340 NATURAL HISTORY OF OTJR SHORES 



Loss by evaporation, causing the salinity of the water 

 to become too great, is to be made up with clean tap water. 

 To ascertain whether the salinity is correct there are little 

 glass balls, sold at the dealers in aquaria, which can be left 

 always in the aquarium. They are so weighted that when 

 all is correct one floats and the other sinks. If the salinity 

 is too great both float, and if you are adding too much 

 tap water both will sink. This is very ingenious, but a 

 simpler plan is to have a salometer. Note upon the scale 

 to what level it submerged in the original stock of sea- 

 water and be guided by this. The too great reduction of 

 density can only occur through carelessness in adding from 

 the tap. 



By this simple plan I once kept a lobster for nearly three 

 years in a flat, bell-shaped, cake-covering glass obtained 

 from a confectioner's. As weeds grew to too great an 

 extent I weeded them out, and in this simple tank I watched 

 the shell- casting, and noted rate of growth of my little 

 lobster, as well as could be done in a scientifically appointed 

 laboratory. 



In regard to the anemones ; strangely enough, a species 

 that occurs in every rock pool, even up to high-water level, 

 that at times gets half boiled in the sun and at others 

 deluged with rain water, and in nature thrives amazingly, 

 will not do at all well in the aquarium. This is the 

 " Opelet " (Anthea cereus), of Gosse. 



Other animals that live well are, firstly, the prawns 

 the whole of these, including the beautifully coloured, little 

 Hippolytes (see page 134). These are active, graceful, and 

 highly interesting little creatures, but it will not do to try 

 and keep them with anemones. 



Of fishes, the fifteen-spined stickle back (page 262) lives 

 well ; so do the pipe-fishes, the blennies. and the gobies. 

 The wrasses also live well, and are exceedingly beautiful. 

 Little grey mullets also live well. 



