BY MEANS OF A CONTINUOUS CURRENT TUBE. 39 



as food supply for Hydroids, then diatoms and other microscopic 

 organisms should be present in the water for the copepods to feed 

 upon. The constituents of the plankton require careful adjustment, 

 and the whole must be kept in a perfectly healthy condition. There 

 should always be a reserve of food in the bell-jar to carry over days 

 of bad weather at sea and other misfortunes. 



Hydroids certainly keep in better condition and live longer in a bell- 

 jar with the water in constant motion than in perfectly still water. 

 They are accustomed in the sea to a current running in a definite 

 direction and carrying along plankton, from which they select their 

 food. In the sea the current is ever running, always fresh and aerated, 

 and always carrying new plankton. The successful rearing of Hydroids 

 in a few gallons of water depends greatly upon imitating, as closely 

 as possible, the natural conditions under which they live in the sea. 

 The current-tube imitates fairly closely these conditions. The Hydroid 

 is placed in a glass tube through which flows a constant current of 

 aerated water carrying along with it the plankton in the bell-jar. 



Description of the current-tube. The power for producing the current 

 within the tube is compressed air. It does not matter by what 

 method the air is compressed, provided that the pressure is kept fairly 

 constant and the air is clean and pure. The latter condition is 

 important, as a considerable quantity of air passes through the sea- 

 water in the course of a day. The supply should be drawn from 

 outside of a building, and then washed or filtered to remove the 

 dust. The air-pump used in the Laboratory at Plymouth is a form 

 of Sprengel's pump, made of metal, and obtainable for about ten 

 shillings.* It is a remarkably cheap, but very efficient piece of 

 apparatus. 



I shall describe the current-tube as it was originally made by me. 

 (Fig. 1). Modifications in size and shape will no doubt be introduced 

 later on to meet special requirements. 



^ is a glass tube, 32 mm. in diameter and about 200 mm. in length. 

 At the lower end a bored cork is inserted, into which is placed the 

 narrow glass tube BCD, having an internal diameter of about 4 mm, 

 B C D is an ordinary T-tube, with one end {D) reduced in length, one 

 end (B) made U-shaped as figured, and the third end (C) remaining 

 perfectly straight. To D is attached by a short piece of rubber-tubing 

 the long glass tube ^, the length of which depends upon the depth of 

 the bell-jar. The next step is to tie a piece of string round the tube A 

 near the top, and lower the current-tube into the bell-jar. The string 

 attached to A is made fast to the top of the bell-jar, and adjustments 

 made to hold A in an upright position. 



* The pump is supplied by Anton Skell, Zinzendorfstrasse 34, Dresden. 



