following the decrease of 

 temperature. At temperatures between 9° and 14.9° the range of individual 

 variations is smaller than it is at higher temperatures and varies from to 2.4 

 liters per hour; at temperatures between 9° and 11.9° the majority of oysters pro- 

 duce a current ranging from to 0.9 liter per hour. At a low temperature between 

 6° and 8.9° the activity of the gills is greatly reduced and the rate of flow is never 

 higher than 0.4 liter per hour. No current is produced below 5°. 



Table 4. — Frequency distribution of the rate of flow of water through the gills at a given temperature. 



The figures indicate the number of obs,ervations 



 '  '      "■■ -...ii J- ;  . -'i; . 



t:. 



The effect of temperature on the ciliary activity of the individual oysters 'cah' 

 be seen in Figure 5. The curves represent the results of seven experiments and cover 

 the-; whole range of individual variations from very slow-working oysters to those 

 producing the highest rate of flow. The results of the remaining eight exiJeriments 

 are not plotted because they represent the tepetition of one of the types of the curve 

 shown in Figure 5. The average results of all 15 experiments ^re shown in a curve 

 drawn in a heavy line. ' 



'In order to draw the average curve all the data were grouped in 14 classes, each 

 having 3° intervals, and the true mean of each class was plotted against the midvalue 

 of the class mterval. Examination of the curves shows that the maximum Activity 

 of the oysters occurs between 25° and 30° G. Exposure to higher temperatures 

 causes a decrease in the rate of flow. Below 40° the process is reversible, but oysters 

 that were kept for 20 minutes at 40° and brought back to 20° failed to recover and 

 produced only irregular and weak currents.^ r'"-'«l-> «■ i '" i-^'^M' - ■!«'. Iftiii mi 



In all the oysters the rate of flow decreases with the drop in temperature, and 

 in the majority of them the current stops at 8°. In a few cases, however, a very 

 weak current was observed at 5.1°. Temperatures below 5° inhibit the ciliary 

 activity of the gill to such an extent that no current is produced by the gill epithelium. 



Analyzing the experimental data, it has been noticed that under the conditions 

 of the experiments every oyster exhibited certain fluctuations ill the rate of flow, 



■ilia aouljum ./I *'vii*Ji 4o.' 



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