ARTIFICIAL CQLTUKE OF MARINE I'LANKTON ORGANISMS. 373 



ill cultures, proportional to the seasonaj variation in the 

 quantity of phytoplankton, would be sufficiently 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) Tank-Water. — "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 higli-water spring- 

 tides, so as to get the least contaminated water, and no water 

 is pumped that does not show" a specific gravity, measured 

 with a hydrometer, of i>^'-' = 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 re- 

 plenished, 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 any- 

 thing 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 algae. 

 Minute naviculoid diatoms, Ectocarpus, Cladophora, 



