348 FIRST FORMS OF VEGETATION. 
parent threads of extreme fineness, packed to- 
gether as closely as the pile of velvet, adhering to 
the surface of the fishes, and covering them as it 
were with a whitish slime. This appearance is 
generally regarded as a kind of decay or consump- 
tion in the animals themselves, and not as an ex- 
ternal clothing of parasitic plants. It is, however, 
a true vegetable growth, as is evident when it is 
placed under the microscope, for the unassisted 
eye can perceive nothing of its true structure ; each 
filament being terminated by a pear-shaped ball, 
about the z45eth of an inch in diameter, and con- 
sisting of a single cell filled with a mucilaginous 
fluid, in which float the reproductive granules. 
These granules move in currents running in vari- 
ous directions, like the circulation of cell-contents 
in the hairs of Tradescantia. As they advance to 
maturity, the mucilage disappears, and the motion 
of the granules becomes more rapid and violent, till 
ultimately they burst their way through the cell, and 
are transferred to the water, there to perform their 
circle of being, and to give birth to new granules. 
All this takes place with such rapidity that one 
observer has remarked that an hour or two suf- 
fices for the complete development and escape of 
the spores ; so that we need not wonder when we 
are told that, once established, the Achlya prolifera 
will often complete the destruction of a healthy 
gold-fish in less than twelve hours. 
