1916 Shelf ord; on Physiological Differences 159 



found that the day lasts 9 hours. It would be much shorter in the 

 northern latitude in which Puget Sound is located. The reversing ther- 

 mometer shows little differences in temperature between the surface and 

 the bottom of this belt. 



At other points than in the lower portions of this belt, red algae 

 dominate and many of the animals are red. Fishes, shrimps, crabs and 

 other invertebrates appear to have taken on the red color of the algae. 

 Gamble (8) found that the young transparent stage of the European prawn, 

 Hippolyte varians, takes on the color of its surroundings within 48 

 hours. If placed in green weed in full sunlight they become green; if 

 placed in red weed they become red. Likewise to a lesser extent the 

 shrimps, crabs and fishes of the green eelgrass are greenish. Gamble 

 showed that fishes also tend to take on the color of their surroundings. 

 The animals upon which Gamble experimented did not take on the 

 color of the light to which they were exposed but took on the color of 

 the background when exposed to sunlight. 



The Coralline Belt (20-100 meters) is characterized by the absence 

 of any form of algae of sufficient size to afford shelter. The water 

 pressure ranges from 2-10 atmospheres. Wave action is not important 

 but tidal currents may keep the bottom clean. The light throughout the 

 belt is very weak. Fol found in his diving experiments in the Mediter- 

 ranean that in 30 meters it was difficult to see animals,, specially red 

 ones, which appear dark or black. Green or blue algae appear bright. 

 The Prince of Monaco's experiments in the Madeiras showed that the 

 day is short at these depths. At 40 meters light of daylight intensity 

 was made evident by Regnard's (22) apparatus for only 15 minutes 

 at about 2 P. M. Puget Sound being much farther north it is not to 

 be expected that a day of more than a few minutes occurs. The lower 

 limit of daylight must be approximately at the lower light of this belt. 

 Our temperature measurements showed differences of a fraction of a 

 centigrade degree. This belt is not very well defined, but since a 

 number of species were not taken below 100 meters and a few were 

 confined below this depth it was thought well to divide the Coralline 

 belt of Forbes as suggested here. No material used in experiments was 

 collected from this belt except in a narrow area studied by Miss Perry 

 whose results are given at the end of this paper as Puget Sound Marine 

 Sta. Pub. 1:175-176. 1916. 



The Suh-coralline Belt (100-200 meters, no specimen taken below 

 165 meters) is a belt with water movement confined to tidal currents. 

 There is probabW no period during the day at which there is any light 

 stronger than twilight. The upper limit is at about the deepest point at 



