302 D. H. WENRICH 



rather than to the possible effect of the two hours additional 

 exposure to light. 



The increase in sensitivity in this experiment was of con- 

 siderable proportions, and its absolute value indicates a rather 

 high degree of sensitivity in these supposedly sluggish animals, 

 for only 4 per cent of the total light intensity needed to be cut 

 off to produce a reaction. 



The increase in sensitivity as a result of repeated stimulations 

 is indicated in another way. When an animal is left undis- 

 turbed it will spontaneously (i.e., without any observed stimu- 

 lation) close its siphons from time to time. After a period of 

 stimulation, however, the frequency of the spontaneous con- 

 tractions of the siphons becomes greatly increased, which increase, 

 it seems reasonable to conclude, is an indication of increased 

 irritability to whatever causes operate to induce such closure 

 when the animals are undisturbed as far as the conditions of 

 the light are concerned. 



(3) Occasionally individuals refused to respond in a typical 

 manner, or perhaps refused to respond at all, although at other 

 times the same individuals reacted in a normal way. Since it 

 was noticed that of the various animals taken from the aquarium 

 for experimentation, some were found lying on their sides and 

 others in a natural position, the idea suggested itself that per- 

 haps an unnatural position of the body tended to interfere with 

 normal physiological activities. Such a result would not be 

 unexpected in view of the presence of equilibration organs, in 

 the form of the so-called otoliths. A series of pairs of animals 

 were tested, one member of each pair having been in a natural 

 position, while the other had been lying on its side or on its 

 dorsum for a period of several days. The animal which had 

 been in a natural position reacted in nearly every case more 

 normally than the one which had been in an unnatural position. 

 In a few cases the differences between the members of a pair 

 were not very great, so that some other causes must have been 

 involved in interfering with normal behavior. 



(4) At other times, during the course of an experiment, an 

 animal would cease to give consistent reactions. In some of 

 these cases the cause seemed to be the accumulation of foreign 

 material in the mantle cavity. For after a period — usually 

 short — of non-responsiveness, a sudden contraction of muscles 



