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A TRAP FOR FREE-SWIMMING ORGANISMS. 



By S. C. Akehurst, F.R.M.S. 



{Bead October 2H(h, 1913.) 



Fig. 3. 



Simply stated this is an arrangement which cuts off the retreat 

 of the creatures after they have been attracted into a small 

 receptacle by light. 



The first trap I used was made of glass in two pieces. The 

 top is funnel-shaped, and holds about 5 ounces of water. This 

 is attached to a horizontally-placed cylinder, 1 inch in diameter, 

 and 1| inch long the whole being mounted on a stem and foot. 



Into the cylinder is fitted a glass spigot, which has been ground 

 in to avoid water passing. There is a hole at the bottom of the 

 funnel flask which allows free access of the water to a small well 

 in the glass spigot. 



When the trap is working, this well opens immediately under 

 the hole at the bottom of the flask, and into this the organisms 

 can enter freely. When desiring to fix the catch, give the spigot 

 a slight turn the mouth of the well then presses against the side 

 of the cylinder and the contents become locked in. 



To set the trap, fill the flask with pond water, cover the entire 

 funnel-shaped flask with some light-proof material, and direct all 

 the light that can be gathered by a bull's-eye on to the cylinder 

 winch contains the glass spigot. Any swimming phototactic 

 organism in the water will at once react and pass into the well, 

 which is brightly illuminated usually 10 to 15 minutes is 

 sufficient to allow for this, but longer time can be given if 

 necessary. Give the spigot half a turn, and, as already explained, 

 this locks the creatures in the well. The water can then be 

 poured off from the flask, the spigot withdrawn, and the rotifers 

 or whatever may have been trapped in the well can be taken 

 up with a pipette and transferred to the slide for examination. 



After the first catch has been taken the trap can be set again and 

 a second lot secured. Work can therefore be carried on without 

 interruption or loss of time until all the water has been dealt with. 



Should there be any sediment, this can be allowed to settle and 

 then trapped off before any attempt is made to catch the organisms. 



There is difficulty in obtaining this trap made in glass ; I have 

 therefore worked out another in metal (fig. 3). This consists of 

 a round box, 1 inch in depth, 3| inches in diameter the top and 

 bottom slightly convex mounted on a tripod. A hole in the 

 bottom allows the water to pass through a short tube, which is in 

 three sections, the first part metal, the second rubber and the 

 third glass. A pinch-cock can be applied to the rubber con- 

 nection, which will prevent water passing when the glass tube 

 has been removed for examination of contents. 



