306 D. H. WENRICH 



2. Relation of Sensitivity to Presence or Absence of Pigment 



It is worthy of note that all the species named in classes a 

 and b had more or less pigment deposited on the ends of the 

 siphons, or along the edge of the mantle, as in Pecten and Ostrea; 

 and these pigmented areas seemed in every case to be the parts 

 sensitive to changes in the intensity of the light. In class c 

 there was a total lack of pigment so far as I could discover by 

 examining the living specimens. The correlation of the pres- 

 ence of pigment with the sensitive areas of these animals is in 

 accordance with a general rule throughout the animal kingdom. 

 That pigment is not necessary to such sensitivity has been well 

 established by Beer ('01 ), R. Hesse ('02), and others. Nagel 

 ('96) finds that Psammobia has no pigment on the siphons, yet 

 it is very sensitive to increases in light intensity. On the other 

 hand, I have found that Cummingia, which has relatively very 

 long, non-pigmented, separate siphons, that move about actively, 

 gave no sign of sensitivity to either increase or decrease in in- 

 tensity in any of the two or three dozen animals that I tested. 

 While the correlation between the presence of pigment and 

 sensitivity is apparently perfect as far as my investigations go, 

 this is not proof but that species may be found on our eastern 

 coast for which this correlation fails to hold. 



3. Differences in Reactions to Increase and Decrease 



of Light Intensity 



Another point may be noted with respect to some members of 

 class a, namely, that in certain cases, and especially in that of 

 Mya, a different set of muscles is involved in the react'on to 

 increase from that involved in the reaction to decrease of light 

 intensity. Time after time specimens of Mya were tested and 

 the reactions followed the same general rule, viz., for decreases 

 in intensity, the movement was that of the muscles involved 

 in closing the siphonal openings, while in responses to increases, 

 the siphon tubes were withdrawn. In the latter case the with- 

 drawal of the siphon tubes was sometimes accompanied by 

 closure of the openings, especially if the reaction was a vigor- 

 ous one, but as a general rule, when the movement of the siphon 

 tube was not great, no closure of the openings resulted. Just 

 why one set of muscles should be used for the reaction to in- 



