362 THE LETTUCE ULVA. 



water would become impure, and unless changed 

 often, your little colony would surely die, at least 

 those of its members who were unable to rise above 

 the fluid, and occasionally breathe the fresh air. 



The secret herein involved, that animal and vege- 

 table respirations counterbalance each other, has only 

 of late years been discovered ; yet it is apparent to 

 any observing eye at the sea-shore ; there we never 

 meet with a rock-pool containing living animals, that 

 is not more or less adorned with sea-weeds. 



The green Lettuce Ulva, so abundant in rock-pools, 

 the sea-grass, which covers almost every fixed object 

 at the sea-shore, or the well-known dulse or Chondrus 

 Crispus, form the only sea-weeds that it is necessary 

 to introduce into an aquarium. In fact, one or two 

 fronds of the Ulva Latissima alone, will answer per- 

 fectly well to purify the water of even a compara- 

 tively large tank. I have often been surprised to 

 find how small a quantity of alg83 was required for 

 the purpose mentioned. After allowing a single 

 frond to float for a few days in a tank, in which some 

 sea- water was newly deposited, I took it out, and for 

 an entire twelvemonth the water remained healthy 

 and as clear as crystal. 



The arrangement of the 'stock' of an aquarium 

 is quite a matter of taste; perhaps no two persons 

 adopt precisely the same plan. It may, therefore, be 

 advisable, as this matter is so arbitrary, for the 

 writer to state how his own tanks are mapped out, 



