78 THE BOOK OF NATURE STUDY 



duced. Great care should be taken not to add too many of 

 these, and the habits of those chosen should be carefully con- 

 sidered. Do not, for instance, introduce any of the big carnivor- 

 ous beetles into a tank already stocked with delicate forms of 

 life. If possible the tank should be self-supporting, that is to 

 say it should contain a proportion of larvae and so forth which 

 can be sacrificed to the appetite of the larger forms. If, however, 

 it is thought necessary to feed any of the inhabitants specially, 

 do this as sparingly as possible, and remove at once any un- 

 utilised particles. 



The Marine Aquarium. The marine aquarium is on the 

 whole more interesting, and can be made to contain a greater 

 variety of animals than the freshwater tank. On the other 

 hand, it must always be difficult to manage except near the sea, 

 for artificial seawater is not always satisfactory, and the main- 

 tenance of a constant specific gravity is not easy. Again, while 

 variety can be added to the freshwater aquarium by the 

 addition of newts, tadpoles, water-beetles, and even water- 

 tortoises, animals which are air-breathers and therefore very 

 tolerant of foul water, all the inhabitants of the saltwater tank 

 are furnished with organs adapted for breathing air dissolved 

 in water, even if some of them can tolerate a temporary removal 

 from the water, and all must therefore have water of great purity, 

 which is an additional difficulty. 



In starting a saltwater aquarium, as in the first case, make 

 sure that your tank is clean and sweet. A layer of washed sea- 

 sand may then be placed at the bottom if desired, but as this 

 is not required for plant roots, it is safer to substitute for it a layer 

 of clean gravel, which is less apt to lodge decomposing particles. 

 Then select some stones or pieces of rock covered with growing 

 weed. Avoid the red weeds, which will not live in the light 

 aquarium and in shallow water. Most of the brown weeds must 

 also be rejected because of their size. The green forms, such as 

 Ulva and Enteromorpha, are most suitable, and if no stone of 

 suitable size can be found bearing them, a piece of rock with 

 weed attached should be broken off with a geological hammer. 

 Place these pieces of rock or stone at the bottom of the aquarium, 



