THE ZOOLOGICAL STATION AT NAPLES. 591 



in dark reservoirs, where the bacteria can most efficiently exert their in- 

 fluence. Thus, at Naples, it was found that the pipes conducting the water 

 from the reservoirs to the rooms were coated internally with a layer of 

 bacterial slime, and that in its passage along these pipes the water under- 

 went considerable purification. For instance, water drawn oft' from one 

 of the taps was found to contain from 26 to 82 per cent, less free ammonia 

 and from 16 to 25 per cent, less organic ammonia than the water in the 

 supplying reservoir. Also, larvte grown in such water were increased about 

 7 '8 per cent, in size. 



Probably, in marine aquaria, a more powerful purifying influence than 

 the bacterial is exerted by the diatoms and minute algaj. Thus sand 

 collected from the bottom of one of the tanks was found to be impregnated 

 with this vegetable growth, which clung to each grain and particle. On 

 filtration of aquarium water through this sand, no less than 91 per cent. 

 of the free ammonia, and 18 per cent, of the organic, were removed. Again, 

 on filtering water in a continuous stream through a layer of sand which 

 originally contained no vegetable matter, this matter was gradually 

 deposited from the water, and the sand gradually developed a capacity for 

 purification which became more and more marked with time. Such a 

 vegetable filter is very sensitive to any variation in the flow of water. 

 Thus, iE the rate of flow were diminished from the maximum of one litre in 

 thi'ee minutes to one litre in fifty minutes or more, there was no longer any 

 purification, but the amount of free ammonia in the water was increased about 

 threefold. This result is of some practical importance. It teaches us that 

 the layer of slime which is deposited on the bottom and sides of the tanks of 

 marine aquaria, and which consists largely of diatoms and algte, is a most 

 valuable purifying agent, provided it be in layers thin enough for an 

 adequate amount of water to circulate to all parts of it. Once, however, 

 the lower layers are insufiiciently supplied, they begin to decompose, and 

 become a source of contamination. 



It was found that a sand filter, even when kept for weeks in darkness, 

 continued to exert a purifying influence. In this case the influence must 

 have been due to bacteria, and not to chlorophyll containing organisms. 

 This bacterial filter acted most efficiently with a slower rate of filtration, 

 and was not nearly so sensitive to a diminution or cessation in the 

 flow of water. With regard to the purifying agency of the macroscopic 

 algsfi, experiments were made on the purifying effects of green weeds such 

 as Viva, and of red weeds such as Gelideum. Their action was not found 

 so favourable as that of diatoms and minute algae, and of bacteria, in con- 

 sequence of the difficulty experienced in getting them to live healthily in 

 the impure aquarium water. 



To the other results obtained it is not possible to refer here at any 

 length. Thus the action of sunlight on the water was investigated, it 

 being found that though an immediate germicidal effect was brought 

 about, the ultimate effect, on withdrawal of the adverse influence, was 

 slight or absent. Again, the effects of increased aeration were investi- 

 gated, but the influence exerted appeared to be but slight. 



Numbei's of experiments were made upon the fouling effects of various 

 animals on the water. As a rule, larvae grown in water fouled by 

 most organisms were increased in size by, on an average, 4*1 per cent., 

 whilst in water fouled by sea-urchins they were diminished in size by 

 6-9 per cent. Fish and crabs appeared to effect about ten times as much 

 contamination «f the water as molluscs and holothiirian?, whilst dead and 



