MANAGEMENT OF LIGHT AND TEMPERATURE. 25 



During tlie last long-continued severe winter it was 

 found very difficult, in an ordinary sitting-room 

 having a south aspect, and a good fire maintained 

 throughout the day, the tanks being also screened 

 at night by a blind, to prevent the powerful cooling 

 effects from radiation on a clear frosty night ; and on 

 three several occasions, marking exactly the three 

 severest frosts that we experienced during the winter, 

 the thermometer immersed in an aquarium contain- 

 ing about thirty gallons of water fell as low as 45 

 Fahr. The Shrimp and Crab tribes, and the Crus- 

 taceans generally, are especially affected by these 

 changes, and on each of the three occasions alluded 

 to one or two individuals perished." 



Excess of heat, and also strong sunlight, are like- 

 wise to be as carefully guarded against, and the expo- 

 sure of a small tank to the direct rays of the sun on a 

 hot summer's day, will speedily result in the destruc- 

 tion of every animal in it. 



Too much light has also the effect of rapidly 

 causing the propagation of microscopic Infusoria of 

 a green colour, such as Euglena and its congeners, 

 which, under this influence, multiply so rapidly as to 

 render the whole water turbid and opaque. These 

 will sometimes subside to the lower part of the tank, 

 as evening approaches, and disappear in the shingle 

 bottom; but immediately the morning light shines 

 strongly upon the aquarium, they will rise like a thin 

 green cloud, and diffuse themselves throughout the 

 water. Although this Infusorial growth is not un- 

 healthy, yet it causes the aquarium to present a very 

 unsightly appearance, and prevents all observation on 



