22 



Moore however, differs sharply from Col ton and others in 

 that he minimizes the importance of wave action and selec- 

 tive agencies, and stresses nutritive processes as the keynote 

 to the problem, particularly the differential utilization of the 

 two main food materials of the snails, barnacles and mussels. 

 Thais feeding exclusively upon barnacles failed to develop 

 color in their shells, while those feeding upon mussels became 

 pigmented. He presented rather striking evidence for this in- 

 terpretation in that on placing highly colored specimens in an 

 aquarium and forcing them to feed exclusively on barnacles, 

 the new shell that formed was unpigmented. When the snails 

 were returned to a diet of mussels pigmentation was restored. 

 There were some discordant results. Yellow coloration, which 

 presumably belongs to the same group of carotenoid pigments 

 producing the darker shades, apparently was not influenced 

 by dietic changes. Pale colored specimens from barnacle crust- 

 ed rocks, fed upon mussels for six months, showed no signs 

 of pigmentation. Unpigmented individuals found in popula- 

 tions of heavily pigmented snails were interpreted as possibly 

 banded specimens in which the area of the mantle incapable 

 of secreting pigment had extended entirely across that organ. 



In connection with the imbricate form which Colton regard- 

 ed as an ecological adjustment to shallow muddy bays, Moore 

 found it to be absent from the intertidal zone, but present on 

 submerged reefs in the sub-littoral area. 



Moore was unable to establish any correlation between the 

 size of the mouth opening and wave action, a point much 

 emphasized by previous authors. 



It will be noted that if we cancel out the divergencies of 

 opinion among these several authors we have little left but the 

 factor determining size, which all seem to be agreed upon as 

 influenced by the relative abundance of an appropriate food 

 supply. This is indicated by the wide prevalence of "giant" 

 forms derived from submerged reefs, where feeding may be 

 continuous rather than intermittent. 



