178 CARNEGIE INSTITUTION OF WASHINGTON. 



filled about three-quarters full and the plug replaced. The sea was quite 

 calm on the day the water was collected and on the three preceding days. 



After the bottles were brought to the laboratory a small piece of floating 

 Sargassum (cooked for 5 minutes at 15 pounds pressure) was added to each 

 bottle, with sterile forceps, and the plug replaced. (To 4 bottles of each of 

 the 4 groups enumerated above, enough peptone was added to make the solu- 

 tion 0.1 per cent peptone.) The bottles were then set aside for 12 days in 

 room temperature which varied from 35° to 40° C. during the day and at night 

 from 28° to 33° C. At the end of this period about 3 c. c. were taken from each 

 bottle with a sterilized pipette and placed in a sterilized watch-glass for micro- 

 scopic examination. Amebas of the species Flabellula mira were found in small 

 number in bottle No. 9 of group No. 3, and in large number in bottles Nos. 13 

 and 14 of the same group. These bottles did not contain peptone. No amebas 

 of any species were observed in any of the other 101 bottles. The indication of 

 this preliminary experiment is therefore that the surface water of the sea is 

 sterile with respect to amebas, but that floating "dust" particles, and of course 

 large pieces of floating Sargassum or other algae, may carry amebas. In this 

 experiment the dust-like particles floating on the surface undoubtedly carried 

 the amebas which came up in the 3 bottles. 



This result is not surprising. Amebas are relatively large organisms and 

 in undisturbed water slowly sink to the bottom if unsupported by some float- 

 ing object. Moreover, the researches of Professor Lipman show that the 

 sea-water in the same region where this water was collected is very poor in 

 bacteria, far too poor to afford sufficient food for the amebas. So that lack 

 of a solid support and lack of food each by itself is sufficient to account for 

 the absence of amebas from surface water. What the range of distribution 

 of amebas is on the floor of the sea with respect to depth, texture, tempera- 

 ture, etc., is wholly unknown, but it is hoped that an investigation bearing 

 on this point may soon be made. 



Many species of amebas, however, do not eat bacteria, but require ciliates, 

 flagellates, and especially diatoms and other small algae of various kinds as 

 food. In order to determine what the chief limiting factor is in the distribu- 

 tion of some of these species, the following preliminary experiment was set up : 



It has long been known that the waters of the cooler oceans are much 

 richer in plankton, especially in phytoplankton, than the waters of the tropical 

 seas, and also that the cooler water contains considerably more nitrogen 

 (about 3 parts per 1,000,000 of water) than the warmer water (about 1 part 

 per 1,000,000). It has frequently been suggested that the difference in the 

 amount of nitrogen accounts for the difference in the richness of the plankton. 

 To test this point experimentally and to determine what effect a plentiful 

 supply of food would have on amebas, I arranged a concrete 1 ank 2.4 meters 

 long by 35 cm. wide by 20 cm. deep. A few irregular concrete blocks, a 

 couple of handfuls of coral sand, and a few pieces of Sargassum were thrown 

 into the tank. A slow stream of water was run through the tank at the rate 

 of about 100 liters per day. To the inflowing water was slowly added enough 

 potassium nitrate in solution to make the sea-water in the tank about 1 part 

 of nitrate to 100,000 parts of water. The tank was out in the open and was 

 exposed to direct sunlight from 11 a. m. to l h 30 m p. m., but during the rest 

 of the day it was partly shaded. On several days the temperature of the 



