the contractions occur, and the speed of rotation 

 of the drum. PubHshed reports frequently fail 

 to mention these significant details. Another 

 feature of importance is the general level cor- 

 responding to the tonus of the muscle to which 

 the valve returns after each brief closing. Under 

 normal conditions the adductor muscle is never 

 completely relaxed. The distance to which the 

 valves are pushed apart by the hinge ligament is, 

 therefore, indicative ot the degree of rela.xation. 

 During my years of study, more than 2,000 

 tracings of shell movements of oysters were 

 obtained under a great variety of conditions. It 

 was possible to group them into five principal 

 types which for the sake of brevity are designated 

 by the first five letters of the alphabet. 



Type A 



The three curves shown in figure 15.3 (,A-1, 

 A-2, and A-3) indicate normal behavior of the 

 oyster. The differences in the appearance of the 

 curves are due primarily to differing speeds of 

 drum rotation. Curve A-2 is a continuation of 

 curves A-1 with the drum movement reduced 

 from 15.3 cm. to 3.6 cm. per hour. The extreme 

 right portion of curve A-2 indicates the summa- 

 tion of sevei'al stmiuli that caused brief closing of 



the valves. The curve A-3 is a variation of type 

 A-1 and is essentially similar to ciu"ves A-1 and 

 A-2. The writing lever in curve A-3 was set in 

 such a way that the magnification of the vertical 

 excursions was only one-third of that used in 

 curves A-1 and A-2. The contractions were, 

 however, more frequent. Several downward 

 excursions of the pen indicate brief attempts to 

 widen the opening of the valves, but the general 

 tonus level of the adductor remained fairly 

 constant. 



Type A shell movement, shown in figure 153, 

 represents movements of an undistiu'bed oyster 

 that maintains a steady current of water for the 

 ventilation of the gills and for the collection of 

 food. The general level of opening of the valves 

 is fairly constant (curves A-1 and A-2). Relaxa- 

 tion of the muscle immediately after rapid con- 

 traction is slow, and the resulting curve slopes 

 down gently (see right parts of curves A-1 and 

 A-2) . Sudden snapping of the valves is associated 

 with the discard of rejected food, mucus, detritus, 

 and other pai tides that accumulate on the inner 

 surface of the pallium. This rejection reaction is 

 an important feature of oyster behavior for it is 

 the principal method of keeping the pallial cavity 

 free from the accumulation of foreign matter. 



A-1 



A-2 



^^Av_J,^ 



A-3 



Figure 15.3. — Shell movcment.s of normally feeding oyster. Type A. Vertical magnification of curve .A.-3 is about one- 

 third of that in A-1 and A-2. In each curve the uppermost point corresponds to the position of the lever when the 

 shells are completely closed. Time interval: A-1, 5 min.; A-2, 30 min.; .\-3, 1 hour. 



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FISH AND WILDLIFE SERVICE 



