BY MEANS OF A CONTINUOUS CURRENT TUBE. 41 



the air from the pump. On the ruljber-tubing near F should be fixed 

 a screw compressor {H) to regulate the flow of air. On allowing the 

 air to enter at F, it forces the water out of tube E down to D, and as 

 the air enters tube C it breaks into bubbles, which pass up tube C and 

 float to the surface. Between every two air-bubbles there is a short 

 column of water. The driving of the water out of tube C by the air- 

 bubbles produces an in-draught of water through tube A. As the air- 

 bubbles follow one another in rapid succession, there flows down tube 

 A a good current of water. 



The Hydroid (7) is suspended inside tube ^ by a silk thread 

 attached to a small glass hook, which hangs over tlie top of the tube ; 

 and the copepods, diatoms, etc., are put into the bell-jar. 



As the current through tube A mainly draws from the upper part 

 of the bell-jar, it is best to keep the top of the tube as low down as 

 possible. The continuous stream of air which bubbles out of tube G 

 not only aerates the water, but sets up a current inside the bell-jar 

 and produces a good circulation. It is therefore advisable to keep the 

 top of tube G low down. This circulation is beneficial to the 

 plankton, and also carries it within the reach of the stream drawing 

 in to tube A. The stream of air-bubbles is in another way of 

 great service. Their continual breaking at the surface prevents the 

 formation of the scum, chiefly due to bacteria, which gradually accumu- 

 lates in small aquaria to form a thick, dirty surface film. This film, 

 when once formed, is difficult to remove, and is often harmful to the 

 inhabitants of an aquarium. 



So long as the water in a bell-jar keeps quite clear there is no 

 necessity to completely change it. I usually siphon off about half a 

 gallon twice a week and fill up with very clean water. 



The f/roivth of Bougainvillia muscus in a airrcnt-tuhe. On Nov- 

 ember 6th a small bush-like colony of Bougainvillia muscus, about 

 20 mm. in length, was suspended inside a current-tube with the root of 

 the colony touching the glass. The colony had a few short stolons 

 growing out from near the distal ends of the branches. It was for 

 observations upon the growth and function of these stolons that the 

 Boiigainvillia was placed in the tube. 



Soon it was apparent that Bougainvillia. liked its new surroundings. 

 The hydranths kept fully extended, and their stomachs were seldom 

 empty. The stolons greatly increased in number and in length, some 

 hanging down 15-20 mm. A few developed, here and there, soli- 

 tary diminutive liytlranths, but there were no signs of active bud- 

 ding of hydranths. The activity of the colony was directed into 

 a stolon which came off from near the root and was able to 

 attach itself to the glass of the tube, along which it grew at a 



