THE HATCHING OF OVA. 91 



it is covered with mucus, to wliich these particles, 

 of course, readily adhere. The greater portion of 

 these impurities are, of course, attracted to the gUls, 

 through which the water passes in the act of respira- 

 tion. The gills, acting as a kind of filter, catch and, 

 detain these particles, which speedily multiply and 

 check the flow of water, and consequently the aera- 

 tion dependent on it ; when, if the fish be unable to 

 rub them off, death from suffocation ensues. The 

 more impure, therefore, the water, of course the larger 

 the loss from this cause. Dr. Davy states that these 

 floating matters consist usually of fibres of the sim- 

 plest form of vegetation, particles of root entangled in 

 them, and granules and nuclei of various kinds. Dr. 

 Davy farther recommends, as a means of keeping the 

 water pure, aquatic plants. But the Doctor did not 

 employ a running stream in his experiments, merely 

 changing the water occasionally. He further adds, 

 that these will increase the infusoria which form the 

 earliest food of the little fish, and in this light the 

 suggestion is a good one ; moreover, the plants may 

 serve to rub off the above parasitic obstructions 

 against. I have always experienced much difficulty 

 in keeping the fish in the tray. They wUl wriggle 

 through any hole or crack, however small. I 

 have usually employed a piece of perforated zinc, 



