4 Hjalmar Théel, 



series of larvge up to the young animal, I repeated my experiments du- 

 ring the summer of 1891, and with satisfactory results. I will here de- 

 scribe somewhat fully the method which enabled me to succeed and the 

 precautions which should be followed, as they will, I hope, prove to the 

 advantage of other investigators. 



1) The aquaria ought to be large and rather high, of a capacity of 

 30 liters or more; the most suitable are those made altogether 

 of glass. They must be covered in. 



2) A sufi&cient number of marine plants, especially Enteromorpha 

 intestinalis and E. compressa, grown upon small stones, must be 

 placed in the aquaria. They supply the water with air, and as 

 soon as they have lost their air- and gas-sacs, they must of course 

 be exchanged for new plants. Before putting the plants in the 

 aquaria, it is of great importance to clean them and wash them 

 carefully, thus preventing the mass of minute animals such as 

 Mollusca, Entomostracans etc., which are attached to and living 

 on them, from filling up the aquarium and fouling its water. 



3) Several times a day the water must be cautiously stirred with a 

 glass-rod, so as to cause a current and prevent the formation of 

 the film on the surface which is well known to characterize stag- 

 nant water. It is evident that this thin membranous film has 

 a pernicious influence on the larvae, hindering them from reaching 

 the surface itself and preventing the water from coming into 

 immediate contact with the air. Besides, the film is a favorite 

 resort of a good many microscopic animals such as Infusoria, 

 minute Entomostracans etc., which rapidly multiply, foul the water 

 and nourish themselves at the expense of the larvœ. If the water 

 be kept in motion, the larvae do not so easily fall a prey to 

 their enemies. 



4) All the water used must be fetched from the open sea, and not 

 from the shore, and it is quite necessary to let it pass through 

 a filter made of a piece of cloth or the finest canvas. 



5) Moreover, it is of importance every other day to let a part of 

 the water at the bottom of the aquarium flow away, which is 

 easily done by means of a fine glass-siphon. While the water is 

 flowing away, it is advisable to change the position of the siphon 

 in such a manner that as much as possible of the foul matter 

 which has sunk to the bottom may be removed. If this foul 

 matter is not taken away, the enemies of the larvœ increase very 



