DEPARTMENT OF MARINE BIOLOGY. 189 



open ocean the slope is covered with small, apparently stunted heads of 

 Acropora, most of which are about 6 inches and very few more than a 

 foot in diameter. Reef corals practically cease at 18.5 fathoms. 



Studies were made of the currents over reef-flats, using an Ekman 

 meter. It was found that a great variety of corals may thrive in pure 

 water at depths of 1 to 8 fathoms where there appears to be no measur- 

 able current ; but silt is soon fatal to them in such regions. 



The limestone sands of Samoan reef-flats begin to be transported 

 when the current moves at the rate of 42 feet per minute; and a cur- 

 rent of 59 feet, observed during a gale, caused a decided disturbance 

 and movement of sand all over the bottom of the Aua reef-flat. 



Our heavy expenses at Tortugas, and in repairs upon the Anton 

 Dohrn, made it necessary to curtail the program of boring through the 

 reef to enable Professor L. R. Gary to study its structure. On this 

 account. Professor Gary did not come to Tutuila this year, but expects 

 to resume his studies in the Pacific in 1920. One boring with the Davis- 

 Galyx drill was, however, made by Mr. Mills off Utelei 200 feet from 

 shore, and hard, wave-worn basaltic rock was found to be underlying 

 the reef at a depth of 68 feet. The upper part of the reef was com- 

 posed chiefly of loose fragments of Pontes, Acropora, and Alcyonaria. 

 Limestone sand was encountered at a depth of 33 feet and continued 

 to the bottom of the boring. Professor Gary will study the material 

 taken from the boring. 



The alcyonaria planted out by Gary in 1918 had made a vigorous 

 growth, and these observations, combined with those upon the growth- 

 rate of stony corals and alcyonaria in Samoa and Fiji, will furnish a 

 basis for an estimate of the age of the living reefs of the Pacific. The 

 rapidity of coral growth is such that these reefs appear to have devel- 

 oped since the last glacial period, and the nearly uniform width of atoll 

 rims over the Pacific suggests that they are all of about the same age. 



Professor Wilham H. Longley continued his observations upon reef 

 fishes at Tortugas, obtaining an excellent series of submarine photo- 

 graphs illustrating the relations between the color patterns of these 

 animals and the general color-scheme of their surroundings. These 

 studies have been pursued for 8 years in the Florida- West Indian region 

 and in the Hawaiian Islands, and it is hoped they may be supplemented 

 in 1920 by further work in the Pacific, in order to give them a world- 

 wide significance. 



Professor J. F. IMcGlendon found that the neuro-muscular system of 

 the scyphomedusa Cassiopea has the same rate of metabolism when 

 etherized as it has when in normal sea-water. He also found that an 

 excess of carbon dioxid in sea-water lowers metabolism in Cassiopea, 

 and this depression is not due to hydrogen-ions in the surrounding 

 water derived from hydrolysis of GO2, but to carbon dioxid per se. 



