ARTIFICIAL BREEDING. 175 



they are in motion amongst the gravel. I may 

 mention that the surface of the fish, however 

 young, is covered with mucus, apt by its ad- 

 hesive quality to retain minute impurities, 

 vegetable and animal organisms and their 

 semina, from which scarcely any water is 

 absolutely free, and which growing, acting as 

 parasites, may, if not rubbed off, have a fatal 

 effect on the young fish. 



AMICUS. Do not these impurities and para- 

 sitical growths collect chiefly about the gills in 

 the instance of the young fish ? If I recollect 

 rightly, you have told me so. 



PISCATOR. They do, and, I believe, for this rea- 

 son, that the gills through which the water passes 

 in the act of respiration, or the function of aera- 

 tion of the blood analogous to it, perform the 

 part of a filter catching at their outer margins 

 and detaining the matters suspended in the 

 water as impurities, thereby proving a check 

 on the flow of water, and the aeration depending 

 on that flow. I have often examined with the 

 microscope the obstructing adhering matter, 

 and have found it commonly of a mixed nature, 

 fibres of the simplest form of vegetation, 

 particles of soot entangled in them, and granules 



