to pass from the container to the tip of the nozzle 

 is recorded by using a colored solution and the value 

 obtained is subtracted from the total time meas- 

 ured, giving the duration of the actual latent 

 period. Since the vessel in the deeper part re- 

 mains afloat, the level from which the solution is 

 withdrawTi remains constant and consequently 

 there is no change in the velocity of current striking 

 the tentacles. 



All parts of this simple apparatus may be built 

 of plastic. The following precautions should be 

 taken: 1) the nozzle in front of the oyster must be 

 firmly mounted and placed on a solid stand to 

 avoid mechanical disturbance when turning the 

 stopcock; 2) water and test solutions should be 

 kept at equal and constant temperature; and 3) 

 levels from which the sea water or the test solu- 

 tion are delivered to the tentacles should be con- 

 stant in order to avoid a change in the velocity of 

 current. 



This method was satisfactorily used in the Woods 

 Hole laboratory in testing the reaction of oysters 

 to various organic compounds and contaminants. 



Sensory stimulation of tentacles by inorganic 

 salt solutions depends on the chemical composition 

 of the substance used and its concentration. The 

 relationship between the concentration of a given 

 substance and the latent period, presented in 

 Hopkins' papers, indicates that the effect, con- 

 sidered as the reciprocal of the latent period, is 

 directly proportional to concentration. Some- 

 times a secretion of mucus covers the tentacles and 

 impedes the reaction. In such cases Hopkins 

 found it necessary to subtract a constant from the 

 latent period values. 



The range of latent period values in Hopkins' 

 experiments varied from a fraction of a second to 

 about 15 seconds. By using 0.5 m solutions of 

 several inorganic salts Hopkins arrived at the 

 conclusion that sensory stimulation is primarily 

 the function of cations, which he listed in the 

 following order of effectiveness: K>Na>NH4> 

 Li. 



Tentacles respond also to chemical stimulation 

 by various organic compounds such as quinine 

 sulphate, cumarin, etc. An odorous compound 

 such as cumarin is detected by the oyster in a 

 concentration of 0.0004 percent. The oyster 

 responds also in a measurable reaction to a 0.004 

 percent solution of quinine sulfate. This con- 

 centration is one-eighth of the strength of the 



solution of quinine that can be detected by man's 

 tongue (Hecht, 1918). 



Cane sugar has little stimulating effect on the 

 tentacles (Hopkins, 1932b). Tests with fructose 

 (in sea water) that I made in the Woods Hole 

 laboratory gave the following results: 



0.05 percent No reaction in 1 minute. 

 1.0 percent Latent period 8 to 12 seconds 



in four out of 10 trials. 

 5.0 percent Latent period 4 to 8.4 sec- 

 onds. 

 Arabinose is more effective: 



50 mg./l Latent period 26 to 49 seconds 



in five out of 10 trials. 

 100 mg./l Latent period 12 to 34 seconds 



in all 10 trials. 

 5.0 percent Latent period 5 to 8 seconds in 

 all 10 trials. 



An interesting reaction was observed when 

 diluted sea water (one part fresh water -\- three 

 parts sea water) was used. The inner lobe of 

 the mantle curled up, but the tentacles were 

 not affected. The gradual curling ended in a 

 sharp contraction of the adductor muscle. Nor- 

 mal sea water added to the edge of the nantle in 

 a control experiment produced no such result. 



A similar protective reaction was observed 

 when the extracts of the meat of the oyster drill 

 {Urosalpinx cinerea) and of the stomach of the 

 starfish (Asteriasf orbed) were used. The tent icles 

 reacted strongly in contact with the undiluted 

 juice of Urosalpinx with the latent periods of 1.5 

 to 3.6 seconds. These experiments suggest that 

 sensory mechanism of the oyster may be sufficient 

 to detect the close proximity of carnivorous 

 gastropods. 



The mechanism of sensory stimulation has not 

 been adequately studied and is not fully under- 

 stood. Its biological significance is, however, 

 apparent. The warning received by the tentacles 

 is transmitted through the circumpallial nerve 

 of the mantle to the ganglia. If stimulation is 

 sufficiently intense either a part or the entire 

 mantle is withdrawn, the entrance to the gills 

 is closed, and the ensuing contraction of the 

 adductor firmly closes the shell. The three 

 steps outlined can be observed under experi- 

 mental conditions; they constitute the three 

 distinct phases of the defense reaction of the 

 organism. 



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FISH AND WILDLIFE SERVICE 



