1916 Shelf or d; on Physiological Differences 171 



IV. DISCUSSION AND CONCLUSION 



These animals, specially those from the greater depths, were taken 

 from deep water and placed in the float car under greatly reduced pres- 

 sure and experimented upon a little later. Was the difference between 

 those taken from the different depths in any way dependent upon the 

 in pressure? To test this a set of animals was collected and kept in the 

 float car covered with canvass for two weeks and the experiments repeated 

 with results in accord with those secured at the beginning of that period. 

 Since there was no essential difference in the character of the animals in 

 these respects after such treatment, effects of pressure, temperature, etc., 

 are in a measure eliminated. If pressure were responsible the removal to 

 shallow water for two weeks would be likely to make some difference. 

 Further the experiments of Regnard (22) tend to show that such small 

 pressures are not of great effect. The few temperature determinations 

 show that there is little difference between the different levels at which 

 such species as the coon-stripe shrimp (Pandalus danae) occurs. Such 

 differences as exist are between the shallower layer over the rocks at the 

 shore and the layers immediately beneath. Temperature responses can 

 hardly be responsible. When one considers the differences in con- 

 ditions in the different habitats he is somewhat at loss to explain the 

 diffrences noted. 



Strong tides and complete circulation of the water would lead one 

 to expect only small differences in the chemical character of the water, 

 but the differences between the two sides of the harbor make such con- 

 clusions unwarranted. It was impracticable to determine the chemical 

 character of the water from the deeper parts because of the absence of 

 & suitable water bottle. 



Of the differences in habitat relations which have been noted the dif- 

 ference in light is the most striking. Turning to table 5, we find that 

 the crabs which live under stones and in the dark on the shore (flat shore 

 crabs), die quicker than crabs of greater depths that live more of the time 

 in the light. 



The color differences in the various Crustacea, which corresponds 

 to the background in individuals from red and green algae, and which 

 according to Gamble (8) is paler from greater depths, makes color the 

 only character correlated with the different physiological states; and this 

 has been demonstrated to be determined by light. Gamble's (8) experi- 

 ments showed that animals take on the color of their background only in 

 the young stages. Thus light may be a correlative of factors causing the 

 differences. The recent experiments of Smith (26) on salmon further 

 show the importance of light. The remarkable differences displayed are 



