316 I). II. WEXRICH 



Thus while some experimental evidence has been produced 



indicating an image-forming capacity for the eyes of Pecten, 



many of the experiments have been uncritical in that they 



have not been conducted so as to distinguish between react ons 



to decreases in light intensity and reactions to the movement 



of an object. The experiments and observations of Nagel, 



Bauer, and Von Uexkull, however, seem to be fairly conclusive. 



It is believed that the experiments described in the present 



paper were so conducted as to avoid any confusion of the two 



kinds of stimulation, and therefore, to establish that Pecten 



gibbus (var. borealis) responds to the movement of an object 



and consequently may be thought to form an image in its highly 



organized eyes. 



IV. SUMMARY 



A. Results on Anodonta: 1. Anodonta is sensitive to de- 

 creases in light intensity but not to increases. Responses were 

 obtained to decreases so slight as not to be detected by the 

 eye of the observer 



2. Sensitivity is influenced by the physiological states of the 

 animals in at least the following ways : (a) Animals kept in the 

 dark a few hours are less sensitive than when kept in the light, 

 even when the light is very weak (e.g , 4 or 5 candlemeters) . 

 (b) Animals are more sensitive after periods during which suc- 

 cessive experimental stimulations were given than after periods 

 of quiet, (c) Unnatural positions of repose for the animals 

 seem to interfere with normal sensitivity, (d) The presence 

 of foreign material in the mantle chamber may temporarily 

 inhibit reactions to stimulation by decreases in light intensity. 

 (e) The presence of eggs or embryos in the gills of gravid females 

 appears to inhibit responses to decreases in the intensity of the 

 light, (f) Inherent differences in sensitivity between individuals 

 probably exists. 



3. By letting A equal the amount of light (expressed in candle- 

 meters) cut off to produce a decrease, and B equal the amount of 

 uninterrupted light (expressed in candle-meters), the degree of 



sensitivity may be measured by the value of the fraction t- — f. 



when A has the lowest value for which a response is given. 



B. Results on marine species: 1. Three classes were dis- 

 tinguishable as follows: (a) Those sensitive to both increase 



