420 H. M. Vernon 



Red weeds as Gelidium generally cause au increase in the free 

 as well as in the org-anic ammonia. They sometimes have a favourable, 

 but as a rule au unfavoiirable, effeet on larvai growth. 



Filtration of water through sand niay remove almost all the free 

 ammonia present and a third or more of the organic ammonia, this 

 purification being elfected by the layer of diatoms and algse on the 

 graius of sand. Sand kept in darkness may also effeet great puri- 

 fication, in this instance through bacterial influence. Larva grown 

 in water thus purified are iucreased in size some A.1%. 



On keeping water in diffuse light for some weeks, nearly all 

 the free ammonia disappears owing to the multiplication of the small 

 quantities of alg?e and diatoms originally present. The organic am- 

 monia is at the same tirae increased, but larvse grown in the wafer 

 are greatly increased in size. 



Larvse grown in water previously exposed under certain cou- 

 ditions to fhe sun, are greatly increased in size. Sunlight, though it 

 has an immediate germicidal action on the water, probably effects 

 no permanent bacterial purification. 



On keeping water in darkness for three or more weeks, nearly 

 all the free, and a third or more of the organic ammonia, are 

 removed by bacterial action. Larva grown in water thus purified 

 are some l.f)% larger than the normal. As far as the ammonia is 

 concerned, water kept in darkness may become as pure as open 

 sea-wafer; but it is not physiologically so pure, for larvai grown in 

 the latter are 16.0_^ larger than the normal. 



A very considerable purification of the Aquarium water is effected 

 by the layer of bacterial slime coatiug the inside of the conducting 

 pipes. Thus half of the ammonia was sometimes removed by this 

 agency, and the larvse grown in the water were 1.%% larger than 

 the normal. 



Larvse grown in water filtered through asbestos, and water 

 previously heated to 50", 76° or 100°, are considerably increased 

 in size. 



Larvse grown in water previously fouled by fish, crabs, Molluscs 

 and Holothurians, are increased in size, but in water fouled by 

 Echinoids and other Plutei, are considerably diminished. Dead 

 Echinoids foul the water about ten times more than most living 

 auimals, whilst fish and crabs foul it ten times more than Molluscs 

 and Holothurians. 



Ammonium chloride acts very iujuriously on larvai growth, but 



