62 THE SEA SHORE 



immediately it is filled, on account of the possibility of the water 

 being contaminated by contact with the cement that has been used 

 to make it water-tight. It is safer to allow the first water to stand 

 for a few weeks, the weeds and all other objects being in situ, and 

 the necessary means employed for perfect aeration during this 

 interval, and then, immediately before the animals are placed in 

 their new home, to syphon off the whole of the water, and refill with 

 a fresh supply. 



In the selection of animals due regard should be paid to 

 two important points first, the danger of overcrowding, and, 

 secondly, the destructive habits of some of the more predaceous 

 species. 



No more than two or three animals should, as a rule, be reckoned 

 for each gallon of water ; and the proportion of animals should be 

 even less than this when any of them are of considerable size. 



As regards the destructive species, these are intended to include 

 both those that are voracious vegetable feeders and also those 

 whose habit it is to kill and prey on other creatures. 



It must be understood that the weeds are to serve two distinct 

 purposes : They are to supply at least some of the oxygen required 

 for the respiration of the animal inmates, and also to serve as food 

 for them. Some marine fishes and molluscs feed on the fronds of 

 the weeds, and among these the common periwinkle may be 

 mentioned as one of the most voracious. If many such animals 

 are housed in the aquarium, it will be necessary to replace at 

 intervals those species of weeds that suffer most from their ravages. 

 The zoospores thrown off by the weeds, particularly in the 

 autumn, are also valuable as food for some of the animals. 



Notwithstanding the destructive character of the periwinkle 

 just referred to, it has one redeeming feature, for it is certainly 

 useful in the aquarium as a scavenger, as it greedily devours the 

 low forms of vegetable life that cover the glass and rocks, thus 

 helping to keep them clean ; and the same is true of the common 

 limpet and other creeping molluscs. Some of these are even more 

 to be valued on account of their partiality for decaying vegetable 

 matter, by devouring which they reduce the amount of the products 

 of decomposition passing into the water. 



Other details concerning the selection of animal and vegetable 

 life for the indoor aquarium must be left to the discretion and 

 experience of the keeper, for it is impossible by written instructions 

 and advice to cover all the various sources of loss and trouble that 



