430 ON THE AETIFICIAL CULTURE OF MARINE PLANKTON ORGANISMS. 



no permanent culture can be obtained, the diatoms soon beginning to 

 lose colour and getting into an exhausted condition. Death takes 

 place in from two to three months after the culture has been started, 

 and in many cases considerably sooner. Long before inability to start 

 new cultures, the test of death, has been established, the valves appear 

 on examination quite colourless and practically empty. 



Samples of outside water, taken at times when the quantity of 

 plankton was widely different, gave no appreciable variation in the 

 results obtained by culture methods. It is, however, doubtful whether 

 differences in the amounts of growth in cultures, proportional to the 

 seasonal variation in the quantity of phytoplankton, would be suffi- 

 ciently marked to be appreciable. 



The total growth under cultural conditions, although small for 

 a culture, is very much greater than any natural plankton that has 

 come within our experience. 



(c) Tanlc-ioater. — " Tank-water " or water taken from the supply of 

 sea-water circulating through the tanks of the Aquarium at Plymouth, 

 shows some striking and interesting differences from " outside water." 

 This water is pumped up from the sea, just below the Laboratory, into 

 two large, covered-in, settling reservoirs, with a capacity of 50,000 gallons 

 each. Pumping is only done at high water, spring tides, so as to get 

 the least contaminated water, and no water is pumped that does riot 

 show a specific gravity, measured with a hydrometer, of p^^"^ = 26"00 

 (S = 34*00) or over. The water is allowed to settle for about a fortnight 

 before being used for the general circulation. 



The tanks themselves are made of slate and glass, and the pipes which 

 convey the sea-water to them are of vulcanite, so that the water does 

 not come in contact with metal, excepting in the pumps, which are of 

 cast-iron. The two settling reservoirs are used alternately, for about 

 a week each. From time to time, tide and water allowing, waste 

 is replenished, and about twice a year each reservoir is emptied, cleaned 

 out and refilled. The aquarium takes about 20,000 gallons, and this 

 is in circulation with one of the two 50,000-gallon reservoirs. An 

 estimate of the amount of life in the tanks of the aquarium must be 

 exceedingly rough, but the intensity of the larger forms of life is 

 far greater than anything met with in natural waters. About 500 

 fish and 2000 invertebrates, including all forms as large as an Actinia 

 equina, might be somewhere near the mark. So it will be seen that 

 the accumulation of excretory products must be a by no means 

 negligible factor. The flora of the tanks is very restricted, and is 

 chiefly composed of minute forms of algte. Minute naviculoid diatoms, 



