1 66 



BULLETIN OF THE BUREAU OF FISHERIES. 



this. The first consisted of a two-bladed propeller fastened in the middle of the bottom 

 of a tub and rotated slowly, there being enough space in the water above the blades to 

 allow the fish room to escape the stroke. This device was not very satisfactory, but 

 as it was operated by hand and the blades roughly constructed, effective use might be 

 made of a more carefully adjusted mechanism of this type. A second and more 

 promising device consists of a branched system of iron pipes bored with many small 

 holes (text fig. 3), through which fine jets of water are forced out at the bottom of a 

 tank. The amount of pressure in these fine jets can be easily regulated from the main 

 supply pipe, and the height to which the glochidia will be driven from the bottom is 

 thus controlled. The tank may be allowed to overflow at the top and the glochidia 



■<^-^S 



Fig. 3. — Apparatus for keeping glochidia suspended in water while fish are being exposed to them for j^ill-infections. Tap 

 water entering at S issues in fine jets through the very small holes placed along the top and sides of the pipes on the bottom 

 of the aquarium, and an even distribution of glochidia throughout the water is thereby maintained. By regulating the 

 force of the water entering the pipes at S the glochidia are prevented from rising to the top of the aquarium and escaping 

 with the overflow. 



prevented from being carried off in the overflow by so adjusting the force of the jets 

 that the glochidia will not rise quite to the surface. This device keeps the glochidia 

 suspended in a very uniform way, and it may prove to be just what is needed for the 

 uniform infection of large numbers of fish. 



CONDITIONS NECESSARY FOR SUCCESSFUL INFECTION. 



Three factors should be considered in attempting the infection of any species of 

 fish with glochidia, namely, the uniform suspension of the glochidia in the water, the 

 reaction of the glochidia when stimulated by mechanical or chemical contact with the 

 fish, and the reaction of the fish's tissues after the glochidium has become attached. 



